Multifrequency Study of
Variability of
Fermi
/LAT
Blazars
by
V´ıctor Manuel Pati˜
no ´
Alvarez
Thesis submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN ASTROPHYSICS
at the
Instituto Nacional de Astrof´ısica, ´
Optica y Electr´
onica
July 2012
Tonantzintla, Puebla
Under the supervision of:
Ph.D. Alberto Carrami˜
nana Alonso
Tenured Researcher INAOE, Mexico
Ph.D. Vahram Chavushyan
Tenured Researcher INAOE, Mexico
Ph.D. Luis Carrasco Baz´
ua
Tenured Researcher INAOE, Mexico
©
INAOE, 2012
The author hereby grants to INAOE permission to
reproduce and to distribute publicly paper and electronic
copies of this thesis document in whole or in part.
©INAOE, 2015
The author hereby grants to INAOE permission to reproduce
and to distribute publicly paper and electronic copies of this thesis
document in whole or in part.
A study of optical and radio
properties of radio-loud AGN
by
Eric Faustino Jiménez Andrade
Thesis submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN ASTROPHYSICS
at the
Instituto Nacional de Astrofísica, Óptica y Electrónica
August 2015 Tonantzintla, Puebla
Under the supervision of:
Dr. Heinz Andernach UGTO
Dr. Jonathan León-Tavares INAOE
Dr. Vahram Chavushyan INAOE
Contents
Abstract IX
1 Introduction 1
1.1 The AGN phenomenon . . . 2
1.1.1 Clasification Schemes . . . 3
1.1.2 Unification Scheme . . . 7
1.2 Radio-Loud AGN. . . 10
1.2.1 Giant Radio Galaxies . . . 10
1.2.1.1 The FR dichotomy. . . 12
1.2.2 AGN - host galaxy coevolution . . . 13
1.3 Motivation . . . 15
2 The Sample 17 2.1 GRG Sample . . . 18
2.1.1 NVSS . . . 19
2.1.2 FIRST. . . 21
2.1.3 SDSS . . . 22
2.2 BL Lac Sample . . . 24
2.2.1 Data reduction . . . 25
3 Methodology 29 3.1 Optical analysis . . . 30
3.1.1 Starlight . . . 30
3.1.2 Star Formation History . . . 32
3.1.3 Black Hole Mass . . . 34
3.1.4 Environmental and morphological classification . . . 37
3.2 Radio parameters. . . 41
3.2.1 Quantitative classification . . . 41
3.2.1.1 Morphological asymmetries . . . 41
3.2.1.2 Flux density asymmetries and radio luminosity . . . 42
3.2.2 Qualitative classification . . . 43
4 Results and Discussion 47 4.1 Unveiling the nature of Giant Radio Galaxies . . . 48
4.1.1 New Catalog of GRGs . . . 48
4.1.2 Correlations between radio and optical parameters of GRGs. . . 50
4.1.2.1 High-z sample of GRG . . . 53
4.2 Exploring the AGN-host galaxy coevolution . . . 57
4.2.1 Star Formation in GRGs. . . 57
4.2.2 Young stellar population in BL Lacs . . . 59
4.2.3 A serendipitous discovery in PKS 0521−36 . . . 60
4.2.3.1 Velocity profiles . . . 62
4.2.3.2 HST optical image . . . 64
4.2.3.3 The central engine . . . 65
4.2.3.4 Mass outflow along the jet . . . 67
4.2.3.5 Concluding remarks . . . 69
5 Conclusions and Future Work 71 5.1 The nature of GRGs . . . 71
5.2 AGN-host galaxy coevolution . . . 72
5.2.1 GRGs . . . 72
5.2.2 BL Lacs . . . 73
5.3 Future Work . . . 74
A Derived optical and radio parameters of GRGs 77
Index of Figures 92
Index of Tables 93
Glossary 95
A mis padres: Carmen y Faustino. Y a todos mis hermanos.
Acknowledgments
This work was carried out during the years 2014-2015 at the Department of Astrophysics of the National Institute of Astrophysics Optics and Electronics (INAOE) and the Depart-ment of Astronomy of the University of Guanajuato (DA-UGTO).
I would like to express my gratitude to Dr. Chavushyan and Dr. Le´on-Tavares, my inter-nal research supervisors, for their patient guidance, advice and assistance in keeping my progress on schedule. I am deeply grateful to my external supervisor, Dr. Andernach, for his guidance into the world of extragalactic radio astronomy. I am grateful for his con-structive suggestions, constant support and helpful comments on the text. His willingness to give his time so generously has been very much appreciated.
My grateful thanks are also extended to Dr. Torres-Papaqui for his guidance with the Starlightcode, and to Dr. Coziol for his support with the classification of Giant Radio Galaxies. I am grateful to Dr. Kotilainen who provided the VLT/FORS2 spectra used in this work.
I acknowledge to the Vahram’s research group for fruitful scientific discussions: Alejandro, Victor and Emmanuel. Finally, I want to express my gratitude to the revisors of the manuscript for their valuable suggestions on the text; Dra. Cruz-Gonz´alez, Dr. Rodr´ıguez and Dr. Carrasco.
This thesis has been possible thanks to the grant from the Mexican National Council for Research and Technology (CONACyT) to carry out the Master Science program in Astrophysics at INAOE, and the grant from CONACyT (Becas Mixtas) to do a research stay at the DA-UGTO.
Abstract
Studies of radio-loud AGN are crucial to address basic questions like how jets are formed, accelerated and collimated, and what is the effect of the presence of such powerful jets in the evolution of the host galaxy and the local environment. In this thesis we have focused on two open issues relevant to the radio-loud AGN phenomenon, the nature of Giant Radio Galaxies (GRGs) and the AGN-host galaxy coevolution.
We present a sample of 197 GRGs with a largest (projected) linear size (LLS) of at least 1 Mpc, divided into 93 low-z (z<0.4) and 104 high-z (z>0.4) sources for which optical spectra from the Sloan Digital Sky Survey (SDSS) are available. We obtained optical and radio parameters of GRGs in order to search for the reason for their having grown to Mpc scales. Moreover, we studied the circumnuclear regions of GRGs and 5 BL Lacs in order to get insights about the coevolution between the AGN and the host galaxy. We put special emphasis in the BL Lac PKS 0521−36 thanks to the remarkable observed features which allow us to obtain further details into the AGN-host galaxy coevolution. The main results of this thesis are:
(I) We found no correlation between the LLS and the environmental type, core radio luminosity nor the mass of the central supermassive black hole; hence, the origin of GRGs is not likely to be due to the environment properties or stronger nuclear activity. Since we see no significant trend for the linear sizes with redshift in GRGs, it would seem that it is neither the longer time scale of radio activity of a small fraction of radio galaxies that makes GRGs grow to their extreme sizes. However, we did find that the percentage of spectral activity type of GRGs changes from high to low-z: the number of QSOs significantly decreases, while the number of dwarf AGNs significantly increases with cosmic epoch (i.e. with decreasing z). This might suggest an evolution with cosmic time from high spectral activity to a low one, which is consistent with a reduction of AGN luminosity with cosmic time.
(II) We found tentative evidence which suggests that the largest GRGs have higher SFR. This supports the idea that large reservoirs of cold gas, which are essential for star forma-tion and an effective accreforma-tion process, might be ubiquitous in the largest GRGs. We did not detect significant young stellar population in most of the BL Lac objects. However, in RX J105782753 we found that at least 1% of the total stellar mass is due to young stars, which suggests ongoing star formation in this object.
(III) We found suggestive evidence of AGN-driven winds and ionized clouds moving in a helicoidal way along the jet in PKS 0521−36. Our data suggests the presence of an outflow moving along the major axis with a maximum velocity of 350 km/s at 3 kpc. The rotation curve of the ionized clouds moving along the optical jet can be well described with a sinusoidal function, in which both, amplitude and period, increase with distance. The farthest detected emission lies at 24 kpc and the maximum projected velocity reaches 100 km/s. Thus, this is the very first evidence of ionized clouds moving along the jet in a helicoidal way on kpc-scales, which suggests a strong interplay between the jet and the host galaxy through the so-called radio-mode AGN feedback.
Chapter 1
Introduction
Active Galactic Nuclei (AGN) are among the most powerful sources of energy in the Universe. They in fact emit in the whole electromagnetic spectrum and some of them are found to be highly variable in all the bands in which they have been observed. Their luminosities are ∼100 times larger than that of normal galaxies and they are detectable up to large distances. As a general definition, AGN are connected to energetic phenomena occurring in the central regions of galaxies that can not be attributed to stellar activity. A variety of objects are grouped under the name AGN: quasars, QSO, Seyfert galaxies, BL Lac, LINERS. The origin of these classification is substantially historical and not always implies a physical difference between objects in different classes.
The central engine is believed to contain a supermassive black hole in the range 106−
1010M
; which, in an accretion process, converts the potential energy of matter to
radia-tion and particle outflow. It is generally believed that the type of object we see depends on the viewing angle, whether or not the AGN produces a significant jet emission, and how powerful the central engine is.
A small fraction of AGN have prominent radio emission, they are called radio-loud AGN. Giant Radio Galaxies (GRGs) are those with a radio emission extending over a projected size of at least 1 Mpc, and are thus the largest structures in the Universe which are associated with radio-loud AGN. The reason for the extreme size of these sources has not been understood. Moreover, understanding the population of radio sources in the Universe, such as GRGs, is not only important for understanding the physical origin of radio emission in AGN, but also for understanding galaxy evolution. Therefore, the study of powerful radio-loud AGN such as GRG and BL Lacs are crucial for the understanding the AGN-host galaxy coevolution.
Throughout this thesis, we assume cosmological parameters of Ω0 = 0.3, Ωm = 0.7 and H0= 72kms−1M pc−1.
1.1
The AGN phenomenon
With typical bolometric luminosities 1045−48erg s−1, AGN are amongst the most luminous
emitters in the Universe, particularly at high photon energies and radio wavelengths. They are characterized by a very large luminosity produced in a very small volume. AGNs can be 100 or in some cases 1000 times more luminous than the stars of their host galaxy. The shape of the continuum emission of an AGN can not be described by a thermal spectrum. It follows approximately a power-law over a wide range of frequencies due to a combination of synchrotron and Compton mechanisms (Figure 1.1).
Recognition that AGN can be sites of very energetic and compact phenomena began with the identification of strong radio sources with galaxies, frequently with the radio emission arising in the form of a pair of sources symmetrically located about the galaxy (double-lobed radio source). Therefore, radio selection was originally the principal method to find AGN (e.g. Matthews et al., 1964). AGN are also luminous IR sources. Typical AGN infrared luminosities are 1044−46 erg s−1 which represents a significant fraction,
∼ 30% on average, of the bolometric luminosity (Treister et al.,2008).
Figure1.1: A schematic representation of an AGN Spectral Energy Distribution (SED), loosely based
on the observed SEDs of radio-quiet quasars (e.g.Richards et al.,2006). The black solid curve represents the total SED and the various colored curves represent the individual components. Also shown is an example radio-UV SED of a starburst galaxy. Grey curve; the SED is of M82 taken from the GRASIL library (Silva et al.,1998) (Credits: Chris Harrison - Centre for Extragalactic Astronomy/University of Durham).
The UV-optical spectra of QSOs, a luminous type of AGN (Section 1.1.1), are distinguished by strong broad emission lines. The strongest observed lines are the hydrogen Balmer-series lines (Hα λ6563 ˚A, Hβ λ4861 ˚A and Hγ λ4340 ˚A), Lyα λ1216 ˚A and prominent lines of abundant ions (Mg II λ2799 ˚A, C III λ1909 ˚A and C IV λ1549 ˚A (Figure 1.2). Star forming regions emit strong emission lines as well; nevertheless, Baldwin et al. (1981) reported that the photoionizing spectrum of a power-law continuum source, such as an AGN, produces very different emission line intensity ratios when compared with that of
The AGN phenomenon 3
typical star-forming regions (mostly due to O and B stars). Hence, emission lines can be used to identify the presence of AGN even in galaxies in which the optical spectrum does not show any direct AGN signature such as broad emission lines and/or an associated double-lobed radio structure.
Figure 2: Composite rest-frame optical/UV spectrum for the optically-selected quasars in the Sloan Digital Sky
Survey, from the work of Vanden Berk et al. [49]. Thedashedanddottedlines show power-law fits to the
continuum emission.
2.3 Optical Emission Lines
As first reported by Baldwin et al. [54], the photoionizing spectrum of a power-law continuum source, such as an AGN, produces very different emission line intensity ratios when compared with that of typical star-forming regions (mostly due to O and B stars). Hence, emission lines can be used to identify the presence of AGN even in galaxies in which the optical/continuum does not show any direct AGN signature, due to obscuration and/or low luminosity. Because the AGN ionizing emission reaches material even a few kilo-parsecs away from the nuclear region, this selection technique is less sensitive to circumnuclear obscuration, and thus provides a more complete AGN view when compared with, for example, optical/UV continuum selection. This technique was used successfully in the SDSS [55, 56] to extend the low-redshift AGN sample to lower luminosities. Emission line ratios and diagnostic regions can be seen in Fig. 3. Emission-line selection can also be used at higher
redshifts, as shown by the DEEP2 galaxy redshift survey, which selected a sample of 247 AGN atz⇠1 from
optical spectroscopy using the DEIMOS spectrograph at the Keck observatory [57].
While this is an efficient AGN selection technique, optical spectroscopy is very expensive in telescope time, and is only feasible for relatively bright emission line regions. This selection may be incomplete at the low luminosity end, if the host galaxy can outshine the high-ionization emission lines. It is currently very
difficult to extend this selection beyondz⇠1, as the relevant emission lines move to observed-frame near-IR
wavelengths, where current-generation spectrographs are significantly affected by atmospheric emission, do not cover wide field of views and have limited multi-object capabilities.
2.4 X-rays
As was found more than 30 years ago, AGN are ubiquitous X-ray emitters [59]. Their X-ray emission extends
from⇠0.1 keV to⇠300 keV and is attributed to inverse-Compton scattering due to high-energy electrons in a
hot corona, surrounding the accretion disk. The high-energy cutoff at⇠100-300 keV is presumably due to a
cutoff in the energy distribution of the electrons in the hot corona. AGN are typically⇠1-5 orders of magnitude
4 -1.58 -0.46
Figure1.2: Composite rest-frame optical/UV spectrum for the optically-selected quasars in the Sloan
Digital Sky Survey, from the work of Vanden Berk et al. (2001). The dashed and dotted lines show power-law fits to the continuum emission.
As was found more than 30 years ago, AGN are strong X-ray emitters (Elvis et al.,1978). Their X-ray emission extends from 0.1 keV to 300 keV. AGN are typically 1-5 orders of magnitude more luminous in X-rays than normal galaxies, which makes them the domi-nant extragalactic population at these wavelengths. The high-X ray luminosities of AGN strongly suggest that they should also be sources of gamma-rays. Indeed, this is now con-firmed by observations. The vast majority of gamma-ray sources detected with Fermi-LAT 1 are associated with AGNs (Abdo et al.,2010).
A striking characteristic of most AGN is that they show variability in their emission line and continuum in every waveband in which they have been studied. This variation could be on different timescales, from months to days even as short as hours (e.g.Pati˜no- ´Alvarez et al.,2013, and references therein). From studying the variability of these objects, it is known that the region where most of the energy of an AGN is produced is rather compact, with a size of the order of parsecs. (e.g.Ulrich et al., 1997).
1.1.1
Clasification Schemes
The taxonomy of AGN is complex and somewhat confusing because the fundamental physical differences between different types of AGN are not clear. Observationally, AGN are classified in various ways such as via luminosity, spectral type or morphology (especially in the optical and radio regimes). In addition, depending upon their properties some types are further divided into several sub-types. However, there are some generally recognized AGN subsets which are summarized and briefly discussed below.
• Seyfert Galaxies
Seyfert galaxies, normally found in spiral galaxies, are lower luminosity AGNs with
MB > 21.3 + 5 log(H0), with quasars defining the higher luminosity AGNs (H0
is the Hubble constant). The luminosity division between Seyferts and quasars is rather arbitrary. Khachikian and Weedman (1974) recognized two subtypes of Seyferts, type 1 and type 2, depending on the presence of a broad component in the permitted lines (Figure 1.3). Osterbrock(1981) further introduced type 1.5, 1.8 and 1.9 Seyferts, according to the presence of broad bases inHα andHβlines.
Narrow Lines
Broad Lines
Narrow Lines
Angstroms
Int
ens
it
y (A
rbi
tra
ry U
ni
ts
)
Figure1.3: Typical spectra of a “normal” galaxy and Seyfert galaxies. (Upper panel) When a galaxy
does not host an AGN, the stellar component dominates in its spectrum (NGC 3368). (Middle panel) Seyfert 1 galaxies have both broad and narrow lines in their spectra (NGC 4151); (bottom panel) while Seyfert 2 galaxies only have narrow lines (NGC 4941). Based on the image from Bill Keel’s slide set (www.astr.ua.edu/keel/agn).
• Quasars
In the 1960s it was observed that certain compact objects emitting radio waves had very unusual optical spectra. These objects were named Quasistellar Radio Sources (QSR, meaning radio sources with a starlike optical identification) which was soon contracted to quasars. Later, it was found that many similar objects did not emit radio waves; these were termed Quasistellar Objects or QSOs. The cosmological distance inferred from the high redshift of quasars implies an extreme luminosity (typically ≥1045ergs s−1, i.e. the most luminous AGNs), which could
not be explained at that time. Nevertheless, now it is clear that quasars form another class of AGNs typically found at higher redshift with higher luminosity than Seyfert galaxies.
In the last few decades the old distinction between QSR and QSO has been aban-doned in favor of using QSO or quasar only, and naming the “old” QSRs as radio-loud QSOs and the others as radio-quiet QSOs.
The AGN phenomenon 5
• Radio Galaxies
Radio galaxies are strong radio sources typically associated with giant elliptical galaxies. Similar to Seyfert galaxies, radio galaxies can also be divided into broad line radio galaxy (BLRG) and narrow line radio galaxy (NLRG). Basically these look like radio loud Seyferts, but they seem to occur in ellipticals rather than spirals. On the other hand, the subclassification of the radio galaxies according to their radio morphology is based on the measurement of the ratio R of the distance between the two brightest spots and the overall extent of the radio emission: Fanaroff-Riley (FR I) with R<0.5 and Fanaroff-Riley (FR II) with R>0.5 (Fanaroff and Riley,1974). The FR I objects (Figure 1.4) have the highest surface brightness along the jets near the core. In contrast, FR II sources (Figure 1.4) show the highest radio surface brightness at the lobe extremities. It is important to note that the cut between FR I and FR II is also somewhat ambiguous: hybrid sources showing jets FRI-like on one side and FR II-like on the other have been observed (e.g.Gopal-Krishna and Wiita, 2000). Further details about this FR dichotomy are given in Section 1.3.1.
Figure1.4: Radio images of the two types of radio galaxies: (a) at low radio luminosity, an FR I radio
galaxy, 1231+674, with diffuse, approximately symmetric jets whose surface brightness falls off away from the center, and (b) at high radio luminosity, an FR II radio galaxy, 1232+414, with sharp-edged lobes and bright hot spots; the jets in this case are often too faint to see. Courtesy of Frazer Owen and Mike Ledlow (Urry and Padovani,1995).
It has been proposed a classification scheme based on the presence of strong emission lines; in this way, Low Excitation Radio Galaxies (LERGs) and High Excitation Radio Galaxies (HERGs) are recognized (e.g.Tasse et al.,2008;Hickox et al.,2009). As the name suggests, in LERGs the strong emission lines normally found in powerful AGN are absent. On the other hand, the HERGs category includes the traditional classes of narrow-line radio galaxies with spectra like those of Seyfert 2, the broad-line radio galaxies and the radio-loud quasars. In recent years it has become clear that the differences between these objects are not only a matter of emission-line strength but also in the observed emission in the optical (Chiaberge et al., 2002), X-ray (Hardcastle et al.,2006) and mid-IR band (Hardcastle et al.,2009). HERGs are typically hosted by lower mass, bluer galaxies (Best and Heckman,2012) located in less dense environments. Despite this preference, LERGs have been identified in blue galaxies and HERGs in red galaxies.
• LINERS
Low Ionization Nuclear Emission Line Regions (LINERs) are the least luminous but most common type of AGNs. They were first identified by Heckman (1980). More than 30% of all spiral galaxies are LINERs, thus they are the most common type of AGN. The optical spectrum of LINERs is similar to Seyfert 2. Compared to Seyferts, LINERs have stronger low-ionization lines such as [SII] λλ6717,6731 ˚A, [NII] λ6584 ˚A, [OII] λλ3726,3729 ˚A and [OI] λ6300 ˚A (Ho et al., 1997). They also have a line luminosity ratio of [OIII]λ5007/Hβ≤3.
• Blazars
Blazars are among the most powerful AGN. Blazars are often very luminous and vio-lently variable over a large range of wave bands from radio to gamma-rays. Spectral Energy Distributions (SEDs) of blazars are characterized by two broad components: a long-wavelength maximum anywhere from infrared to X-ray frequencies, and a second peak at higher energies, from hard X-rays to TeV gamma-rays (e.g.Bonning et al., 2012). Two types of blazars are recognized according to their optical prop-erties: (1) flat-spectrum radio quasars (FSRQs), which show strong, broad emission lines in their optical spectrum; and (2) BL Lacertae (BL Lac) objects, which are characterized by an optical spectrum which at most shows weak emission lines, some-times displays absorption features and in some cases can be completely featureless (Giommi et al.,2012).
In a more general scheme, AGN can be divided into objects that are radio-loud and those that are radio-quiet. A useful criterion to distinguish carefully between radio-loud and radio-quiet objects, appears to be the radio-optical ratio Rr−o of specific fluxes at
6 cm (5 GHz) and 4400 ˚A (680 THz) (Kellermann et al., 1989); for radio-loud objects
Rr−o is generally in the range 10−1000, and most radio-quiet objects fall in the range
0.1< Rr−o <1. While this still leaves some ambiguous cases near the demarcation line,
a criterionRr−o≥10 for “radio-loudness” appears to be appropriate.
The radio-quiet objects can be further categorized as Seyfert galaxies or quasi-stellar objects (QSOs); the radio-loud objects encompass radio galaxies, quasars and blazars. In fact, only a small percentage (∼10%) of all AGN are known to be radio-loud.
At this point it is clear that the AGN family is diverse. In the next section, I aim to describe the way in which the AGN taxonomy has been unified.
The AGN phenomenon 7
1.1.2
Unification Scheme
The attempts to explain the AGN phenomena were driven by two main arguments: (a) different components of the AGN are responsible for the observed emission and (b) the orientation of the AGN with respect to the line of sight. In this way the radio, infrared, optical, UV, X-ray andγ-ray observations of AGNs can be explained with a single model of the central AGN region. The different types of AGN result from different viewing angles towards the symmetry axis, and from the absence or presence of certain compo-nents. In the following I briefly explain the most important emission components and their interpretation (see Figure 1.5):
• AGN components
It is now generally accepted that AGN are powered by the release of gravitational en-ergy from a supermassive central black hole (SMBH). Enen-ergy linked to the black hole spin (e.g. Blandford and Znajek, 1977) or rotating accretion disks (e.g. Blandford and Payne,1982) may be instrumental for forming prominent jets which transport material from the innermost region of the AGN out to kpc scales, sometimes even Mpc-scale distances with relativistic speeds (Section 1.3.1). On these scales, such jets are identified via the detection of non-thermal radio emission attributed to syn-chrotron radiation of relativistic electrons in the jet (Begelman et al., 1984).
X-ray emission from a large number of kpc-scale AGN jets has been observed ( Mar-shall et al.,2005). This emission has been explained either as synchrotron emission from several distinct populations of relativistic electrons, or as Inverse Compton emission from electrons scattering of the Cosmic Microwave Background (CMB), assuming that the jet plasma moves highly relativistically towards the observer (e.g. Harris and Krawczynski, 2006). It has been observed that the extragalactic γ-ray sky is dominated by radio-loud AGN, being mostly blazars, some radio galaxies and a few radio-loud narrow-line Seyfert 1 (NLSy1) galaxies (e.g. Le´on-Tavares et al., 2014). In particular, the radiation mechanism of the γ-ray emission in blazars is widely believed to be inverse Compton scattering of ambient photons, either from inside the jet (synchrotron-self-Compton - SSC;Bloom and Marscher,1996), or from an external source (B la˙zejowski et al., 2000).
In the vicinity of the central region of an AGN matter is accreted from a disk onto the black hole. Optical/UV continuum emission is often attributed to the thermal radiation with temperatures ∼30,000 K originating in the accretion disk (Malkan and Sargent,1982). The accretion disk is partially covered by a corona of hot (still thermal) material. The hot corona Comptonizes some of the emission, producing a high-energy tail that extends into the hard X-ray regime (e.g.Galeev et al.,1979).
In the so-called broad line region (BLR) dense and fast-moving clouds of matter, orbiting above or below the accretion disk, exhibit broad optical emission lines (e.g. Holt et al.,1980). In contrast, in the narrow line region (NLR) larger clouds of mat-ter, moving at slower speeds and farther away from the black hole, exhibit narrow-line emission. The BLR clouds have distances on the order of 10 light days from the SMBH; the NLR clouds orbit the SMBH at distances of a few hundred parsecs. A large torus of gas and dust (≈1 pc diameter) resides outside of the accretion disk
and the BLR. This torus tends to obscure the accretion disk and the BLR along most lines of sight, and emits reprocessed emission from the central engine in the infrared (Nenkova et al.,2008).
• Orientation effectsThe AGN phenomenon Volker Beckmann
BL Lac FSRQ
BLRG, Type I QSO
NLRG, Type II
QSO BLRG
NLRG
Seyfert 2
Seyfert 1
transmitted
scattered
absorbed reflected
narrow line region broad line region
black hole accretion disc electron plasma dusty absorber
ra
d
io
-l
o
u
d
(R
L
)
A
G
N
ra
d
io
-q
u
ie
t
(R
Q
)
A
G
N
high power low power
jet
Figure 1:Schematic representation of our understanding of the AGN phenomenon in the unified scheme [1]. The type of object we see depends on the viewing angle, whether or not the AGN produces a significant jet emission, and how powerful the central engine is. Note that radio loud objects are generally thought to display symmetric jet emission. Graphic courtesy of Marie-Luise Menzel (MPE).
binaries and the super massive black holes. The Ultra-luminous X-ray sources (ULX) are candi-date examples of intermediate mass black holes (IMBH; [16]) which could help bridge the gap, but further study and in particular improved classification of their non-X-ray counterparts will be necessary to settle this question. Other LLAGN classes need to be separated beyond ambiguity
from the non-active galaxies. In particular HIIgalaxies and LINER tend to become
indistinguish-able below some signal-to-noise threshold [17]. The forthcoming large survey telescopes surveys should bring clarification. Finally, the illusive link between AGN and non-active super massive black holes, like Sgr A* in our very own galaxy, needs to be understood.
AGN research remains a rich field, worthy of our investments of time, energies and talents that will continue to provide unexpected future insights into the nature of the Universe we live in.
Acknowledgement: We thank the anonymous referee for the constructive comments.
References
[1] Beckmann, V. & Shrader, C. R. 2012, “Active Galactic Nuclei”, 380 pages, Wiley-VCH
[2] Fabian, A. C., Iwasawa, K., Reynolds, C. S., & Young, A. J. 2000, PASP, 112, 1145
5
Figure 1.5: Schematic representation of our understanding of the AGN phenomenon in the unified
scheme. The type of object we see depends on the viewing angle, whether or not the AGN produces a significant jet emission, and how powerful the central engine is. The central black hole (for anM= 108M
black hole, the Schwarzschild radius isRS = 3×1013 cm) is surrounded by an accretion disk (of size
∼1−30×1014cm). The broad emission lines originate in clouds orbiting above the disk (at∼2−20×1016
cm). A thick dusty torus (inner radius∼1017 cm) obscures the BLR when the AGN is seen from the
side; a hot corona above the accretion disk probably plays a role in producing hard X-rays; narrow lines are produced in clouds much farther from the central source (1018−1020cm). Radio jets (extending from
∼1015cm to several times 1024cm) emanate on one of both sides from the region near the black hole in
the case of radio loud AGNs (Krawczynski and Treister,2013). Graphic courtesy of Marie-Luise Menzel (MPE) (Beckmann and Shrader,2012).
It has been suggested (e.g.Antonucci and Miller,1985) that the different classes of AGN can be unified within a single scenario in which the orientation of the system with respect to the observer is the main factor in producing the different observed properties. At larger viewing angles, the gas torus conceals the BLR and only narrow lines are observed, resulting in Type-2 Seyferts, narrow line FRI and FRII radio galaxies. Closer to the line of sight, the torus no longer obscures the BLR and the AGNs appear as Type-1 Seyferts, radio-quiet quasi-stellar objects (QSOs), broad-line steep-spectrum radio quasars and FSRQs. For viewing angles<10◦, the relativistically beamed non-thermal continuum emission from the relativistic jet dominates and the object is a blazar. The blazar class encompasses flat spectrum radio quasars (FSRQs), which have FR type II morphology jets (FRII,Fanaroff and Riley,1974), and BL Lac objects, which are thought to have FR type I morphology jets (Urry and Padovani,1995). Based on orientation-independent properties
The AGN phenomenon 9
like extended radio emission, host galaxy types and magnitudes, galaxy environments,the
FRI radio galaxies are thought to be the parent population of the BL Lac objects(Wardle
et al.,1984;Antonucci and Ulvestad,1985).
Despite the general acceptance of the unification scheme of AGN as sketched in Figure 1.5, this scheme fails to explain all the different varieties of observed AGN phenomena. Other important considerations may be the mass of the central black hole, the accretion rate, or the specific geometry of the absorber (Beckmann and Shrader, 2012). Recently, this classification scheme has been criticized on the grounds that it introduces strong selection effects (Ghisellini et al.,2011;Giommi et al.,2012). In addition, very little is known about the degree to which the unified model applies to low luminosity AGN (LLAGN) for which
Lb≈(10−5
−10−3)LEdd2(e.g. Istomin and Sol,2011).
The observational properties of LLAGNs are quite different from those of more luminous AGNs. In fact, the low emission-line luminosities in LLAGN can be modeled in terms of photoionization by hot, young stars (e.g. Terlevich and Melnick, 1985), by collisional ionization in shocks (e.g. Koski and Osterbrock, 1976) or by aging starbursts ( Alonso-Herrero et al., 2000). However, some fraction of LLAGNs has been identified as the counterparts of strong double-lobed radio structures (e.g.Nagar et al.,2002).
Moreover, the SED of LLAGN does not seem to have the thermal continuum prominence in the ultraviolet (UV) - the “big blue bump” - which is one of the signatures of the presence of an optically thick, geometrically thin accretion disk (Eracleous et al.,2010). LLAGNs typically have weak and narrow Fe Kαemission (Terashima et al.,2002) which suggests the absence of a thin accretion disk. These observations favor the scenario in which the accretion flow in LLAGNs is advection-dominated (ADAF; Narayan et al., 1998) or radiatively inefficient (RIAF).
Thus far, we are quite certain about a number of characteristics of AGN; however, a lot of questions remain open. As a result, the study of radio-loud AGN has become crucial for the understanding of the AGN phenomenon.
2L
eddis a natural limit to the luminosity that can be radiated by a compact object of massM. This
limit arises because both the attractive gravitational force acting on an electron-ion pair and the repulsive force due to radiation pressure decrease inversely with the square of the distance from the black hole. Thus,Ledd= 4πGM mpc/σT; whereGis the gravitational constant,mpis the mass of a proton,cis the
1.2
Radio-Loud AGN
As mentioned before, AGNs have been divided into radio-loud and radio-quiet objects, i.e. those with and without prominent radio emission. A lot of research has been done but still we do not know what makes an AGN radio-loud. Some authors have argued that radio-loud AGN represent a fleeting active phase in the evolution of the subset of giant elliptical galaxies that have recently undergone galaxy interactions and mergers (e.g. Ramos Almeida et al.,2012). Moreover, there is a lack of knowledge about how these jets are launched, how they remain collimated over distances of∼100 kpc and how they reach distances of Mpc as seen in Giant Radio Galaxies (GRGs). However, it appears certain that all these questions are tightly connected to the accretion process powering the AGN and the central engine itself.
Understanding the population of radio sources in the Universe is not only important for understanding the physical origin of radio emission in AGN, but also for understanding
galaxy evolution. Radio-loud AGN are particularly important for understanding the link(s)
between galaxy evolution and nuclear activity, because they are invariably associated with early-type host galaxies3, allowing relatively clean searches to be made for the signs of the triggering events. In radio-loud AGN, the powerful radio jets may also shock-accelerate the gas, driving bipolar winds at speeds up to thousands of km/s (e.g. Nesvadba et al., 2008). On a larger-scale, radio-loud AGN have been invoked as the solution to both the cooling flow and the “entropy floor” problems in the intra-cluster medium of groups and clusters (McNamara and Nulsen,2007, and references therein).
In the following, I describe the open issues related with radio-loud AGN that we aim to address in this thesis.
1.2.1
Giant Radio Galaxies
Giant Radio Galaxies (GRGs) are those radio galaxies in which the radio emission exceeds a projected largest linear size (LLS) of 1 Mpc (see Figure 1.6). These extended radio sources constitute the largest known physically connected structures in the Universe which are associated with AGN; therefore, it is possible that they could play a special role in the formation of the large-scale structure. They are extremely useful for studying a number of astrophysical problems. For instance, since a number of known GRGs exhibit asymmetric structures over their megaparsec extents (e.g.Saripalli et al.,2005) these objects are ideal candidates to investigate the distribution and physical state of the gas in the Warm-Hot Intergalactic Medium (WHIM) (e.g.Malarecki et al.,2013,2015). In addition, GRGs can be used to study the evolution of radio sources, constraining orientation-dependent unified schemes, probing the intergalactic medium at different redshifts, etc (e.g.Subrahmanyan and Saripalli, 1993; Subrahmanyan et al., 1996; Mack et al., 1998; Schoenmakers et al., 1998).
Systematic studies of GRGs as a population have always been hampered by the small number of sources available and by non-uniform selection effects. Since GRGs are large
Radio-Loud AGN 11
Figure1.6: 3C236 at 608MHz observed with the WSRT, Mack et al. (1998). Size = 38.4×38.4 arcmin.
It is among the largest known radio galaxies, with the radio structure having a total linear size of 4.2 Mpc.
and their radio emission is not very powerful their surface brightness is relatively low. This makes them difficult objects to detect or recognize in most large-scale radio surveys. As a result, a large fraction of the known GRGs have been discovered serendipitously (e.g. Hine,1979;de Bruyn,1989), hence the difficulty in obtaining a uniformly selected sample. However, recent efforts have focused on the identification of GRGs in radio surveys such as Westerbork Northern Sky Survey (Rengelink et al.,1997), NRAO VLA Sky Survey (NVSS; Condon et al.,1998), Faint Images of the Radio Sky at Twenty centimeters (FIRST;Becker et al., 1995) and Sydney University Molonglo Sky Survey (Bock et al.,1999). These new sources have been included in several samples (e.g.Schoenmakers et al.,2001;Lara et al., 2004;Machalski et al., 2006).
It has been argued that GRGs could be the result of normal radio galaxies expanding in very low density environments permitting them to reach their overwhelming sizes (e.g. Subrahmanyan et al.,2008; Kuligowska et al.,2009). Nevertheless, for GRGs within the footprint of SDSS DR7 and with z<0.3,Ortega-Minkata et al.(2013) applied an algorithm to count the number of neighbor galaxies within 1 Mpc projected radius from the GRG host, finding no relation between LLS of the radio emission and ambient galaxy density. On the other hand,Komberg and Pashchenko(2009) proposed that the giant sizes of radio sources can be attributed to the population of long-lived radio-loud active nuclei, which in turn can evolve into GRGs. Thus, these extremes objects are probably old objects in later stages of their evolution.
GRGs could also result from very powerful core activity since it is believed that strong jet activity in an AGN is related to the pc/sub-pc scale condition of its host galaxy, more precisely to the properties of its central black hole (e.g.Blandford and Rees,1974). Ku´zmicz and Jamrozy(2013) found a correlation between the mass of the central black hole with the LLS of 21 giant radio sources (LLS > 0.7 Mpc) associated with quasars (i.e Giant Radio Quasars - GRQs). On the contrary, it has been found that the GRGs have core strengths similar to that of smaller sources of similar total luminosity, hence their large sizes are unlikely to be caused by stronger nuclear activity as was proposed by Ishwara-Chandra and Saikia(1999).
GRGs are also interesting objects to study the interplay between the AGN, host galaxy and the surrounding environment. They allow us to study their radio structures in detail and to use them as probes of the gaseous environment of their host galaxies on scales of a few hundred kpc to a few Mpc. Moreover, as noted byJamrozy et al. (2005), such radio galaxies may affect the processes of galaxy formation, since the pressure of gas, outflowing from the radio source, may compress the circumnuclear cold gas clouds thus initiating the formation of stars on one hand, and quenching the formation of galaxies on the other hand.
1.2.1.1 The FR dichotomy
The previous sections described that radio-loud AGN can be classified according to their radio morphology as proposed byFanaroff and Riley (1974). However, the origin of the FR dichotomy has been a long-standing open question in AGN astrophysics (e.g.Saripalli, 2012). A large number of explanations for the FR dichotomy have been proposed. These include intrinsic differences in the central engine and properties of the intergalactic medium (IGM). It has been argued that: 1) FR Is have higher mass black holes than FR IIs (e.g. Ghisellini and Celotti,2001); 2) FR IIs have faster-spinning black holes (e.g.Meier,1999) and higher accretion rates (e.g.Marchesini et al.,2004); 3) FR I jets comprise electrons and positrons and FR II jets comprises electrons and protons (e.g.Celotti and Fabian,1993). It has also been speculated that FR IIs evolve into FR I radio sources (e.g.Maraschi and Rovetti,1994;Jackson and Wall,1999).
As mentioned in Section 1.1, a relationship is known between the FR classification and the existence of high-excitation (HE) emission lines in the optical spectra of their host galaxies (Hine,1979;Laing et al., 1994). In this scheme, objects without high-excitation emission lines (i.e. no strong emission lines) are referred to as low-excitation (LE) radio galaxies, and they are most common at low radio luminosities. Almost all FR I radio galaxies are LE sources, while optical hosts of the most powerful radio sources, i.e., FR IIs, usually have strong emission lines. However, many FR II galaxies have been found to be LE radio galaxies (e.g.Evans et al.,2006). It has been proposed (e.g.Hardcastle et al.,2006,2007) that low- and high- excitation radio AGNs (LERAGN and HERAGN, respectively) reflect different modes of black hole (BH) accretion. LERAGN are a class of radio-luminous AGNs that accrete radiatively inefficiently (e.g.Hardcastle et al., 2006); Bondi accretion (spherical accretion) of the hot X-ray emitting IGM is sufficient to power the jets of low-power radio galaxies in the centers of galaxy clusters (Allen et al.,2006). On the contrary, HERAGN sources are powered by radiatively efficient accretion (at Eddington rates) of cold gas (e.g.Merloni and Heinz,2008).
Since the work of Fanaroff and Riley (1974) the FRIs were to have lower radio powers than FRIIs on average; however,Ledlow and Owen(1996) showed that the radio luminos-ity boundary between FRI and FRII sources seemed to increase with increasing optical luminosity (Mabs) of the host galaxies. In the case of radio galaxies Mabs is dominated by the stellar light of the host galaxy. Recently, using a sample of over 2000 radio-loud AGN, Best (2009) did not find a clear segregation in radio luminosity between the FRI and FRII radio sources as proposed byLedlow and Owen(1996).
Radio-Loud AGN 13
Another theory to explain the FR dichotomy holds that very similar jets are ejected in both FR classes but interactions with the environment determine their large-scale morpholo-gies (Gopal-Krishna and Wiita,2000). Studies of hybrid sources displaying both FRI and FRII jet characteristics (with one lobe being edge brightened and another edge darkened), strengthened the view that the environment in which the jets propagated may be respon-sible for the kind of structure developed on large scales (e.g.Kharb et al.,2014).
In particular, some studies have pointed out the advantages of using GRGs to address this issue. Lara et al. (2004) found that two samples of FRI and FRII type GRG are separated by a break in the total radio luminosity, at logP1.4GHz = 25.0 W/Hz. In
addition, by analyzing the FRI and FRII abundances as a function of redshift they did not find evidence of a possible FRII to FRI evolution of radio galaxies. Further studies with a larger compilation of GRGs which include high-z sources might give new insights into this issue.
1.2.2
AGN - host galaxy coevolution
In recent years it has become apparent that AGN are not only interesting objects to study in their own right, but may also play an important role in the process of galaxy formation and evolution (seeBest et al.,2005). These ideas have emerged due to the tight correlation between black hole mass and that of the stellar bulge component of their host galaxies, as estimated from the velocity dispersion (Ferrarese and Merritt, 2000; Gebhardt et al., 2000), the absolute magnitude of the bulge (Kormendy and Richstone, 1995; Magorrian et al.,1998) or using virial methods (Marconi and Hunt,2003). As a result, it has been suggested a close relationship between the growths of black holes and galaxies.
During the growth of a BH huge amounts of energy can be liberated. The total accretion energy is two orders of magnitude higher than the binding energy of the host galaxy spheroid. If even a small fraction of the accretion energy can influence the gas on scales of 0.1 to 1000 kpc, growing BHs have the potential to regulate their own growth and impact upon the gas in their host galaxies and that in the larger-scale environment (Harrison, 2014, and references therein). In fact, the feedback from AGNs in star-forming galaxies could quench further star formation and black hole growth at the same time (e.g.Springel et al., 2005). For instance, based on their estimated molecular gas contents, Nesvadba et al.(2010) found that radio galaxies in their sample are∼10−50 times less efficient in forming stars compared to normal galaxies.
The AGN feedback may take place through two mechanisms: one is AGN radiation (known as radiative or quasar mode of AGN feedback), while the other is interaction of the jet with the ISM (known as kinetic or radio mode). The most intense effects of the kinetic mode are seen in powerful radio galaxies. With jets up to several Mpc in size, they can inject enormous amounts of energy, not only into their host galaxy ISM, but also into the IGM of the galactic group or cluster in which they reside (Labiano et al.,2013,2014; Russell et al., 2013). Studies of different populations of young and evolved radio galaxies and accurate measurements of their SFR are the key to understanding how AGN feedback affects the properties of their hosts.
Figure 1.Density volume rendering of the central part of the domain (32 kpc box length) att= 14 (left) andt= 22 Myr (right). Only thez >0 half is shown and densities close to the ambient X-ray gas are transparent to give a tomographic view. The colour bars show
logρin units ofmpcm−3.
Figure 2.Density volume rendering of the high density gas in the disc (face-on view) att= 14 (left) andt= 22 Myr (right). The box length is 32 kpc, lower density regions are transparent and the colour scale is the same as in Fig. 1.
cylindrical orifice of jet plasma (density 5×10−5m pcm−3,
speed 0.8c) with the same pressure as the environment, a radius ofrj= 0.4 kpc and an initial length in both directions of 3rj, respectively.
We include radiative cooling by a tabulated cool-ing function for atomic processes down to 104K as
Sutherland & Dopita (1993) with a metallicityZ= 0.5Z⊙ (Erb 2008), which is a very important effect in the disc due to the high densities and correspondingly short cooling times. No strong dependence on the metallicity or the exact form of the cooling function is expected since the mechanism found in our simulations can simply be understood by the fact that the cooling becomes more efficient (shorter cooling times) for denser gas. To avoid the need to include a fine-tuned feedback recipe to stabilize the cooling disc against the vertical gravity component we exclude gravity in the simu-lations and stabilize the disc by imposing a minimum tem-perature of 104K, which mimics a strong radiative heating
from the stars that prevents any cooling below the
thresh-old although this might still be expected in dense, shielded regions.
Since cooling is included, no truly static setup of the disc is possible and we run control simulations of the disc without the jet for comparison in order to monitor the evo-lution of the undisturbed disc. In contrast to Paper I, we use constant pressure for the initial conditions of the en-tire domain, however enforcing a pressure corresponding to a minimum temperature ofTmin/µ= 104K where
neces-sary (µ: mean particle mass in proton masses). Although the disc gas cools rapidly toTminand drops out of pressure balance, it shows only little evolution in the fluid variables with little impact on the dynamics (however, resulting in a different jet asymmetry than in Paper I). To allow for a more relaxed disc state, the jet of powerLkin= 5.5×1045
erg s−1
is only started att= 10 Myr. The simulations are carried out using the RAMSES 3.0 code (Teyssier 2002), a non-relativistic second-order Godunov-type shock-capturing adaptive mesh refinement code. The grid is refined to the maximum of 62.5 pc cell size in all regions of interest. For the
c
⃝??? RAS, MNRAS000, 1–14
Figure 1.Density volume rendering of the central part of the domain (32 kpc box length) att= 14 (left) andt= 22 Myr (right). Only thez >0 half is shown and densities close to the ambient X-ray gas are transparent to give a tomographic view. The colour bars show
logρin units ofmpcm−3.
Figure 2.Density volume rendering of the high density gas in the disc (face-on view) att= 14 (left) andt= 22 Myr (right). The box length is 32 kpc, lower density regions are transparent and the colour scale is the same as in Fig. 1.
cylindrical orifice of jet plasma (density 5×10−5m pcm−3,
speed 0.8c) with the same pressure as the environment, a radius ofrj= 0.4 kpc and an initial length in both directions of 3rj, respectively.
We include radiative cooling by a tabulated cool-ing function for atomic processes down to 104
K as Sutherland & Dopita (1993) with a metallicityZ= 0.5Z⊙ (Erb 2008), which is a very important effect in the disc due to the high densities and correspondingly short cooling times. No strong dependence on the metallicity or the exact form of the cooling function is expected since the mechanism found in our simulations can simply be understood by the fact that the cooling becomes more efficient (shorter cooling times) for denser gas. To avoid the need to include a fine-tuned feedback recipe to stabilize the cooling disc against the vertical gravity component we exclude gravity in the simu-lations and stabilize the disc by imposing a minimum tem-perature of 104K, which mimics a strong radiative heating
from the stars that prevents any cooling below the
thresh-old although this might still be expected in dense, shielded regions.
Since cooling is included, no truly static setup of the disc is possible and we run control simulations of the disc without the jet for comparison in order to monitor the evo-lution of the undisturbed disc. In contrast to Paper I, we use constant pressure for the initial conditions of the en-tire domain, however enforcing a pressure corresponding to a minimum temperature ofTmin/µ= 104K where
neces-sary (µ: mean particle mass in proton masses). Although the disc gas cools rapidly toTminand drops out of pressure
balance, it shows only little evolution in the fluid variables with little impact on the dynamics (however, resulting in a different jet asymmetry than in Paper I). To allow for a more relaxed disc state, the jet of powerLkin= 5.5×1045
erg s−1is only started att= 10 Myr. The simulations are
carried out using the RAMSES 3.0 code (Teyssier 2002), a non-relativistic second-order Godunov-type shock-capturing adaptive mesh refinement code. The grid is refined to the maximum of 62.5 pc cell size in all regions of interest. For the
c
⃝??? RAS, MNRAS000, 1–14
Figure1.7: Simulations of AGN feedback (in a box of 32 kpc on a side) at 14 Myr (left) and 22 Myr
(right) after the onset of the jet, in edge-on (top) and face-on (bottom) views of log density (mpcm−3). In
this simulation we observe the interaction of a powerful AGN jet with the massive gaseous disc (1011M)
of a high-redshift galaxy. Following the system over more than 107 years, we can observe that the jet
activity excavates the central region, but overall causes a significant change to the shape of the density probability distribution function and hence the star formation rate due to the formation of a blast wave with strong compression and cooling in the ISM. This results in a ring- or disc- shaped population of young stars. At later times, the increase in star formation rate also occurs in the disc regions further out since the jet cocoon pressurizes the ISM (Gaibler et al.,2011).
It has been observed that powerful radio galaxies show evidence of ongoing AGN feedback, mainly in the form of fast, massive outflows (e.g.Labiano et al., 2013,2014). Analytical models have used the idea of galaxy-wide outflows to explain the relationship between BH-mass and galaxy BH-mass (Alexander and Hickox, 2012). AGN-driven outflows (in addition to supernovae) may be required to unbind gas from their host galaxies to fully explain the chemical enrichment of ICM and the IGM (e.g.Fabjan et al., 2010) and it has also been proposed that, in some cases, AGN-driven outflows could also cause positive feedback by triggering star formation (SF) episodes through induced pressure of the cold gas, see Figure 1.7 (Gaibler et al.,2011;Nayakshin and Zubovas,2012;Ishibashi and Fabian,2012). AGN outflows on sub-parsec scale with high-velocity winds (up to∼0.1c) are observed in a large fraction of AGN and may even be ubiquitous (Fabian, 2012). However, while significant, these studies provide little direct insight into what effect these outflows have on the gas and SF over galactic scales (Harrison, 2014). In addition, broad (> 500−1000 km/s), high-velocity and spatially extended [O III] λ5007 ˚A emission has revealed galaxy-wide ionized outflows via integral field spectroscopy (IFS) of low and high redshift AGN (e.g. Harrison et al., 2012; Liu et al., 2013). These IFS observations have demonstrated that galaxy-wide ionized outflows exist and have the potential to drive gas out of their host galaxies; however, they are typically of small and inhomogeneous samples of AGN (see Harrison et al.,2012, and references therein). Therefore, as cited byHarrison (2014): we must appeal to observations to look for direct evidence that AGN have an impact on their host galaxies and larger scale environment and to constrain the details of how, when and where this impact occurs.
Motivation 15
1.3
Motivation
Studies of radio-loud AGN are crucial for addressing basic questions as how jets are formed, accelerated and collimated, and what is the effect of the presence of such powerful jets in the evolution of the host galaxy and the local environment. In this thesis we focus on two important issues relevant to the radio-loud AGN phenomenon, the nature of GRGs and the AGN-host galaxy coevolution. We believe that an effective way to address these issues is:
(1)To obtain optical and radio parameters of the largest sample of Giant Radio
Galaxies reported so far in order to search for the reason for their having grown to
Mpc scales.
(2) To study circumnuclear regions in GRGs and BL Lacs in order to get
insights about the coevolution between the AGN and the host galaxy. We put
special emphasis in the BL LacPKS 0521-36thanks to the remarkable observed features which allow us to shed light on this matter.
This dissertation addresses the above issues as follows,
InChapter 2I describe the GRG samples, which include low and high redshift sources,
and the BL Lac sample. I provide details about the SDSS, NVSS and FIRST surveys from which the GRG samples were extracted. Also, the reduction process of the BL Lacs spectra is described.
Chapter 3describes the methodology. First, I explain the way we obtained information
from the host galaxies (e.g. SFH) and parameters from their central engine (black hole mass). Then, I describe the methodology followed to obtain radio parameters from all the GRGs studied in this work.
InChapter 4presents the results. I provide general information about the new catalog of
GRG and all significant correlations between optical (including SFR) and radio parameters of these objects; in addition, I present the comparison between radio properties of low and high-z GRG. Furthermore, I present the results of a detailed analysis of the BL Lac PKS 0521-365, which is found to have a strong interaction episode with its host galaxy.
Chapter 2
The Sample
In this thesis we focus on the study of 93 GRGs with LLS≥1 Mpc andz≤0.4. In order to provide insights about the evolution of GRGs across cosmic time we use a comparison sample of 104 GRGs with LLS≥1 Mpc andz >0.4. Furthermore, this thesis is concerned with the study of circumnuclear regions in GRGs and BL Lacs in order to get insights about the AGN-host galaxy coevolution. Therefore, the data used in the present investigation comprise radio and optical observations of GRG and optical spectra of BL Lacs.
In general, we employ the NVSS and FIRST radio surveys (at 1.4 GHz) to study basic radio properties in GRGs; around 1 GHz the spectral distribution of continuum flux density is synchrotron like. The angular resolution of the NVSS (θ= 4500) allows to observe extended
emission from the lobes; on the other hand, the one of the FIRST (θ= 5.400) offers adequate
angular resolution to isolate the core emission and to obtain accurate positions for optical counterparts.
In order to study optical properties of the host galaxies we use SDSS spectra (GRG) and spectroscopic data obtained with the VLT/FORS2 (BL Lacs). Since the active nucleus in radio galaxies does not outshine the galaxy’s optical emission it is always possible to obtain optical spectra where the stellar component dominates, e.g SDSS spectroscopy data. On the other hand, intermediate spectral resolution observations of BL Lacs allow to detect the weak absorption lines of their host galaxies, which is often outshined by the non-thermal continuum of the point-like central synchrotron source.
2.1
GRG Sample
The sample of GRGs has been derived from the compilation obtained by H. Andernach and collaborators (including the present author). They inspected the entire image atlases of NVSS, SUMSS and WENSS, aided by FIRST when available, using both visual inspection and automated methods1. In addition, this compilation includes all the sources reported in published samples (about 100 until now), several sources reported in the January 2014 version of “The Million Quasars” (MILLIQUAS) catalogue (Flesch,2014) and some GRGs recently reported in the Radio Galaxy Zoo (RGZ)2. As of January 2015, this compilation comprises more than 1500 objects with LLS above 0.5 Mpc. Nevertheless, about 35% of these objects have only photometric redshift available in the literature. In particular, they almost doubled the number of GRGs with LLS beyond to 1 Mpc known so far, including the finding of the largest one currently known, and a few of the largest ones at higher redshift (z>0.5). This catalog includes 372 objects reported for the first time as GRGs; these were found through the inspection described above and some of them had been recognized as radio galaxy, but their large size had not been noticed by other authors before.
0.05 0.1 0.2 0.5 1 2
z
2 5 10 15 20 30
L
A
S
(
ar
cm
in
)
low z
high z
New known 1 Mpc 3 Mpc 5 Mpc
Figure2.1: Size-Redshift distribution of 197 GRGs with LLS≥1 Mpc and available spectrum in the
SDSS, 93 with z≤0.4 and 104 with z>0.4. For a given linear size, e.g. 1 Mpc, the lines represent the expected angular size as a function of redshift. Filled red circles represent sources reported for the first time as GRGs in the compilation of H. Andernach, while blue circles show the sources previously reported (either in the literature or from sources like RGZ or Milliquas).
This thesis is concerned with the largest ones; thus, GRGs with LLS beyond to 1 Mpc. Furthermore, we selected those objects with spectrum available in the SDSS (see Figure 2.1). The main sample comprises objects with z ≤0.4; thus, only sources with Balmer emission lines (Hβ λ4861 ˚A, Hα λ6563 ˚A) falling within the SDSS spectral range were
1adsabs.harvard.edu/abs/2012sngi.confP...1A, www.astro.ugto.mx/∼carlos/isantiago2013ab.php 2http://radio.galaxyzoo.org
GRG Sample 19
selected. This assures that the spectra can be fitted with Starlight (Cid-Fernandes et al.,2005) and theHα λ6563 ˚A line profile can be deconvolved for further analysis (e.g. BH mass estimation). Objects with z > 0.4 were also considered; however, we did not perform a detailed analysis like the one with sources with z≤0.4. We use this sample to compare the radio properties and the AGN activity type between high- and low-z GRGs. In short, the low-z sample comprises 93 objects while the high-z one comprises 104. The redshift distributions of GRGs in these two samples are shown in the Figure 2.2.
Figure2.2: Redshift distributions of GRGs with LLS≥1 Mpc; (left panel) 93 objects with z≤0.4 and
(right panel) 104 objects with z>0.4. It is noticeable that those objects associated with QSOs dominate at z>1; however, we see 5 cases of QSOs for z<0.4.
In order to study the radio emission properties of these objects we employ the NVSS and FIRST radio surveys. Since the sky coverage of the SDSS and the above radio surveys are similar we can get radio and optical data simultaneously. In the following, I briefly describe the NVSS, FIRST and SDSS.
2.1.1
NVSS
The NRAO VLA Sky Survey (NVSS) is a 1.4 GHz continuum survey covering the entire sky north of −40 deg declination (82% of the celestial sphere) (Condon et al., 1998). One of the main NVSS data products is the set of 2326 continuum image “cubes” each covering 4 deg ×4 deg with three planes containing the Stokes I, Q, and U images. For this survey the D and DnC configurations of the Very Large Array (VLA) were used. The survey images have an angular resolution ofθ= 4500FWHM. This, together with the short
baselines provided in the D configuration, allow a very good surface-brightness sensitivity needed to detect and collect the full flux of very extended sources (Figure 2.3). The rms uncertainties in right ascension and declination vary from<1 arcsec for relatively strong (S>15 mJy) point sources to 7 arcsec for the faintest (S = 2.3 mJy) detectable sources. The source density is∼54 sources per square degree while the completeness limit is about 2.5 mJy.
The catalog of sources extracted from the images contains∼1.8×106 objects, including two astrophysically distinct populations of extragalactic radio sources (see Figure 2.4).
Figure2.3: The synthesized beam has a nearly Gaussian main lobe whose FWHM is about 4500. The
nearby sidelobes are fairly small and have nearly zero mean (Condon et al.,1998).
First, over 99% of the strongest (S >60 mJy at 1.4 GHz) sources in previous large-scale surveys are likely to be classical radio galaxies and quasars; second, low-luminosity AGN and star-forming galaxies containing HII regions ionized by massive (M > 8M) short
lived stars and relativistic electrons accelerated by their supernova remnants (Condon, 1992).
For the first time, NVSS detected thousands of the nearest AGN over most of the sky (Condon et al.,1998). Detection of nearby radio galaxies is scientifically valuable because these can be studied in greater detail; their optical morphologies can be determined, their optical spectra and redshifts are easy to obtain, a given angular resolution yields better linear resolution, etc.
0.0001 0.001 0.01 0.1 1 10
0.1 1 10 100 1000
S (Jy)
Figure2.4: Weighted source counts at 1.4 GHz (data points) and models (Condon,1984) indicating
contributions of powerful AGN (dotted curve) and normal galaxies plus “starbursts” and low-luminosity AGN (dashed curve) to the total (continuous curve). The quantityn(S)dsis the number of sources per steradian with flux densities betweenSandS+dS, and weighting byS5/2divides by the counts expected
in a static Euclidean Universe (Condon et al.,1998).
The NVSS was made as a service to the astronomical community. For more than 15 years, products and user software have been available via internet3.
GRG Sample 21
2.1.2
FIRST
Faint Images of the Radio Sky at Twenty cm (FIRST) is a survey designed to produce the radio equivalent of the Palomar Observatory Sky Survey over 10,000 square degrees of the North and South Galactic Caps.
Radio observations were made with the NRAO Very Large Array (VLA) in its B - con-figuration. A final atlas of images was produced by coadding the twelve images adjacent to each pointing center. These maps have 1.800 pixels, a typical rms of 0.15 mJy, and a resolution of 500. The noise in the coadded maps varies by only 15% from the best to the worst places in the maps, except in the vicinity of bright sources (>100 mJy) where sidelobes can lead to an increased noise level. At the 1 mJy source detection threshold, there are∼90 sources per square degree,∼35% of which have resolved structure on scales from 200−3000(Becker et al.,1995).
A source catalog including peak and integrated flux densities and sizes derived from fitting a two-dimensional Gaussian to each source was generated from the coadded images. The survey area has been chosen to coincide as much as practically possible with that of the Sloan Digital Sky Survey (SDSS). Both the images and the catalogs constructed from the FIRST observations are available to the astronomical community via internet4.
0 1 2 3 4 5
Source Density (104
deg-2
) 0.0
0.2 0.4 0.6 0.8
Identification Error Rate (fraction)
D
C
B
19 20 21 22 23 24 25 26 V Magnitude
0.0 0.2 0.4 0.6 0.8 1.0
Identification Error Rate (fraction)
D
C
B
Figure2.5: (Left panel) The radio source identification error rate as function of density of candidates
on the sky for 1.4 GHz VLA observations. The RMS position error for a radio source is taken as 1/10 FWHM beam-width, appropriate for sources near the survey limit (0.600, 1.800and 5.400for B, C and D
configuration respectively). (Right panel) The identification error rate at the Galactic pole as function of optical brightness (considering star and galaxy densities)(Becker et al.,1995;Helfand et al.,2015).
There were two main considerations for the design of the FIRST survey. First, positional accuracy and sensitivity sufficient to achieve a large number of optical identifications. Approximately 50% of the radio sources in the FIRST survey have optical counterparts brighter than 23rd magnitude; at this level, sub-arcsecond positions are necessary for reliable identifications (see Figure 2.5).
In addition, since the morphology of the brightness distribution of a radio source is also important for making a correct optical identification, the FIRST survey offers an angular
resolution sufficient to provide a morphological classification of the radio emission. In fact, many sources are complex: for example, radio triples are common. In such cases, the optical counterpart will be located at the position of a weak central component, far from the brighter radio lobes. Without high resolution, misidentification is likely. The radio morphology is also important for distinguishing between FR I and FR II radio sources (Becker et al.,1995).
2.1.3
SDSS
The Sloan Digital Sky Survey (SDSS) is a major multi-filter imaging and spectroscopic survey using a dedicated 2.5 m f/5 modified Ritchey-Chretien altitude-azimuth telescope located at Apache Point Observatory, in south east New Mexico (Latitude 32◦460 49.3000 N, Longitude 105◦ 490 13.5000 W, Elevation 2788m). A 1.08 m secondary mirror and two corrector lenses result in a 3◦ distortion-free field of view (Gunn et al.,2006).
Data Release 12 (DR12)5is the final data release of the SDSS-III, containing all SDSS ob-servations through July 2014. It includes the complete dataset of the BOSS and APOGEE (IR) surveys, and also newly includes stellar radial velocity measurements from MAR-VELS. DR12 directly follows DR11, which was an internal release only to the SDSS col-laboration, as well as the prior public release, DR10 (Ahn et al., 2014). DR 12 includes five types of data: images, optical spectra, infrared spectra, stellar interferometry, and catalog data (parameters measured from images and spectra, such as positions, magni-tudes and redshifts). In the present work, we are interested in using images and optical spectra.
SDSS has imaged about one-third of the night sky in five broad bands (ugriz). The resulting catalog includes photometry for almost 5×109 unique objects. In addition to
catalog data, SDSS also makes all its imaging available through the Science Archive Server (SAS)6, which can return FITS files either for single SDSS fields or in bulk.
DR12 includes hundreds of thousands of new galaxy and quasar spectra from the Baryon Oscillation Spectroscopic Survey (BOSS), in addition to all imaging and spectra from prior SDSS data releases. In total, DR12 contains 2,401,952 galaxy and 477,161 quasar spectra; with a sky coverage of about 14,500 square degrees. SDSS makes all its spectra available as FITS files through the SAS.
All previous SDSS spectroscopic data (Data Releases 1-8) were taken with the SDSS spec-trograph. With Data Release 9, the first data from the Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph were released. In Table 2.1 I present some important infor-mation about SDSS spectra measured with both spectrographs.
The instrumental dispersion of the SDSS spectrograph is 69 kms−1 per pixel (and ∼
60kms−1 for BOSS); nevertheless, this value may vary from pixel to pixel and therefore
the measurements of the velocity dispersion are affected. The aperture of an SDSS spec-troscopic fiber (2 and 3 arcsec for the SDSS and BOSS spectrograph respectively) samples
5http://www.sdss.org/dr12/ 6http://dr12.sdss3.org
