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Chapter 1
INTRODUCTION
Aim of this thesis is to model the stellar content of Elliptical Galaxies as it results from the past
history of star formation and chemical evolution in these systems, and to compare observational
integrated magnitudes, colors, and line strength indices with those from theoretical models suitably
developed to this purpose.
Understanding the present day properties of elliptical galaxies and their evolution as a function
of time bears very much on several topics of modern cosmology, on stellar evolution theory, on
stellar populations and on theories of galaxy formation and evolution.
Twenty years after the seminal papers on the formation of elliptical galaxies by Larson (1975)
and by Toomre (1977) there is still much disagreement among astronomical community on both
the process of formation and evolution of early-type galaxies. Is an elliptical galaxy formed in
a single collapse or via merging ? Was there a short epoch of formation or have ellipticals been
formed continuously by hierarchical merger at similar levels ? What is the in
uence of density
environment ? Once created, is the stellar population of elliptical galaxies just passively or do
minor merging/accretion events drastically change their characteristics frequently ?
The two major competing scenarios for the formation and evolution of elliptical galaxies are:
(i) hierarchical clustering: In this scenario elliptical galaxies are supposed to be formed by mergers
(one to several) of smaller units. Each episode may induce star formation and chemical
enrichment. In addition to this, at any time elliptical galaxies may suer from dynamical
interactions with other systems perhaps inducing additional stellar activity.
(ii) monolithic star formation in isolation: Elliptical galaxies are supposed to suer from dominant star forming activity and consequent enrichment at very early epochs followed by
quiescent evolution. Also in this case marginal episodes of star formation of both internal
and external origin may occur under suitable circumstances (interactions).
Common to any mechanism of galaxy formation and evolution there are three main aspects
to be investigated: (1) morphology, which is related to dynamical processes (2) chemistry and
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CHAPTER 1. INTRODUCTION
spectro-photometry, which are related to the past history of star formation and chemical enrichment.
The dynamical aspect of the problem is investigated by means of numerical simulations (N-Body
and Three-SPH) which lead to a satisfactory modelling of the morphological properties, but are
still somewhat unable to follow the chemical and photometric evolution of the stellar content in
these systems. Only recently a few attempts exist in literature in which chemical and photometric
properties are taken into account (Carraro et al., 1998 Kaumann & Charlot, 1996 Kaumann
& Charlot, 1997).
Starting from the pioneering studies by Toomre & Toomre (1972) and Barnes (1988) up to the
more recent ones by Kaumann & Charlot (1996), and Kaumann & Charlot (1997) ellipticals are
viewed as being formed by mergers of disk galaxies in a universe where structures are built through
hierarchical clustering. In the local universe, ongoing merging is observed and it is generally
assumed that most of the \ultra-luminous" IRAS galaxies represent merging process (Schweizer,
1990). In brief, if two disk galaxies merge together, the cold gas of the parent galaxies is turned in
to stars via a burst mode of star formation, and an elliptical-like galaxy is formed. Merging of two
disk galaxies can of course occur for a large range of ages and metallicities of their stellar content.
Studies to investigate the evolution of the stellar population and gas content of the merging objects
are by von Alvensleben & Gerhard (1994), Moller et al. (1997) and von Alvensleben (1996).
Despite its success in modelling the dynamical structure of elliptical galaxies, the hierarchical
scenario suers from a point of diculty as far as the chemical properties are concerned. Indeed,
elliptical and spiral galaxies have dierent ratios between the abundances of some chemical species.
Elliptical galaxies are in general more metal-rich and enhanced in -elements with respect to spiral
galaxies. This implies that during the merging process additional chemical enrichment with strong
enhancement in -elements must take place.
The chemical and spectro-photometric aspect are investigated by means of the so-called Evolutionary Population Synthesis (EPS) originally due to Tinsley (1978) (see Tinsley (1980a), and Bruzual
(1993) for review), and since then used by many authors (see below for complete referencing).
The great advantage of the EPS method resides in its simplicity, thanks to which the implementation of the EPS technique in rather complicated models of chemical evolution (Closed-box and
infall schemes) has been possible. In these models (see below) star formation, chemical enrichment,
and photometric properties are simultaneously followed as a function of time.
Aim of this study is to investigate the physical nature of a pattern of properties of elliptical galaxies with the aid of suitably designed models in which both the chemical and spectrophotometric aspects are followed in great detail.
1.1. ELLIPTICAL PROPERTIES
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1.1 Observational properties of Elliptical Galaxies
Elliptical galaxies show many features requiring theoretical explanation. Among others we call
attention on the color-magnitude relation (CMR), the enhancement in -elements suspected to
exist in the brightest (most massive) ellipticals and in the very central regions of these systems,
the relationship between the Ultraviolet Excess and central velocity dispersion and Mg2 index
(Burstein et al., 1988), and the so-called Fundamental Plane.
THE COLOR-MAGNITUDE RELATION
The elliptical galaxies obey the CMR (Matthews & Baker, 1971 Larson, 1974 Bower et al.,
1992a Bower et al., 1992b), i.e. colors get redder at increasing luminosity. This relation is
conventionally interpreted as a mass-metallicity sequence (Faber et al., 1977 Dressler, 1984a
Vader, 1986a) which means that massive galaxies reach higher mean metallicities than the less
massive ones. Starting from the original suggestion by Larson (1974), the mass metallicity sequence
results from the so-called Supernova Driven Galactic Wind mechanism which implies that the
overall duration of the star forming activity increases with the galactic mass (tSF / Mgal ).
This is not the only interpretation because the CMR can also be interpreted as due to an
increase in the mean stellar age with the galaxy luminosity (Worthey, 1995 Kodama & Arimoto,
1997).
THE -ENHANCEMENT-MASS RELATION
The line strength indices Mg2 and hFei are observed to get stronger at increasing galactic
luminosity and for a given galaxy going towards the center and the gradient in Mg2 is steeper
than the gradient in hF ei. All this is currently interpreted as indicating that the abundances of
the abundances of the -elements (Mg, O, etc.) with respect to iron are enhanced in the central
region of galaxies and in the brightest systems.
Understanding of these properties stands on stellar nucleosynthesis, because all heavy elements
observed in the interstellar medium (ISM) are produced and ejected by stars, in particular via
SN explosions. Since iron is mainly produced by Type Ia SN (accreting white dwarfs in binary
systems in the most popular scheme) and only the Type II SN contribute to -elements, to get
an enhancement in -elements there are several avenues:
dierent ratio between Type Ia and Type II Supernov
dierent time scale of star formation
pre-enriched interstellar medium (Prompt Initial Enrichment, PIE).
As far as the rst assumption is concerned, to get enhancement the number of SN II should be
higher than the Type Ia events, and the ratio NumSNII =NumSNIa should decrease at increasing
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CHAPTER 1. INTRODUCTION
galaxy mass. This can be obtained by adopting a suitable initial mass function (IMF). Specically,
to guarantee the observed enhancement of -elements in the brightest galaxies, the percentage of
massive stars should increase with the galaxy mass. Recently Chiosi et al. (1998) have investigated
the hypothesis of an IMF changing with the temperature, density and velocity dispersion of the
star forming medium (Padoan et al., 1997), which is likely to vary with galaxy mass.
In regard to the second alternative, it is worth recalling that the mean lifetime of a binary
system (Type Ia progenitors) is 1 Gyr, and therefore the contamination by Type Ia supernov
occurs later as compared to Type II supernov. It follows that with the standard supernova
driven galactic wind model and classical IMF, the time scale of star formation over there must be
shorter than about 1 Gyr. So, to reproduce the observed trend of an enhancement increasing with
;1, which is in contradiction with the expectation
the galaxy mass, we expect that: tSF / Mgal
from the CMR.
The last hypothesis is the so-called Prompt Initial Enrichment (examined by Vazdekis et al.
(1996), and Vazdekis et al. (1998)). A variation of the PIE models consist in assuming that the
rst stellar generations are formed with a conveniently at IMF, so that they contribute metals
at early times, but non to light at the present epoch. PIE of the gas in the nuclei of ellipticals is
easily obtained by relaxing the hypothesis of instantaneous complete mixing of the gas, and letting
the enriched material sink towards the center. This is indeed a natural result of the dissipational
galaxy formation process (Larson, 1975).
THE ULTRAVIOLET EXCESS
UV observations of elliptical galaxies have shown that these apparently old stellar system
exhibit a strong upturn short-ward of about 2,000
A, dubbed the UV-upturn or UV-excess. Although the UV ux may vary from galaxy to galaxy, the remaining spectrum down to the infrared
is virtually identical in all galaxies, apart from some eects short-ward of about 5,000
A. This UV
emission has a minimum level in M32 and reaches intensities which are almost an order of magnitude higher in other galaxies such as NGC 4486 and NGC 4649 (Code & Welch, 1979 Bertola
et al., 1980 Bertola et al., 1982 Burstein et al., 1988), ranging from 2.05{4.50 mag. The source
of this emission seems to be distributed across the galaxies in the same way as the normal, cool
component. The study of Burstein et al. (1988) has shown that the UV-optical color (1550{V),
dened as the dierence between the average ux in the range 1,250 and 1,850
A and in the V
band, is positively correlated with the strength of Mg2 line absorption in the V band, in the sense
that the (1550{V) color is bluer at higher line strengths, opposite to the behaviour of optical
color indices. Opposing theories have been devised to explain this correlation. From one side,
Lee (1994) and Park & Lee (1997) suggest that the UV ux originates in the low metallicity tail
of an evolved stellar population with a wide metallicity distribution. On the oder side, several
groups (Bressan et al., 1994 Tantalo et al., 1996 Bressan et al., 1996 Greggio & Renzini, 1990
Horch et al., 1992) propose that metal-rich horizontal branch (HB) stars and their progeny are
1.1. ELLIPTICAL PROPERTIES
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responsible for the UV ux. These dierent metallicity scenarios in turn argue for dierent ages
for the stellar populations in these galaxies.
THE FUNDAMENTAL PLANE
One of most studied and nowadays debated properties of elliptical galaxies is the Fundamental
Plane. Elliptical galaxies do not populate uniformly the three-dimensional parameter space having as coordinates the central velocity dispersion (0 ), the eective radius (Reff ) and the mean
eective surface brightness (Ie = LB =2R2eff , where LB is the total galaxy luminosity in the
blue band). They rather closely around a plane (Dressler et al., 1987 Djorgovski & Davis, 1987
Bender et al., 1992 Djorgovski & Santiago, 1993) thus called the Fundamental Plane (FP). The
discovery of the FP for ellipticals was soon perceived as relevant to our understanding of galaxy
formation, and soon the FP has been used to map galaxy distances and deviation from the Hubble
ow. Although the universality of this FP is still a matter of debate with regard the slope and
the zero point, its existence is not in question.
The FP can be projected onto any two axes of the three variables. Examples of these projections
are the color-magnitude relation, the Kormendy radius-surface brightness relation (Kormendy,
1977) and the Faber-Jackson relation between luminosity and velocity dispersion (Faber & Jackson,
1976). The rst more interesting analysis of the FP has been proposed by Bender et al. (1992)
for the cluster galaxies of Virgo and Coma. They choose a new orthogonal coordinate system in
the 3-space of the observable parameters, specically the new coordinates are: k1 / log (M=c2 )
k2 / log (c1=c2 )Ie3 (M=L) k3 / log (c1 =c2)(M=L), where c1 and c2 are representing structural
constant. In this new coordinate system the k1 ; k2 projection gives an edge-on view of the plane.
Looking at this projection is soon evident that elliptical galaxies in Virgo and Coma clusters are
well constrained to a plane. The main properties of the FP are the so-called tilt, i.e. the systematic
increase of k3 along the FP, and its tightness, i.e. the nearly constant and very small dispersion
of k3 at every location on the FP.
Since the discovery of the FP, a great deal of work has been spent to understand the physical
cause of tilt and tightness for which the following suggestion have been put forward:
1) systematic variation of the IMF (this implies that the FP tells perhaps more about star formation than about galaxy formation) (see Renzini & Ciotti (1993))
2) trend in the relative properties and distributions of bright and dark-matter, than the FP may
tell on the amount of concentration experienced by dissipational baryonic component relative
to the dissipation-less component (see Ciotti et al. (1996)).
In both cases major changes and ne tuning are required. The eects of structural and dynamical
causes (such as orbital radial anisotropy, relative bright/dark matter distributions, shape of the
light proles) have been also investigated under the assumption of a constant (M/L) ratio. Also
in this case ne tuning is however required to preserve the tightness of the FP.
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CHAPTER 1. INTRODUCTION
As nowadays there is no single solution to this problem because there are also hints for changes
in the FP, de Carvalho & Djorgovski (1992) suggesting a systematic dierence between eld and
cluster ellipticals both in the zero point and scatter of the FP. There are also evidences for
variations with the bandpass of the FP slope which are inconsistent with any reasonable stellar
population model (Pahre & Djorgovski, 1996). Finally, there seems to be a variation of the FP
tilt with the age (Pahre & Djorgovski, 1996).
New models have been developed by the Padova group, in which relaxing some of the standard
assumption, a new IMF (Padoan et al., 1997) has been included, and its eects on galaxy evolution
explored, to nd a viable solution of the tilt of the FP, color-magnitude relation and chemical
enhancement of -elements at the same time see Chiosi et al. (1998) for all details].
1.2 Dissertation outline
In the following I will present the new chemical-spectro-photometric models of ellipticals galaxies
which improve upon the previous ones developed by Bressan et al. (1994) with the aim of reproducing the observed properties of these systems, and solving some of the major contradictions
pointed out by the authors.
The rst step towards the new generation of models presented in this thesis has been to
analyze the eects of infalling of primordial gas in the potential well of the galaxy on the
galaxy evolution itself (open-box approximation). The infall has been included to explain the
so-called analog of the G-Dwarf problem pointed out by Bressan et al. (1994) and to adopt
a model physically consistent with the observations (see chapter x 2 and section x 5.1 for all
details).
The second improvement to these models has been to described a galaxy as subdivided in
shells to reproduce the recent observations are metallicity, color and line strength indices
gradients in elliptical galaxies (see section x 5.4).
Third, all the results have been compared with the observational data in order to explain
the main properties of the elliptical galaxies (see x 6).
Finally, these models have been utilized to disentangle the questions posed by the agemetallicity degeneracy, and on the abundances-mass relationship (see chapters xx 8 and 7).
Shortly chapters xx 3 and 4, describe the key ingredients that have been utilized to construct the
new chemical-spectro-photometric models presented.
Summary and conclusions are given in chapter x 9.