Stellar populations analysis along the Hubble sequence

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Stellar populations analysis

along the Hubble sequence

by

Anaely Pacheco Blanco

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

January 2011

Tonantzintla, Puebla

Under the supervision of:

Dr. José Ramón Valdés

INAOE

Dra. Olga Vega

INAOE

Dr. Fabi´an Rosales-Ortega

Consejo Superior de Investigaciones Cient´ıficas,

INTA-CAB, Madrid, Spain

c

INAOE 2011

The author hereby grants to INAOE permission to

reproduce and to distribute publicly paper and electronic

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Abstract

In this thesis work a stellar populations study is proposed for two different galaxy

sam-ples. The first galaxy sample comprises 18 Early-Type galaxies from the bright end of

the Color-Magnitude Relation for the Virgo Cluster (Bower et al.,1992). For this

sam-ple, panchromatic data are available: ultraviolet broad-band photometry, optical spectra

and broad-band points and infrarred spectroscopy and photometry. These data will be

used to construct the panchromatic spectral energy distribution to be used in order to

calibrate the stellar population models. Also the spectral indices in the Lick/IDS system

were measured to charactrize the stellar populations. The observations were performed

with long-slit spectroscopy.

The second sample comprises 17 late-type galaxies observed by the PINGS project

(PPAK IFS Nearby Galaxies Survey) (Rosales-Ortega et al., 2010). The observations

were performed by the PPAK instrument (Verheijen et al.,2004; Kelz & Roth, 2006;

Kelz et al.,2006) at Calar Alto Observatory. The advantage of these observations is that

they were performed by means of integral field spectroscopy, giving spatially resolved

spectra.

The stellar populations analysis will be done with two different models, that were

se-lected in order to compare their results. TheBruzual & Charlot(2003) model performs

a full-spectrum fitting, while theBarbaro & Poggianti(1997) model performs the fitting

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Resumen

En este trabajo de tesis se presenta un estudio de poblaciones estelares y de la

histo-ria de formaci´on estelar en dos muestras diferentes de galaxias. La primera muestra

comprende 18 galaxias tempranas de la regi´on brillante de la relaci´on Color-Magnitud

para el C´umulo de Virgo (Bower et al., 1992). De dicha muestra se cuentan con

ob-servaciones en diversas frecuencias del espectro: datos fotom´etricos en el ultravioleta,

espectros y puntos fotométricos en el óptico y espectros y datos fotométricos en el

in-frarrojo. Estos datos se usar´an con el objetivo de construir una distribuci´on espectral de

energ´ıa abarcando dichas frecuencias, la cual nos servir´a para calibrar los modelos de

sintes´ıs de poblaciones a utilizar. Para esta muestra tambi´en se calcularon los ´ındices

de Lick/IDS para poder caracterizar a la poblaci´on estelar. Las observaciones para la

muestra de galaxias tempranas fueron realizadas con rendija larga.

La segunda muestra a estudiar comprende 17 galaxias tard´ıas y fueron observadas como

parte del proyecto PINGS (PPAK IFS Nearby Galaxies Survey) (Rosales-Ortega et al.,

2010). Dichas observaciones fueron realizadas con el instrumento PPAK (Verheijen

et al.,2004;Kelz & Roth,2006;Kelz et al.,2006) en el observatorio de Calar Alto. La

ventaja de estas observaciones consiste en que son espectros realizados con fibras de

campo integral, dando resoluci´on espacial de la informaci´on.

El estudio de las poblaciones estelares se llevar´a a cabo con dos modelos distintos, los

cuales fueron seleccionados para comparar los resultados obtenidos de ellos. El

mod-elo de Bruzual & Charlot (2003) realiza un ajuste a todo el espectro mientras que el

modelo deBarbaro & Poggianti(1997) realiza el ajuste en intervalos y caracter´ısticas

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Acknowledgments

• To Consejo Nacional de Ciencia y Tecnolog´ıa (CONACYT) for the economic support provided for my studies.

• To my family for their inconditional support.

• To my advisors and collaborators for allowing me into their group.

• To my friends and collegues.

• To my mentor, Germ´an Mart´ınez Hidalgo, for being the light into my suddenly dark path.

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For all the ones that always stand next to

me, even when they are not physically

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List of Figures

1.1 Hubble sequence . . . 2

1.2 Fundamental Plane . . . 4

1.3 Color-Magnitude Relation for Coma . . . 5

1.4 Tully-Fisher Relation . . . 6

1.5 Galaxies and their spectra . . . 8

1.6 Age-metallicity degeneracy . . . 10

1.7 IFUs technique . . . 12

1.8 Color-Magnitude Relation for the Virgo Cluster . . . 14

1.9 Example of IFS data . . . 16

3.1 ETGs NIR spectra . . . 36

3.2 Passive ETGs MIR spectra . . . 41

3.3 Active ETGs MIR spectra . . . 43

3.4 NGC 1058 . . . 50

3.5 NGC 1637 . . . 52

4.1 Absorption lines . . . 55

4.2 ETGs optical spectra . . . 56

4.3 Index bands representation . . . 62

4.4 Indices central bands . . . 65

4.5 Indices correlation plots . . . 70

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List of Tables

3.1 Virgo sample . . . 32

3.2 GALEX photometry . . . 33

3.3 OAGH observational data . . . 34

3.4 2MASS photometry . . . 38

3.5 PINGS sample . . . 46

4.1 Extraction aperture for the ETGs . . . 54

4.2 Equivalent apertures for HST photometry . . . 59

4.3 HST photometry . . . 61

4.4 Index bands . . . 63

4.5 LSS sample . . . 67

4.6 Lick/IDS FWHM . . . 68

4.7 Instrumental indices . . . 76

4.8 ETGs centralσ . . . 78

4.9 Transformation Coefficients . . . 79

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Chapter 1 Introduction

Hubble Sequence

In the early 20s Edwin Hubble introduced a galaxy classification based on the

morphol-ogy observed from optical imaging, providing the following types:

1. Elliptical (E0-E7)

2. Lenticular (S0 or SB0)

3. Spiral (Sa-c or SBa-c)

4. Irregular (Irr)

The morphological classification is sketched in Fig. 1.1. Starting from the left

hand-side the elliptical galaxies run to the irregular ones, passing from the lenticular to the

simple and barred spirals.

G´erard de Vaucouleurs and Allan Sandage expanded to the Hubble morphological

classification in 1959. The Hubble morphological classification considered the

tight-ness of the spiral arms and the presence of a bar in the spiral galaxies, but they included

3 features in order to classify galaxies:

1. Bars: where the notation goes as follows, A for galaxies without bars, B for

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Figure 1.1: Hubble sequence also known as the tuning fork

2. Rings: the galaxies showing rings are denoted by (r), (s) stands for the ones

without them and (rs) are called ”transition” objects.

3. Spiral arms: these are denoted as Sd (SBd) if they show diffuse spiral arms and

a weak bulge, Sm (SBm) for irregular galaxies without a bulge and Im for very

irregular galaxies.

Despite the origin of the Hubble sequence as a morphological classification, other

physical properties and observational trends of the galaxies are reflected in it, due to

their stellar population content (optical colors, optical linear sizes, optical luminosity,

optical surface brightness affected by extinction, far infrared emission, radio continuum

emission, X-ray emission, neutral hydrogen mass and content, carbon monoxide (CO),

chemical abundances and total masses) as mentioned byRoberts & Haynes (1994). A

brief description of some main characteristics of each galaxy type is given below.

Elliptical Galaxies

These galaxies are characterized by their elliptical and smooth light profile known as

the de Vaucouleurs profile:

I(R) =I(Re) exp

"

−b

R Re

1/4

−1

#

, (1.1)

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notation En is given by their apparent flattening n = 10(a₋b)/a running from 0 to 7, whereais the semi-major axis andbis the semi-minor axis. Their oblate shape is a consequence of the velocity dispersion anisotropy. These galaxies are characterized by

their lack of significant amount of cold gas and dust, and therefore, recent and intense

star formation is not expected. As a consecuence their stellar populations are considered

to be old and their colors red. Their optical spectra is dominated by absorption features.

Elliptical galaxies are bright sources of X-rays due to their hot ionized gas (T_∼107_K).

These galaxies are mainly found in dense environments like clusters. It is interesting

that these galaxies follow scaling relations such as:

The Fundamental Plane: Faber & Jackson found in 1976 a relation that was later

recognized as a projection of the Fundamental Plane (FP). They related the

Lu-minosity L of the galaxy with their velocity dispersionσ. The Fundamental Plane

later introduced byDressler et al.(1987) andDjorgovski & Davis(1987) relates

the effective radiusRe, the effective surface brightnessµB and the central

veloc-ity dispersionσ. This relation is shown in Fig.1.2.

Color-Magnitude Relation (CMR): Found for Virgo and Coma Clusters in 1972 by

A. Sandage, it is also related to the mass of the galaxy. This relation can set

con-straints on galaxy formation and evolution models (Kodama & Arimoto, 1997),

since the dispersion will give information about the formation process and the

slope may constrain the merging history (Renzini, 2006). An example of this

relation for the Coma Cluster is shown in Fig. 1.3.

Lenticular Galaxies

This kind of galaxies share elliptical and spiral properties. They have a disc and a bulge

and could show a bar, but they lack of spiral arms, dust and gas. They are denoted as S0

(or SB0, if a bar is seen). In the disc, the stars follow circular rotating orbits, but in the

bulge they move randomly. The light profile of the disc follows the same exponential

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Figure 1.2: Fundamental Plane for field Early-Type Galaxies (squares) and for Coma ellipticals (triangles,Jørgensen et al.(1995)) taken fromRenzini(2006). Blue squares are fromTreu et al.(2005) while red squares and black circles are fromdi Serego Alighieri et al.(2005)

Spiral Galaxies

This name came from the spiral pattern present in these type of galaxies. There is also

a bulge and a disc and in some cases a bar. These galaxies are denoted by the notation

S (or SB; for barred spirals) followed by a letter (a, b or c) depending on the tightness

of the spiral pattern and the size of the bulge. The disc follows an exponential light

profile:

I(R) = I0exp (−R/R0). (1.2) where I0 is the central surface brightness and R0 the scale-length. The stars of the

disc in these galaxies move around the galactic center in nearly circular orbits. This

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Figure 1.3: CMR (U-V) vs MV for the Coma Cluster taken fromRenzini(2006). Here the

colors represent different galaxies types. Red ones represent elliptical galaxies, orange are S0, blue comprises spiral and irregular galaxies and open circles are unclassified galaxies.

luminosity L (or absolute magnitude M) with the rotational velocity Vrotas it is shown

in Fig. 1.4. In the case of spiral galaxies they present a variety of stellar populations due

to their active star formation history. From the colors, the reddish bulge is dominated

by older stars while the blueish arms are dominated by gas, dust and younger stars.

Thus a stellar population analysis in this case is more complex due to the presence of

various generations of stars and the effects of dust extinction. The optical spectrum of

spiral galaxies show intense emission lines due to the gaseous component, but also a

stellar contribution of the subjacent older population.

Irregular Galaxies

Those galaxies without a distinctive structure fall here. They are denoted by Irr and

the dwarf irregulars are the most common in the universe. Examples of these are the

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Figure 1.4: Tully-Fisher relation (I-band) taken fromBlanton & Moustaka(2009)

Stellar Populations of Early and Late Type Galaxies

The division in early and late type galaxies is related to the erroneous interpretation

that this classification corresponded to an evolutionary sequence. Nevertheless, the

Hubble sequence is related to observable and physical parameters of the galaxies like:

bulge/disc luminosity ratio, stellar population, integrated spectra, star formation

histo-ries (SFHs), relative HI/HII content and chemical abundances of the ISM (Blanton &

Moustaka,2009).

Basically, Elliptical and Lenticular galaxies belong to the Early-Type Galaxies (ETGs)

and the Spiral and Irregular galaxies to the Late-Type Galaxies (LTGs). The main

classification parameter is their stellar populations. Traditionally, it is thought that the

ETGs are dominated by an almost homogeneous population of old and solar-metallicity

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and high-metallicity stars and, their higher abundance of neutral and molecular

hydro-gen makes them favorable for recent star formation episodes (e.g. Strateva et al.,2001;

Blanton & Moustaka,2009).

Therefore, the integrated spectra of an ETG is mostly dominated by red giants and

AGB stars, with intense absorption and metalic lines (e.g. Renzini, 2006, and

refer-ences therein). For the LTGs, on the contrary, the nebular emission is intense compared

to the underlying stellar continuum. An example of the differences in their spectra is

shown in Fig. 1.5.

Only the nearest galaxies can be resolved into individual stars, therefore, the study

of their stellar populations can be done only by the analysis of their integrated light.

This limitation made necessary the development of stellar population synthesis

mod-els. They are tools used to interpretate the observations (like colors or spectral features)

and to determine the fraction of stars contributing to the integrated light. Nowadays

there are plenty of population synthesis tools at the disposition of the scientific

com-munity, provided byWorthey(1994);Buzzoni(1995);Bressan et al.(1996);Maraston

(1998, 2005);Bruzual & Charlot(2003);Fioc & Rocca-Volmerange (1997);V´azquez

& Leitherer (2005); Vazdekis et al.(2003); Gonz´alez-Delgado et al. (2005) and

Cid-Fernandes et al. (2005), but all are in continuous improvement by the inclusion of

new stellar libraries, the effects of non-solar element abundances and enrichment (e.g.

Pietrinferni et al.,2006), the advanced stellar evolutionary stages (e.g. Marigo,2007)

and binary stars (e.g. Eggleton,2009;Li et al.,2006). ETGs are important in the

con-text of galaxy formation and evolution, because a considerable fraction of their

bary-onic mass (60-70%) is assembled into stars (Fukugita & Peebles, 2004). As a result

of their scaling relations (FP and CMR) these galaxies are considered as an almost

ho-mogeneous old stellar population and then treated as a simple stellar population (SSP).

A SSP is a coeval (born at the same time) and chemically homogeneous (same

ini-tial chemical abundances) population. However, recent observations have showed that

these galaxies are not as passive as first thought, revealing disturbed structures (e.g.

Rampazzo et al.,2005;Panuzzo et al.,2007), dust and shells (e.g.Colbert et al.,2001),

UV excess (e.g.Bertola,1986;Bica et al.,1996) and infrared emission lines (e.g.

Bres-san et al., 2006, PAH), suspecting the evidence of a recent rejuvenation event of the

ETGs.

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interest-Figure 1.5: Spectra of the different morphological classes, image fromStrateva et al.(2001). Where z is the redshift, r∗

and u∗

are the measured magnitudes from the SDSS filters r and u, respectively.

ing conclusions have been made. Bower et al.(1992) from the analysis of the CMR of

the Virgo and Coma Clusters showed that cluster ellipticals have formed the majority

of their stars at z & 2. Also,Bernardi et al.(2003, 2006) from the analysis of the FP found that ETGs in low-density environments are in average 1 Gyr younger than those

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cluster and field ellipticals show the same mean age.

Balmer absorption lines (Hγ, Hδ and Hβ) are commonly used as age estimators for old stellar populations, while for the metallicity the <Fe> and Mg2 indices are often

used. Notice, however that an absolute determination of the ages and the metallicities

is not possible due to the age-metallicity degeneracy (Worthey,1992). This degeneracy

makes necessary the searching of new spectral indices to break it (e.g. Yamada et al.,

2006). In Fig. 1.6(a) I show the optical spectra of a SSP for two different ages and

metallicities (10 Gyr with Z=0.008 and 4.5 Gyr with Z=0.02). It is evident that they are

indistinguishable, this is the age-metallicity degeneracy effect. In Fig. 1.6(b)the same

SSPs are shown but in a wider spectral range, from the ultraviolet to the infrared. From

this plot, it is shown that in order to make a precise study of the stellar populations in

ETGs, a panchromatic approach is necessary.

The most massive galaxies present enhanced Mg abundances relative to Fe, that

arises from theα₋enrichment (including Mg) produced by the Type II supernovae (e.g Wheeler et al.,1989). A lower limit to the formation redshiftzF &2.5of ETGs was set

byKelson et al.(2001) using Balmer absorption lines.Thomas et al.(2005) found that

the metallicity and the [α/Fe] ratio correlates to the velocity dispersion (or mass). They also find that lower-mass ellipticals show evidence of recent (.1Gyr) star formation. This was confirmed byKaviraj et al.(2007b) using GALEX observations.

From all the above, we can conclude that the study of the stellar populations in ETGs is

not finished yet. The main present problems are the age-metallicity degeneracy and the

evidence of rejuvenation episodes, implying that an ETG can not be treated as a SSP.

The study of the stellar populations in LTGs is even more complicated due to their

more complex SFHs and the presence of dust extinction (e.g Leitherer et al., 1996).

Therefore, they can not be treated as a SSP, and they must be analyzed with the

com-posite stellar population (CSP) method (e.g. Serra & Trager, 2007). A combination

of different generations of stars (each one treated as a SSP) would reproduce the

in-tegrated characteristics. So, LTGs are still unexplored objects due to their complexity

and their substructure (Ganda et al.,2006), and mainly the two-dimensional kinematics

and stellar population analysis.

For a long time the spectroscopy has been done with long-slit spectrographs. The

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(a) Optical spectra

(b) Spectrum from the ultraviolet to the infrared

Figure 1.6: Age-metallicity degeneracy in the optical wavelength range can be broken using other wavelength ranges such as Ultraviolet or Infrared

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of the galaxy it is positioned. A great variety of previous studies used this technique

in the nuclear regions of the galaxies, sampling a small region (Trager et al., 1998;

Worthey,1992;Annibali,2005) and providing global parameters of the galaxy, lacking

of a truly spatially resolved study. The recent development of new instrumentation,

as Integral Field Spectroscopy, has allowed the analysis of spatially resolved stellar

populations. In the next section a brief description of the Integral Field Spectroscopy is

given.

Integral Field Spectroscopy

Integral Field Spectroscopy (IFS) consist of a simultaneously spatially resolved spectra

over a 2D field, producing a datacube of a scalar quantity related to a flux density as a

function of spatial coordinates in the field and wavelength: I(x, y, λ). There are several

instruments and techniques to achieve IFS. A brief explaination of each one follows.

Integral Field Units

Besides the spectrograph, an Integral Field Unit (IFU) is necessary for obtaining IFS.

The IFU will divide the 2D spatial plane into a continuous array and this can be done

in three different ways:

1. Lenslet array: A microlens array (MLA) is the dividing element. The incoming

light will be concentrated in a small dot that will be dispersed by the

spectro-graph. The MLA can be tilted from the optical axis of the system to avoid that

the spectra of each dot overlaps.

2. Fibres: This is the most used technique. The dividing element is a bundle of

optical fibres which transfer the light into the spectrograph. A disadvantage of

this technique is that the sampling is not contiguous due to the gaps between the

fibre cores, this can be corrected by adding an array of lenslets in front of the

fibre bundle.

3. Image-slicer: The dividing element is a segmented mirror in thin horizontal

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Fabry-P´erots or tunable filters

Fabry-P´erot imaging spectrographs can cover a large FoV (_∼5 arcmin) with high

spa-tial and spectral resolution in a single exposure but only for a single wavelength, so for

completing the datacube you must scan along the wavelength range of interest.

Multi-Object Spectroscopy

This technique is designed to obtain spectra of multiple and separated targets in the

FoV by using a slit mask or fibres or lensets that can be positionated in the region of

interest.

The IFS techniques described are illustrated in Fig.1.7.

Figure 1.7: Schematic view of the different IFUs techniques, image taken fromIntegral Field Spectroscopy Wiki(2010)

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Objectives and Methodology

The principal aim of this thesis is to study the stellar content and star formation history

in different types of galaxies along the Hubble sequence. The stellar populations in a

galaxy provide information on their formation and evolution. In order to do the

analy-sis, we will perform the stellar population synthesis method by using the state of art of

stellar atmosphere models and stellar evolutionary tracks.

In order to reach this goal we have selected two different samples of nearby

galax-ies. The ETGs sample is composed of galaxies belonging to the bright end of the

color-magnitude relation of the Virgo cluster (Bressan et al.,2006). For this sample we

already have infrared data (SPITZER/IRS spectra and broad-band images, NIR

spec-troscopy and 2MASS photometry), optical data (OAGH spectra and HST photometry)

and ultraviolet data (GALEX photometry). The available data will allow us to

con-struct, for the first time, the complete panchromatic SED for these galaxies. The SED

will allow us to study the energetic sources at different wavelengths (e.g.Boselli et al.,

2003) and to calibrate our stellar population synthesis models. We will also calculate

spectral indices in the Lick/IDS system and their correlation to the SED, being useful

for models recalibration. The description of the Lick/IDS system and the technique will

be given in more detail in Section2.2.

The LTGs sample is composed of a variety of nearby spiral galaxies (normal,

lop-sided, interacting and barred). We will use the advantage of IFU observations

per-formed with the PMAS/PPAK spectrograph at Calar Alto as part of the PINGS (PPAK

IFS Nearby Galaxies Survey) project (Rosales-Ortega et al., 2010). Two galaxies of

the PINGS sample were selected: NGC 1058 and NGC 1637 as the initial sample for

the LTGs study. The stellar population synthesis will be initially performed with two

different models: Bruzual & Charlot(2003) model and theBarbaro & Poggianti(1997)

model. These models were in principle selected because the first one is a full-spectrum

fitting while the second fits specific wavelength intervals and spectral features. This

will allow us to compare the results derived from both models and to inspect the

ad-vantages or disadad-vantages of using this kind of methods. These galaxies were chosen

mainly because the mosaic covers the full optical size (defined by the B-band 25 mag

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(a) CMR (U-V) (b) CMR (V-K)

(c) CMR (J-K)

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spatially resolved stellar population study.

The first population synthesis model (Bruzual & Charlot, 2003) was selected because

one of the galaxies in the sample (NGC 628) has been already studied with it (S´anchez

et al., 2010). The other population synthesis model (Barbaro & Poggianti, 1997) is

adopted in order to compare the results and taking advantage of the capability of the

model to work with selective extinction and non-continuous SFHs. It is also desirable

to conduct a panchromatic study for the LTGs sample, starting with infrared

photome-try using CANICA at the OAGH, that will allow us to study the older population and

the extinction. The details regarding to the observational program are yet to be

anal-ysed.

To show the advantages of the IFS observations that will be used to obtain the spatially

resolved stellar population study for our LTGs sample, the optical spectra for different

regions inside a galaxy of this sample (NGC 1058) are shown in Fig. 1.9.

The thesis is structured as follows:

• Chapter 2: This gives a description of the tools to be used in order to study the stellar populations of our samples of galaxies.

• Chapter 3: Provides a description of the samples (ETGs and LTGs) and their available observational data (long-slit spectroscopy and IFS).

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(a) Image atλ= 4050A and selected spaxels˚

(b) Optical spectra from the selected spaxels

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Chapter 2 Stellar Populations Properties

Diagnosis

The evolution of a galaxy is imprinted in their stellar populations, as a result of their

Star Formation History (SFH). Since the analysis made by Roberts (1963) the Hubble

sequence has been seen as a monotonic sequence in present-day Star Formation Rates

(SFRs) and past SFHs. This lead to the development of several diagnostic methods

for determining the global SFRs. The first attempt based on the integrated colors was

done by Tinsley (1968), Tinsley (1972) and Searle et al. (1973). Direct diagnostic

methods based on integrated emission-line fluxes (Cohen, 1976; Kennicutt Jr., 1983),

near-ultraviolet continuum fluxes (Donas & Deharveng,1984) and infrared continuum

fluxes (Harper & Low, 1973; Rieke & Lebofsky, 1978; Telesco & Harper, 1980) are

commonly used to determine very recent (_∼ 107 _{yr) star formation events. This work} will be devoted to study the SFH of our sample of galaxies, and then, the earliest star

formation events as a function of time. The following sections in this Chapter will be

dedicated to explain the different methods used to analyze the SFHs.

2.1 Stellar Populations Indicators

Generally, galaxies can not be resolved into individual stars which limits their study to

the integrated light where the contributions of all their components are mixed. For this

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properties of a galaxy in order to infer the age, metallicity and star formation history

(SFH) of its population. We can distinguish between two approaches of population

synthesis: an empirical and an evolutionary one.

In the empirical approach, the observed spectrum of a galaxy is reproduced by a

combi-nation of spectra of individual stars or star clusters with different ages and metallicities

from a library (Faber,1972;O’Connell,1976;Pickles,1984). This approach does not

consider stellar evolution and as a consequence is not possible to study the past or future

of the galaxy spectrum. On the other hand, the evolutionary approach takes into account

the stellar evolution (Tinsley,1978;Bruzual,1983;Renzini,1986;Renzini & Buzzoni,

1986;Arimoto & Yoshii,1987;Worthey,1994;Bressan et al.,1994;Maraston,2005).

Basically, two main ingredients are necessary for the latter: a stellar evolutionary

pre-scription and a stellar spectral library.

Ideally, the stellar evolutionary prescription must include all the evolutionary phases

(from the earliest to the latest) for all mass ranges. In the case of the stellar spectral

library, this should provide a complete coverage of the Hertzprung-Russell diagram,

accurate atmospheric parameters (effective temperatures, surface gravities and

metal-licities), and the most complete wavelength coverage with good spectral resolution.

The library can be theoretical (based on atmosphere models, limited by the knowledge

of the physics of stellar atmospheres) or empirical (based on observations of stars,

lim-ited by the quality of the observations and biased by the targeted stellar population).

Other ingredients are: the Initial Mass Function (IMF), that will populate the zero-age

main sequence according to its slope and their lower and upper mass limits, and the

SFH, that will give the time dependence of the stellar population spectrum.

In order to determine the SFHs from the integrated spectra of the galaxy, you can

use the spectral indices (e.g. Dressler & Gunn, 1983; Buzzoni et al., 1992; Bressan

et al., 1996) or a full-spectrum fitting technique (e.g.Tinsley & Danly, 1980;Cardiel

et al.,2003;Cid-Fernandes et al.,2005). The spectral indices consider some interesting

spectral features from which the stellar populations age and metallicity can be derived,

while the full-spectrum fitting technique consider all the wavelengths involved in the

observational data available. In the following sections I give a more detailed description

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2.2. Spectral Indices

stellar populations models will be used by means of two different models: the simplified

model of Barbaro & Poggianti(1997) (hereafter BP97Sim) and GALAXEV byBruzual

& Charlot(2003). Both models are described in the following subsections.

2.2 Spectral Indices

A spectral index, or line-strength, is a measure of the intensity of a spectral feature

of interest. Since first introduced by Faber et al. (1977) and Burstein et al. (1984),

the Lick/IDS system has been widely used in order to trace the stellar populations of

intermediate-old systems from their optical integrated spectra. The Lick/IDS system

consists of 25 spectral indices defined for a spectral dataset of 460 stars, 35 globular

clusters and 400 galaxies observed between 1972 and 1984 at the Lick Observatory

with a Cassegrain spectrograph in the wavelength range between 4000-6400 ˚A and

∼8_{A resolution.}˚

The determination of the stellar populations ages, metallicities and abundances is done

from an adequate combination of the line-strength indices sensitive to the parameter of

interest and then compared to the modeled line-strengths. This method relies on the

de-pendence of the spectral features present in the optical spectra with the temperature and

luminosity class of the stars. Due to the small wavelength interval between the spectral

features and that are used as flux ratios, this technique is not affected by extinction and

distance.

The Balmer lines (Hβ, HγA, HγF, HδA, HδF) had been widely used as age indicators,

while a combination of Fe and Mg indices are used for the metallicity estimation (e.g.

O’Connell, 1980;Dressler & Gunn, 1983;Worthey & Ottaviani, 1997;Ferreras et al.,

1999; Wu et al., 2005; Jimenez et al., 2007; Harrison et al., 2010). Some of these

combinations are:

< F e >= (F e5270 +F e5335)/2 (2.1)

[MgF e] =pMgb < F e > (2.2)

The detailed process on the measurement and calibration of the spectral indices is given

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2.3 Spectral Fitting

2.3.1 BP97Sim

Model Description

A simplified model of the original described by Barbaro & Poggianti(1997) is prefered

in order to reduce the model parameters, to study discontinuous star formation histories

and to allow the implementation of selective extinction for different stellar

popula-tions, also because the basic physical parameters that describe the integrated spectra

are included. In this approach the integrated spectra is a combination of 10 stellar

pop-ulations of different ages. Each generation was born with a Salpeter IMF in the mass

range 0.1-100 M⊙. The ages of the populations are:

1. Youngest population (3_×106_, ₈_×₁₀6 _and ₁₀7 _{yr) responsible for the ionizing} photons that produce the emission lines.

2. Intermediate population (5_×107_,₁₀8_, ₃_×₁₀8_,₅_×₁₀8 _and₁₀9 _{yr) responsible} for the strongest absorptions of the Balmer lines (EW(Hδ)>4A)˚

3. Older population (>1 Gyr) contributing significantly in the spectral continuum affecting the spectral lines equivalent widths.

These populations were chosen taking into account observational constraints for the

evolutionary timescales.

For each single generation the integrated and composite spectra are produced using

the original model ofBarbaro & Poggianti(1997). This is an evolutionary population

synthesis model that computes the integrated spectra of a galaxy from the far UV to

the infrared and includes the contributions from the stellar populations and the ionized

interstellar gas. The non-thermal gaseous emission (from non-stellar ionizing sources)

and the far IR emission of dust are not included. A brief description of the original

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2.3. Spectral Fitting

• All the evolutionary phases -up to the AGB and post-AGB- are included, except the pre-main sequence. Different metallicities are allowed.

• For stars with Tef f > 5500 K and in the UV region, the spectral library

is based on the stellar atmospheres ofKurucz(1993). Covering the

wave-length range<14500 ˚A.

• For stars with Tef f < 5500 K observed spectra from Lanc¸on &

Rocca-Volmerange(1992) were adopted. Wavelength range: 14500-25000 ˚A, with

a resolution of 25-70 ˚A and a unique calibration independent of Z.

• The region forλ > 25000_{A is not included because it is strongly affected}˚ by dust emission.

• For the fitting of the visible and infrared spectra an interative procedure was performed using as an aid a blackbody spectrum.

• In the far UV region, for Tef f > 50000K, the spectra were approximated

using a blackbody distribution.

• In the wavelength range 3500-7500 ˚A , for all the luminosity classes at solar metallicity with a resolution of 4.5 ˚A , the spectral library ofJacoby et al.

(1984), which covers the spectral types O-M was used.

2. Ionized gas emission (line and continuum spectrum):

• This contribution is added for objects younger than 107 yr.

• Ionizing flux of the young population is computed with the stellar atmo-spheres model ofKurucz(1993).

• Luminosities of the nebular hydrogen lines (Balmer series) are derived us-ing recombination case B.

• Luminosities of other lines are derived using HII region models byRubin (1985) andStasinska(1990).

Technique

For producing the synthetic spectra in this case, we will follow the procedure given in

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with the observations, the modeled spectra (_∼4_{A) have to be broadened for matching}˚ the resolution of the observations (7 ˚A for ETGs and 8 ˚A for LTGs). The samples are

described in more detail in Chapter3. Each generation is assumed to be extincted by

a uniform screen of dust with a Galactic extinction lawRV = AV/E(B−V) = 3.1

(Cardelli et al.,1989), butE(B₋V)varies for different populations. The best-fit model will be chosen as the one that minimizes the differences between observed selected

features and the modeled ones. A merit function will be constructed for N=12 features

( EW: [OII]3727, Hδ, Hβ, Hα, Relative Intensities; between them, of the continuum flux in the wavelength ranges: 3770-3900, 4020-4070, 4150-4250, 4600-4800,

5060-5150, 5400-5850, 5950-6250 and 6370-6460 ˚A):

(MF)2 =

N

X

i=1 W_i2

Oi−Mi

Ei+E0

2

(2.3)

where i refers to the features of interest, Mi is the value predicted by the model, Oi

is the observed value, Ei is the accuracy; being 1% for the continuum features and

10% for the EWs, E0 is the minimum error, Wi is the weight of the feature, being

1 for all the features except for 3770-3900 ˚A (0.4) and [OII]3727 (0.5) due to the

flux calibration uncertainty of Jacoby’s spectra below 4000 ˚A and that [OII]3727 is not

related in a simple way to the star formation rate being sensitive to factors like: hardness

of the ionizing spectrum, local conditions of the nebula, the density and metallicity

of the gas (Poggianti et al., 2001) . The merit function will be minimized using the

simulated annealing method with the Numerical Recipes’ routine AMEBSA available

fromNumerical Recipes Third Edition Webnotes(2007).

2.3.2 GALAXEV

Stellar Evolution Prescription

This model allows the use of three stellar evolution prescriptions:

1. Padova 1994:

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• Initial chemical compositions Z1= 0.0001, 0.0004, 0.004, 0.008, 0.02, 0.05 and 0.1, where Y=2.5Z+0.23 with Z⊙=0.02 and Y⊙=0.28.

• Initial masses in the range0.6_≤m_≤120M⊙, except for Z=0.0001 where

0.6 _≤ m _≤ 100M⊙ and Z=0.1 where 0.6 ≤ m ≤ 9M⊙, because for

more massive stars mass-loss by stellar winds domines and for this high

metallicity mass-loss rates were not available.

• Radiative opacities ofIglesias et al.(1992).

• Includes phases of stellar evolution from the zero-age main sequence (ZAMS) to the beginning of the thermally pulsing regime of the asymptotic giant

branch (TP-AGB) for low and intermediate masses and core-carbon

igni-tion for massive stars.

• For solar composition, the models are normalized to the temperature, lumi-nosity and radius of the Sun at age of 4.6 Gyr.

• Overshooting included for convective cores of M > 1.5 M⊙ in a mild

regime (Λc ≤0.52), for the mass range 1-1.5 M⊙with a reduced efficiency,

also included for the envelopes of low and intermediate mass stars.

2. Padova 2000:

• New version of the Padova 1994 library.

• Revised equation of state.

• Low-temperature opacities.

• Inclusion of masses down to m=0.15 M⊙, but no more massive than 7 M⊙

because the new equation of state affects mainly the evolution of stars with

M<0.6M⊙.

• Chemical abundances Z=0.0004, 0.004, 0.008, 0.019 and 0.03, where Y=2.25Z+0.23 with Z⊙=0.019 and Y⊙=0.273.

3. Geneva:

1 _{here X+Y+Z=1, where X is the H relative mass abundance, Y is the He relative mass abundance}

and Z is the relative mass abundance of higher elements

2_{where the free path of the convective elements is given as}_Λc_×_H

p, whereHpis the local pressure

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• Only for solar metallicity.

• Tracks ofSchaller et al.(1992) form_≥2M⊙,Charbonnel et al.(1996) for

0.8_≤m <2M⊙andCharbonnel et al.(1999) for0.6≤m <0.8M⊙.

• Abundances X=0.68, Y=0.30 and Z=0.02.

• Opacities ofRogers & Iglesias (1992) form _≥ 2M⊙, Iglesias & Rogers

(1993) for0.8_≤m <2M⊙.

• Phases of stellar evolution: ZAMS to the beginning of the TP-AGB or core-carbon ignition according to the initial mass.

• Overshooting included for convective cores of m > 1.5 M⊙ in a mild

regime.

• Differences of this model to the Padova 1994 one: absence of overshooting for the mass range 1-1.5 M⊙and in the envelopes of low and intermediate

mass stars, higher helium fraction, inclusion of mass loss along the red giant

branch, convection treatment in the core-helium burning, internal mixing

and mass loss of masive stars.

4. Supplements:

• Evolutionary tracks of the low and intermediate mass stars beyond the early-AGB with TP-early-AGB and post-early-AGB. Adoption of the effective temperatures,

bolometric luminosities and lifetimes of the TP-AGB stars of the

Vassil-iadis & Wood (1993) models for the metallicities Z=0.001, 0.004, 0.008

and 0.016.

• Transition from the M-type star (Oxygen rich) to C-type (Carbon rich) due to the Carbon dredge-up defined by Groenewegen & de Jong (1993) and

Groenewegen et al.(1995) models, requiring that, for a given initial

main-sequence mass the relative durations of the two phases are the same given

by these models.

• For the post-AGB evolution, the tracks are taken from the models of Vassil-iadis & Wood(1994) for the metallicities in the range0.001 _≤Z _≤0.016. For lower-mass stars, the Sch¨onberner (1983) track of a 0.546 M with

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• For ages greater than 20 Gyr, white dwarf cooling models for0.4 _≤ m _≤ 1.0M⊙ofWinget et al.(1987) were adopted.

• Unevolving main-sequence stars of masses0.09_≤m <0.6M⊙.

Stellar Spectral Library

1. Low Resolution:

• Model atmospheres for stars in the metallicity range10−5 _. _Z _. ₁₀ _Z

⊙

fromLejeune et al.(1997) andLejeune et al.(1998) in the wavelength range

91 ˚A to 160µm, withλ/∆λ _≈ 200₋500. AsBruzual & Charlot(2003) mentioned they used Kurucz (1995) theoretical spectra for the hotter stars

(O-K), observed spectra fromBessell et al.(1989),Bessell et al.(1991) and

Fluks et al.(1994) for M giants,Allard & Hauschildt(1995) for M dwarfs.

There are three available versions of this library:

(a) BaSeL 1.0: spectra rebinned on homogeneous scales of

fundamen-tal parameters (effective temperature, gravity, mefundamen-tallicity) and

wave-length.

(b) BaSeL 2.2: Corrections for systematic errors evident in the

UBVRI-JHKL colour-temperature relations (Lejeune et al., 1998) applied to

BaSel 1.0. Correction derived for solar metallicity applied to all

metal-licities.

(c) BaSeL 3.1: Semi-empirical corrections made by Westera (2001) and

Westera et al.(2002) for model atmospheres at non-solar metallicities

using metallicity-dependent UBVRIJHKL color calibrations.

• The models cover temperatures2000 _≤Tef f ≤50000K

• For stars hotter than 50 000 K (50000_≤Tef f ≤106K), the non-LTE model

atmospheres ofRauch(2002) for Z=Z⊙and Z=0.10 Z⊙ was adopted. This

model covers the wavelength range 5-2000 ˚A with a resolution of 0.1 ˚A.

• For Z=0.0004 and 0.0001, Tef f ≥ 50000 K pure blackbody spectra was

assumed, also for stars cooler than 2000 K, for all metallicities.

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• STELIB, a library of 249 observed spectra of stars (LeBorgne et al.,2003) in a wide range of metallicities (₋2.0<[F e/H]<+0.50)3_{. In the}

wave-length range: 3200-9500 ˚A, with a FWHM of 3 ˚A(λ/∆λ _≈ 2000) and a sampling interval of 1 ˚A, S/N_∼50pixel−1_{. Covering the spectral types from} O5 to M9 and luminosity classes from I to V. Due to contamination from

tel-luric features the wavelength intervals 6580-6950 ˚A and 7550-7725 ˚A were

not considered in the fits.

3. Wider Spectral Coverage:

• Based on an observational spectral library of 131 Galactic stars, (Pickles, 1998). Covering the spectral types: O5-M10 and luminosity classes: I-V.

• The spectral library is divided in three metallicity groups: 11 considered metal-weak, 12 metal-rich and 108 with solar metallicity.

• Wavelength range: 1150 ˚A-2.5 µm, with a sampling interval of 5 ˚A /pixel, givingλ/∆λ_≈500.

• Main sequence and subgiant stars hotter than 40000 K, giant stars hotter than 32000 K, supergiants hotter than 26000 K and cooler than 4000 K are

not included.

• Ultraviolet spectra based on International Ultraviolet Explorer (IUE) obser-vations, with a sampling interval 1-1.2 ˚A for the wavelength range

1205-1935 ˚A, 2 ˚A for 1935-3150 ˚A. For the Extreme UV (91-1195 ˚A) is completed

with BaSeL 3.1 spectra.

• In the infrared (2.5-160µm) for M0-M10 giant stars synthetic spectra from Fluks et al.(1994). For 5000 ˚A -10 µm, 10 equally spaced stellar

temper-atures in the range 2600 _≤ Tef f ≤ 4400 K based on theSchultheis et al.

(1999) model.

4. Carbon stars and super wind phase:

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• For C-type TP-AGB stars, solar metallicity models for carbon stars in the range 2600 _≤ Tef f ≤ 3400 K ofH¨ofner et al. (2000), wavelength range:

2500 ˚A -12.5µm.

• Empirical corrections for the UBVRIJHK broad-band colors were applied.

• For the superwind phase, 16 TP-AGB stars with VRIJHKL data and partial UB information were used.

Technique

In this Section, we will briefly describe the technique used for producing synthetic

spectra, called Isochrone synthesis. This is based on the assumption that any star

for-mation history can be expanded in series of instantaneous bursts called Simple Stellar

Population (SSP).

As mention byCid-Fernandes(2007) the integrated light of a LTG can be described

by the contribution ofN∗different generations of stars (different ages and metallicities)

given by

Lgal_λ (~x, AV) =Lgalλ0

N∗

X

j=1

xjlλ(tj, Zj)⊗LOSV D×10−0.4AVrλ (2.4)

wherelλ is the spectrum assigned from the spectral library of the population j

normal-ized at λ0, tj and Zj are the ages and metallicities of each generation, xj is its light

fraction, LOSV D is the line-of-sight velocity distributions andrλ = (Aλ −Aλ0)/AV

is the reddening law. The SFH is encoded in the population vector~x. The population of

the different evolutionary phases along the isochrone is given by the IMF. To each star

populating the isochrone, a spectrum is assigned from the stellar spectral library in use.

The IMF is an adjustable parameter, but theChabrier(2003) parametrization is used.

φ(logm)_∝







exph₋(logm−logmc)2 2σ2

i

m_≤1M⊙

m−1.3 _{m >}_1M

⊙

(2.5)

wheremc = 0.08M⊙is the mean mass,σ = 0.694, with lower and upper mass cut-offs

mL = 0.1M⊙ and mU = 100M⊙. The SED of a SSP is normalized to a total stellar

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mass of 1 M⊙at aget ′

= 0, sampled in 221 unequally spaced time-steps from 0-20 Gyr and covering the wavelength range 90 ˚A-160µm.

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Chapter 3 Galaxy Samples and Observations

In this Chapter I will describe the selection criteria defining the galaxies samples

di-vided in early and late types. The observational data available for each one is also

ex-plained, emphasizing the fact that for the ETGs sample the optical spectra is obtained

with the long-slit technique while for the LTGs one is done with an IFU technique.

3.1 Early-type Galaxies: The Virgo Cluster Sample

In this Section a description of the main features of the observational data available

for the sample of ETGs is given. The set of data includes broad-band photometry

in the UV (GALEX), optical (HST), NIR (2MASS and CANICA/OAGH) and MIR

(SPITZER) spectral ranges, and also spectroscopic low-resolution data in the optical

(OAGH), NIR (TNG) and in the MIR (SPITZER) spectral ranges. The wide spectral

range and the quality of the data will allow to perform and compare different standard

population synthesis analysis, for instance, Lick indices (Buzzoni et al., 1992;

Bres-san et al., 1996;Annibali et al., 2007), broad-band photometric data fitting from UV

to the NIR (Mayya et al., 2004;Bianchi et al., 2005;Kaviraj et al., 2007b,a; Bridˇzius

et al., 2008; Rodriguez-Merino et al., 2010), evolutionary synthesis modelling and/or

full-spectrum modelling, by fitting the optical spectrum with a linear combination of

multiple SSPs (Tinsley & Danly,1980;Cardiel et al.,2003;Cid-Fernandes et al.,2005;

Sarzi et al.,2006;S´anchez et al.,2010). The wide spectral range of the data will allow,

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allow a good calibration of the panchromatic theoretical SSPs, to correlate the SEDs

with spectral indices, and the identification of the features of fully passively evolving

ETGs (Bressan et al., 2006). Furthermore, to have a good calibrated panchromatic

SEDs allows to check the method proposed byBressan et al.(1998,2006) to broke the

age-metallicity degeneracy in ETGs by using the optical to MIR spectral ranges.

Notice that one of the main problems of these kind of work, from the observational

point of view, is to compare data within the same aperture. Special care was taken in our

data set, from UV to MIR, ensuring the homogeneity of the data by the use of a unique

aperture of 5 arcsec in radius, which, at the distance of the sample, corresponds to the

emission region of the central 390 parsecs. The election of this aperture was actually

imposed by the effective aperture of the SPITZER spectra, which were the starting

points for the panchromatic data set. In the following Subsections, a description for

each spectral range, the sources and the main features of the data to be used are given.

In Chapter4.1a description of the reduction and calibration process already performed

in the data and the current status of the same ones is detailed.

3.1.1 Galaxy sample description

The galaxy sample covers 18 bright galaxies from the Virgo cluster color-magnitude

relation (Bower et al.,1992) observed in three SPITZER observing programs resulting

in a data set of low-resolution spectra from the IRS instrument:

• Cycle 1: Breaking the age metallicity degeneracy in local early-type galaxies: clues about formation and evolution of spheroids, P.I. A. Bressan.

• Cycle 2: The evolution of early-type galaxies in nearby clusters: breaking the age metallicity degeneracy with Spitzer IRS Blue Peak-up imaging, P.I. A. Bressan.

• Cycle 3: Tracing the eventful life of field early-type galaxies with the silicate emission feature of evolved stars, P.I. R. Rampazzo.

A sample of 65 ETG galaxies, predominantly at low-density environments, have

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3.1. Early-type Galaxies: The Virgo Cluster Sample

age, metallicity and [α/Fe] of the stellar population, and also to study if there are gra-dients. They found a wide spread in the age distribution, from a few Gyr to 15 Gyr.

They also analyzed possible correlations between age, metallicity and α₋enrichment with the central velocity dispersion and the local galaxy density. In the case of the

cen-tral velocity dispersion, they found that the chemical enrichment is more efficient and

that the star formation process is shorter in more massive galaxies. They also conclude

that very young objects are found in low-density environments while at high-density

environments the ages are never younger than 5 Gyr. They did not find environmental

effects for theα₋enrichment. In single galaxies, the central regions are more metallic than the outer ones. I will use the same method for the analysis of our galaxy sample,

more detailed explanation is given in Section ??. The details of the SPITZER

observa-tions and the reduction process are presented later in this Chapter (Section 3.1.2). The

main properties of these galaxies are sumarized in Table 3.1, where Col. (1) gives the

galaxy name under the New General Catalogue (NGC), Cols. (2) and (3) list the

equa-torial coordinates from NED1 _{, Col. (4) provides the V magnitudes taken from NED,}

while Col. (5) provides the morphological and spectral classification of the galaxy,

when available, from NED. Finally, Cols. (6) and (7) list the radial velocity and the

central velocity dispersions from NED and HYPERLEDA2 , respectively. In this

the-sis, I will adopt an average distance of 16.1 Mpc to all of galaxies of the Virgo Cluster

(Kelson et al.,2000).

3.1.2 Panchromatic Data

One of the aims of the project is to construct for the first time the panchromatic SED

for these ETGs. Therefore, observations at several wavelengths are required. Most of

them were taken from the literature and databases. A brief description of the available

data divided by wavelength range is given below.

1_{NASA/IPAC Extragalactic Database} 2_Hyperleda₍₂₀₁₀₎

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Table 3.1: Main properties of our ETGs sample

Name RA(2000) Dec(2000) V Type z [km/s] σ[km/s] NGC4339 12h23m34.95s 06d04m54.3s 12.26 E0,Sy2 1289 113.73 NGC4365 12h24m28.23s 07d19m03.1s 10.52 E3 1243 256.15 NGC4371 12h24m55.43s 11d42m15.4s 11.79 SB0 943 134.64 NGC4377 12h25m12.27s 14d45m43.8s 12.76 SA0 1375 144.06 NGC4382 12h25m24.05s 18d11m27.9s 10.01 SA0,pec 729 178.67 NGC4406 12h26m11.74s 12d56m46.4s 9.83 S0/E3 -244 235.01 NGC4435 12h27m40.49s 13d04m44.2s 11.74 SB0,Liner 801 156.68 NGC4442 12h28m03.89s 09d48m13.0s 11.38 SB0 532 186.73 NGC4473 12h29m48.87s 13d25m45.7s 11.16 E5 2244 179.25 NGC4474 12h29m53.54s 14d04m07.1s 12.38 S0,pec 1588 87.67 NGC4486 12h30m49.42s 12d23m28.0s 9.59 E0-1,Sy 1307 334.44 NGC4550 12h35m30.60s 12d13m15.3s 12.56 SB0,Liner 381 96.42 NGC4551 12h35m37.97s 12d15m50.4s 12.97 E 1172 106.75 NGC4564 12h36m26.99s 11d26m21.5s 12.05 E6 1142 157.4 NGC4570 12h36m53.40s 07d14m47.9s 11.84 S0/E7 1730 187.85 NGC4621 12h42m02.32s 11d38m48.9s 10.57 E5 410 225.15 NGC4636 12h42m49.87s 02d41m16.0s 10.43 E/S0 938 203.14 NGC4660 12h44m31.98s 11d11m25.9s 12.16 E5 1083 188.59

Ultraviolet data

The UV data were provided by A. Marino3_{. They consist of photometric data in the}

Far UV (FUV: 1350-1750 ˚A) and the Near UV (NUV: 1750-2800 ˚A) bands taken with

GALEX (Galaxy Evolution Explorer) telescope. This is a modified Ritchey-Chretien

telescope with a diameter of 50 cm. The aperture is 4.5 arcsec FWHM for the FUV and

6.0 arcsec FWHM for the NUV. The AB magnitudes were measured from the GALEX

intensity images within a 5arcsec-radius circular aperture.

The UV data are presented in Table3.2. The first column indicate the galaxy name,

the second and third columns are the FUV and NUV AB magnitudes that can be

con-verted into fluxes by using the following expressions:

F UV :m(AB) =₋2.5 logflux/1.40_×10−₁₅

erg s−1cm−2A˚−1

+ 18.82 (3.1)

NUV :m(AB) =₋2.5 logflux/2.06_×10−16_{erg s}−1_cm−2_A_˚−1

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Table 3.2: Ultraviolet AB magnitudes measured within a 5 arcsec radius circular aperture.

NGC FUV NUV

4339 20.96_±0.40 19.36_±0.10 4365 18.41_±0.07 18.12_±0.04 4371 20.20_±0.12 19.04_±0.05 4377 20.39_±0.37 19.08_±0.12 4382 19.37_±0.22 17.76_±0.07

4406 - - -

-4435 - - -

-4442 19.33_±0.09 18.41_±0.04 4473 19.30_±0.09 18.31_±0.04 4474 21.35_±0.17 19.35_±0.06 4486 17.27_±0.10 16.70_±0.05 4550 19.43_±0.09 18.57_±0.04 4551 21.03_±0.15 19.63_±0.05 4564 18.78_±0.08 18.29_±0.05 4570 19.26_±0.09 18.35_±0.05 4621 18.44_±0.08 18.00_±0.04 4636 18.95_±0.18 18.43_±0.09 4660 19.74_±0.10 18.58_±0.04

Optical data

Spectroscopy data were obtained in the observing runs of april 2005; january, february

and march 2006; february 2007; march and april 2008 at the ”Guillermo Haro”

Astro-physical Observatory (OAGH) located in Cananea, Sonora (+31o_{03‘ 10“ N, 110}o_23‘

05“ W) with the 2.12 meter Ritchey-Chretien telescope and the Boller and Chivens

spectrograph with two gratings (150 l/mm and 600 l/mm). The spectrograph was

mounted at the f/12 Cassegrain focus of the telescope. The plate scale of the

spec-trograph with a CCD TK1024 is 0.463 arcsec/pixel and 0.382 arcsec/pixel with a

Ver-sarray CCD detector (1324 x 1300 pixels) along the spatial direction. The slit length is

∼3arcmin.

The observations were done with the 150 l/mm grating (_∼7A) covering the wave-˚ length range from 3800 to 7000 ˚A and a slit aperture of 2 arcsec sampling a physical

size of _∼ 156 pc (assuming a mean distance to the Virgo cluster of 16.1 Mpc taken fromKelson et al.,2000). The slit was oriented in order to match the position angle of

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observa-Table 3.3: Observational data for the acquisition of the optical spectra at OAGH.

NGC Obs Date UT exp time[s] airmass 4339 22/02/06 09:01 5400 1.12 4365 12/04/05 05:48 2400 1.18 4371 30/01/06 10:48 3600 1.06 4377 24/02/06 09:48 5400 1.06 4382 29/01/06 09:17 3600 1.14 4406 02/04/08 07:57 5400 1.08 4435 01/04/08 06:56 5400 1.05 4442 12/04/05 06:58 3600 1.08 4473 29/01/06 10:29 3600 1.07 4474 25/02/06 09:39 5400 1.05 4486 10/02/07 08:57 5400 1.14 4550 02/04/08 06:19 5400 1.08 4551 25/02/06 11:43 5400 1.22 4564 27/02/06 09:47 5400 1.06 4570 27/02/06 11:31 5400 1.25 4621 12/04/05 08:07 3600 1.06 4636 13/03/08 08:52 5400 1.14 4660 12/04/05 09:15 3600 1.14

tions of the galaxy sample are shown on Table3.3, where the column 1 is the name of

the galaxy, column 2 the date of the observation in the format day/month/year, column

3 is the UT of the observation, column 4 the exposure time of the observation in seconds

and column 5 the airmass of the observation. The final (reduced and calibrated) spectra

and the reduction process will be presented and explained in Section4.1. In order to

correct for aperture differences between the data, we need broad-band photometry.

The broad-band optical images were obtained from the HST database at

Multi-mission Archive at STScI(2009) in the available filters at WFPC1-2(F300W, F450W,

F555W, F547W, F606W, F675W, F702W and F814W) and ACS(F475W, F658N and

F850LP). The detailed process of the reduction and calibration of the images, as well as

the integrated magnitudes or fluxes within the central 5 arcsec are presented in Section

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Near-Infrared (NIR) data

NIR Spectroscopic observations were provided by A. Bressan 4 _{and they were}

ob-tained with the 3.58m Alt-Az active optic Galileo National Telescope (TNG-INAF)

located at La Palma, Spain (+28o45‘ 28.3“ N, 17o53‘ 37.9“ W), by using the NICS

instrument and the AMICI grism, with a slit width of 2 arcsec giving low-resolution

spectra (R_∼ 25) in the wavelength range from 0.8 to 2.4µm. The data reduction was performed with the standard IRAF procedures as detailed in Section 4.1.1. The NIR

spectra for some of the galaxies in the sample are shown in Fig. 3.1. Due to bad

ob-serving conditions some of the galaxies (NGC 4551, NGC 4564, NGC 4570, NGC

4636) were not observed. Currently, these spectra are not corrected for aperture effects.

That will be done in the same way as the optical data by using NIR broad-band data,

within the central 5 arcsec. In Table 3.4the available 5 arcsec radius circular aperture

2MASS (Two Micron All Sky-Survey) magnitudes at 1.25 µm (J-band), 1.65µm

(H-band) and 2.17µm(K-band) collected from IRSA are diplayed. In order to correct for

aperture differences those galaxies without 2MASS data, we are currently carrying out

a CANICA (OAGH) observational program. The 2MASS magnitudes can be converted

into fluxes using the following expressions:

J = ₋2.5 log F

1.594_×10−20_{erg cm}−2_s−1_Hz−1 (3.3)

H = ₋2.5 log F

1.024_×10−20_{erg cm}−2_s−1_Hz−1 (3.4) Ks = −2.5 log

F

6.667_×10−₂₁_{erg cm}−₂_s−₁_Hz−₁ (3.5)

Mid-Infrared (MIR) data

As mentioned before, the spectroscopic MIR data defined the final number of objects

of our ETGs sample and also the working aperture for the remaining observational data.

The data set consist of low-resolution SPITZER-IRS MIR spectra from 5 to 21.3µm

for all the galaxies listed in Table3.1, except for NGC 4406 because of the low S/N of

its spectrum. The spectra were observed by SPITZER in cycles 1, 2 and 3 and reduced

and calibrated by A. Bressan5, who provided them for this thesis.

4_{private communication, (2010)} 5_{private communication, (2009)}

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Table 3.4: 2MASS magnitudes for the ETGs in our sample.

NGC J H K

λ( ˚A) 12500 16500 21600

NGC4339 11.289_±0.004 10.588_±0.004 10.336_±0.006

NGC4365 – – –

NGC4371 10.840_±0.003 10.141_±0.003 9.899_±0.004 NGC4377 10.855_±0.002 10.158_±0.003 9.918_±0.003

NGC4382 – – –

NGC4406 – – –

NGC4435 10.288_±0.001 9.547_±0.002 9.247_±0.002

NGC4442 – – –

NGC4473 – – –

NGC4474 11.164_±0.003 10.473_±0.003 10.218_±0.003

NGC4486 – – –

NGC4550 11.193_±0.003 10.484_±0.003 10.277_±0.004 NGC4551 11.463_±0.003 10.772_±0.004 10.519_±0.005 NGC4564 10.440_±0.002 9.709_±0.002 9.444_±0.003 NGC4570 10.167_±0.002 9.447_±0.002 9.190_±0.002

NGC4621 – – –

NGC4636 – – –

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The observations were performed in Standard Staring mode with low-resolution

(R_∼64-128) modules SL1 (7.4-14.5 µm), SL2 (5-8.7 µm) and LL2 (14.1-21.3 µm). These spectra were reduced and flux-calibrated with a pipeline specially developed for

ETGs, that exploits the large degree of symmetry that characterizes the light

distribu-tion of these galaxies. Finally, the extracdistribu-tion of the spectrum was done with a fixed

aperture of 3.6 x 18 arcsec2. The reduction procedure is fully described in Bressan et al.(2006), here only the main features are summarized.

1. We first carried out the e−

/sec to Jy conversion corrected for aperture losses

(ALCF) and slit losses (SLCF). To estimate the ALCF four calibration stars were

used (HR 2194, HR 6606, HR 7341 and HR 7891) and defined as the average

ratio of the fluxes extracted within the standard aperture and twice the standard

aperture. The SLCF is defined as the wavelength-dependent ratio between the

whole flux of a point source on the FoV and the flux selected by the slit. This

was done by simulating the PSF of the system and adopting a hat beam

transmis-sion function of the slit.

2. Recovery of the intrinsic SED of the galaxy by the convolution of a surface

brightness profile model with a PSF taking into account the relative position

an-gles of the slits and the galaxy to obtain the received flux within the aperture. The

profile considered is a wavelength-dependent two-dimensional modified King’s

law:

I =I0

1 + X 2

R2

c

+ Y

2

[Rc(a/b)]2

−γ/2

where X and Y are the coordinates along the major and minor axes of the galaxy,

b/a is the axial ratio (from the literature), I0, Rc and γ are free parameters and

functions of the wavelength obtained by fitting the observations to the simulated

profile.

3. We estimated the S/N by considering two sources of noise: instrumental and

background noise (measuring the variance of pixel values in background-substracted

images) and a Poissonian noise of the source (estimated as the square root of the

ratio between the variance of the number of e−

extracted per pixel in each

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Following Bressan et al.(2006) the ETGs are classified in two groups depending on

the different ”degree” of activity seen in their MIR spectra. They claim that a proper

definition of passive ETG can be only established from the analysis of the MIR

spec-tral features (see alsoPanuzzo et al.,2010). These authors define as passively evolving

ETG those whose MIR spectra do not show any emission lines or PAH emission

fea-tures, while active ETG refers to objects that show at least, one of the features. The

main characteristic of a passive MIR spectrum is the presence of the broad emission

feature around _∼ 10µm that is attributed to silicate emission arising from the dusty circumstellar envelopes of Oxygen rich AGB stars (Athey et al., 2002;Molster et al.,

2002;Sloan et al.,1998;Bressan et al.,1998) superimposed on the photospheric stellar

continuum from red giant stars. Passive ETGs of our sample are showed in Fig. 3.2

while the 4 objects presenting emission lines and/or PAH emission features are showed

in Fig.3.3.

Broad-band MIR data are also available at MIPS (24, 70 and 160µm) and IRAC

(3.6, 4.5, 5.8 and 8.0µm) in the central 5 arcsec provided by A. Bressan6_.

3.2 Late-type Galaxies: The PINGS Sample

PINGS is the acronym for PPAK IFS Nearby Galaxies Survey, P.I. F.F. Rosales-Ortega

(Rosales-Ortega et al., 2010). This project is a two-dimensional spectroscopic

mo-saicking that obtains a continuous spectral coverage of the 17 nearby disk galaxy

sam-ple. More than 50 000 individual optical spectra and a total observed area of almost

80 arcmin2 represents the final data set of this project. Because of the 2-dimensional

nature of the project it was designed to study the distribution of the chemical properties

of the gas in the galaxies, and the spatially resolved variations of the stellar continuum

across the surface area of the sample.

The PINGS project uses observations from the Potsdam Multi Aperture

Spectro-graph (PMAS,Roth et al.,2005) mounted at the Cassegrain focal station of the 3.5 m

telescope located at the German-Hispanic Astronomical Centre at Calar Alto (CAHA),

Spain. The PMAS instrument is an integral field spectrophotometer covering the optical

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3.2. Late-type Galaxies: The PINGS Sample

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3.2. Late-type Galaxies: The PINGS Sample

Stellar populations analysis along the Hubble sequence