LA CALIDAD EN EL MUNDO EDUCATIVO

As mentioned at the end of Section1.3.3, a key evolutionary phase in merger-driven evolutionary scenarios is the ignition of AGN activity that, through outflows, can inhibit both star formation and its own fueling. Such objects would be expected to have luminous quasar activity, starburst or post-starburst signatures, along with indications of a recent merger (e.g., companion, tidal tails, asymmetries, etc.).

One of the objects that more clearly meets all these requirements is called UN J1025-0040, the

1.5 Post-Starburst QSOs 27

Figure 1.13 Optical spectrum of the PSQSO UN J1025-0040. An Instantaneus Starburst (ISB) model of 400 Myr old is also shown, matching the high order Balmer lines and H/K CaII. The bottom panel shows the residual quasar spectrum obtained by subtracting the ISB model from the UN J1025-0040 spectrum, revealing broad Hβ. Extracted fromBrotherton et al. (1999).

most evident transition QSOs discovered in Sloan Digital Sky Survey (SDSS) data byBrotherton et al.

(1999). Its optical spectrum shows simultaneusly a strong blue continuum with broad Hα and Hβ emission lines, typical of classical type 1 QSOs, and a red Balmer jump and high order Balmer lines in absoption, characteristic of an post-starburst population of ∼ 400 Myr (see Figure 1.13). Attending to its spectral properties it was called Post Starburst QSO (PSQSO). Moreover, it is morphologically classified as a merger remnant (Brotherton et al., 2002), and has a companion galaxy in a post-starburst phase (Canalizo et al.,2000).

It is not an isolated case, in the SDSS data release 3 (DR3), ∼ 600 PSQSOs where identified. It is important to note that, tecnically speaking, not all of them are QSO (M_B < -23), but a significant fraction are lower luminosity AGN (M_B > -23). In fact, in Cales et al.(2011) sample, only 1 out of 29 reaches the QSO luminosity.

The morphology of PSQSOs was analyzed by Cales et al. (2011) using HST images of a sample of 29 PSQSOs with redshifts between 0.25 and 0.4 (z ∼ 0.3). They found a heterogeneous population with a variety of morphologies including host galaxies that are both early-type (13/29) and spiral (13/29). Disturbances such as tidal tails, shells, star-forming knots, and asymmetries are seen in 17/29 as signposts of interaction/merger, and are evenly distributed among early-type and spiral galaxies. Two of these systems are clearly merging with their companions.

Also, using optical spectroscopy,Cales et al. (2013) fully characterize PSQSOs properties. They model the spectra of 38 PSQSOs to determine ages and masses of the host stellar populations, and BH masses and Eddington fractions of the AGNs. They found PSQSOs have M_BH = 10^7,5−8,5 M_⊙ and are accreting at a few percent (∼ 1 % - 10 %) of L_Edd. Hosts have stellar masses of around 10¹⁰⁻¹¹ M_⊙ and stellar populations with several hundred Myr to few Gyr old (∼ 200 - 2000 Myr). No clear correlations were found between ages/masses of the stellar populations and the AGNs properties when considering the sample as a whole. However, when sub-classifying in morphological types, they found early type galaxies have significantly higher AGN luminosities (< L_AGN > ∼ 10^43,82 erg s⁻¹) and younger starburst ages (< SBage > ∼ 960 Myr) while the spiral hosts have a more complex and extended SFH (< SBage > ∼ 1.6 Gyr) and lower luminosity AGN (< L_AGN > ∼ 10^43,51 erg s⁻¹).

Using MIR spectra, Wei et al. (2013) found that PSQSOs in early-type host galaxies tend to have relatively strong AGN activities, while those in spiral hosts have stronger PAH emission, indicating more star formation.

Cales et al.(2013) hypothesize that early type PSQSOs are likely the result of a major merger and were likely LIRGs in the past. They may represent the low-redshift counterparts to the high-luminosity quasars much more common at z = 2 - 3. While spiral PSQSOs show more complex SFHs triggered by secular evolution or less dramatic events, as harassment or bars. Simulations also agree with two mechanisms responsible for mutual BH-bulge growth (Hopkins & Hernquist, 2009). The brightess AGN must be triggered in major mergers: the galactic supply of gas must lose angular momentum in less than a dynamical time to be converted to new bulge mass. However, lower luminosity AGN require little bulge growth and small gas supplies, and could be triggered in more common nonmerger events. For example, bars in spiral hosts may be sufficient to fuel the nuclear activity of Seyfert galaxies (Jogee,2006).

1.5.2. Evolutionary picture revisited and the Starburst / AGN timing

In a recent work,Cales & Brotherton(2015) made a global comparison of their 38 PSQSOs sample with post-starburst galaxies and quasars at similar redshift (z ∼ 0.3), and with similar luminosity (M_r

∼ -23). They found that post-starburst galaxies have elliptical and disturbed / post-merger morpho-logies similar to those of the more luminous PSQSOs, display similar spectral properties, but also can have younger stellar populations for a given starburst mass (∼ 400 Myr younger). Quasars at similar redshifts and luminosities around the Seyfert / quasar transition posses similar AGN characteristics, but do not appear to be hosted by galaxies with significant post-starburst populations. On the other hand, recent studies of more luminous quasarsCanalizo & Stockton(2013) find hosts consistent with luminous PSQSOs. They proposed an scenario where the luminous / elliptical PSQSOs may be in transition between post-starburst galaxies and a more luminous quasar stage. The scenario proposed

1.6 Post-Starburst QSOs 29

by them is shown in their Figure 9. Basically, is the same as Hopkins et al. (2008) (see Figure 1.8) but including between (e) and (f) stages first the post-starburst galaxies and then the PSQSOs.

It seems that observationally and theoretically there is growing consensus regarding the AGN fuelling in the context of merger driven evolutionary scenarios; in which bursts of star formation happen early and coincide with the close approach passes of the two nuclei, while the AGN is trigge-red later on in the evolution of the merger around coalescence (Wild et al.,2007; Schawinski et al., 2010; Alexander & Hickox, 2012; Van Wassenhove et al., 2012; Canalizo & Stockton, 2013). At the sub-kpc scales, competing effects (e.g. inflow, outflow, star formation) cause the gas to pile up (Alexander & Hickox, 2012; Fathi et al., 2013). At any rate, this suggests that getting fuel all the way down to the BH is not straightforward and takes time. Our current view of merger triggered AGN fuelling is one in which torques from the merger are efficient drivers for bringing gas down to a critical distance from the BH (∼ 100 pc). Then a series of small-scale internal instabilities (stochastic fuelling) finally brings the gas to the BH, where the time it takes the gas to reach the BH may be comparable to or longer than the AGN duty cycle (∼ 10⁸ yr).

1.5.3. Other transition QSOs/AGNs

Age dating the stellar populations in transition AGNs samples could be used as a constrain for the major merger driven scenario. If the proposed scenario is correct, the peak of star formation would occur while the AGN is still inactive / deeply buried, and therefore, by the time the AGN becomes visible in the optical, the stellar population would be detected as an ageing starburst of several hundred Myr to few Gyr.

This seem to be the case in the sample of transition QSOs at z ∼ 0.2 (previously classified as passive ellipticals) of Canalizo & Stockton(2013), with important contributions of intermediate age post-starburst populations of 0.7 - 2.4 Gyr. Also, recentlyYesuf et al.(2014) broaden the definition of post-starburst galaxies to include transiting systems possibly hosting AGN. The AGN fraction of these post-starbursts, as estimated from optical line ratios, is about 3 times higher than that of normal star-forming galaxies of the same mass. Moreover, these post-starburst AGN have mean stellar population ages between 350 - 700 Myr, and their spectral properties are analogous to previously discovered PSQSOs.

However, in other transition systems there is not a delay between star formation and the AGN ac-tivity, and both appear quasi-simultaneously (Canalizo & Stockton,2000,2001;Bessiere et al.,2014).

Some scatter in the delay between the starburst and the AGN activity is always expected, as the ti-mescales of the different processes depend on the details of the merger: mass ratios and gas fractions of the progenitors, orbital configuration, etc. Moreover, the amount of dust in the nuclear region also determines the possibility of simultaneously detecting the SB and the AGN. If there is little dust, both phenomena could be seen simultaneously. If dusty, the AGN will not be visible until much later.

According to the previous observational samples the scatter is really huge, ranging from 0 - 2 Gyr.

A detailed analysis of the stellar populations in greater statistical samples of transition objects is still needed to clear up this picture.

In anycase, post-starburst QSOs/AGNs as those found byBrotherton et al.(1999) andYesuf et al.

(2014) are ideal laboratories to study connections between the starburst and AGN phenomenon, as the properties of both could be derived simultaneusly for each individual system.

The primary scientific objective of this thesis is to find new observational evidence that test the validity of the major gas rich merger scenario, in order to understand the genesis events that lead to the formation of quasar activity, and to place galaxies with nuclear activity in the context of the formation and evolution of galaxies. A significant contribution can be done to this field by studying and comparing the stellar populations and the extended ionized gas properties along the whole merger sequence from U/LIRGs to QSOs.

To approach this goal, we have characterized and compared the stellar populations and the ionized gas properties in two small samples of galaxies in different evolutionary stages: 4 U/LIRGs (2 pre-merger and 2 pre-merger) and 9 PSQSOs. This study requires high quality optical data with resolved spatial and spectral information, like the one provided by Integral Field Spectroscopy (IFS). Our IFS covers the rest-frame optical range 3700 - 7000 ˚A, from which we can derive:

1. The spatial distribution of the stellar populations. The high-order Balmer lines, the CaII H and K, G band, Mgb and the Balmer break, together with our evolutionary synthesis models and spectral synthesis techniques, like Starlight, will be used to determine the average stellar population properties: age, metallicity, mass, stellar mass surface density and stellar extinction.

2. The spatially resolved SFHs in light and mass, and the SFR maps derived from the spectral synthesis.

3. The spatial distribution of the extended ionized gas by means of the most important emission lines, such as Hα, Hβ, [NII]λ6583, [OIII]λ5007, [OII]λ3727, [OI]λ6300 and [SII]λλ6717,6732.

The ionization conditions determined using the well-known and most updated forbidden-line diagnostics in the oxygen and nitrogen lines, used to distinguish between gas photoionized by the AGN or by young stellar components.

4. Ionized gas extinction maps will be used to deredden the observed Hα maps and obtain extinction-corrected maps of the SFR.

We intend to test, for example, whether there is an evolution in the merger-induced star formation, the SFR and the ionization structure between U/LIRGs in different merger stages. Also, ages can be used as a clock to test if PSQSOs are part of the post-merger evolutionary sequence between U/LIRGs and normal elliptical galaxies. Another important goal is to found if the properties of the starbursts (age, mass) in the PSQSOs are somehow correlated with the BH properties (accretion rate, mass).

A brief outline of the content of each chapter in this thesis is as follows:

Chapter 2: this chapter presents the data samples and the characteristics of the observations.

Chapter 3: this chapter gives a summary of integral field spectroscopy (the major observational technique used in this thesis) and summarize the data reduction process.

Chapter 4: this chapter gives a summary of the spectral synthesis methodology and models applied to study the stellar populations.

Chapter 5: this chapter presents the results of the analysis of the two pre-merger (class IIIb) LIRGs IC 1623 and NGC 6090.