Understanding Galaxy Formation and
Evolution
Vladimir Avila-Reese1
Instituto de Astronom´ıa, Universidad Nacional Autónoma de México, A.P. 70-264,
04510, México,D.F. avila@astroscu.unam.mx
The old dream of integrating into one the study of micro and macrocosmos
is now a reality. Cosmology, astrophysics, and particle physics intersect in a
scenario (but still not a theory) of cosmic structure formation and evolution
called Λ Cold Dark Matter (ΛCDM) model. This scenario emerged mainly to
explain the origin of galaxies. In these lecture notes, I first present a review
of the main galaxy properties, highlighting the questions that any theory of
galaxy formation should explain. Then, the cosmological framework and the
main aspects of primordial perturbation generation and evolution are ped-
agogically detached. Next, I focus on the “dark side” of galaxy formation,
presenting a review on ΛCDM halo assembling and properties, and on the
main candidates for non–baryonic dark matter. It is shown how the nature of
elemental particles can influence on the features of galaxies and their systems.
Finally, the complex processes of baryon dissipation inside the non–linearly
evolving CDM halos, formation of disks and spheroids, and transformation
of gas into stars are briefly described, remarking on the possibility of a few
driving factors and parameters able to explain the main body of galaxy prop-
erties. A summary and a discussion of some of the issues and open problems
of the ΛCDM paradigm are given in the final part of these notes.
arXiv:astro-ph/0605212v1 9 May 2006
1 Introduction
Our vision of the cosmic world and in particular of the whole Universe has
been changing dramatically in the last century. As we will see, galaxies were
repeatedly the main protagonist in the scene of these changes. It is about
80 years since E. Hubble established the nature of galaxies as gigantic self-
bound stellar systems and used their kinematics to show that the Universe as
a whole is expanding uniformly at the present time. Galaxies, as the building
blocks of the Universe, are also tracers of its large–scale structure and of its
evolution in the last 13 Gyrs or more. By looking inside galaxies we find
that they are the arena where stars form, evolve and collapse in constant
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Vladimir Avila-Reese
interaction with the interstellar medium (ISM), a complex mix of gas and
plasma, dust, radiation, cosmic rays, and magnetics fields. The center of a
significant fraction of galaxies harbor supermassive black holes. When these
“monsters” are fed with infalling material, the accretion disks around them
release, mainly through powerful plasma jets, the largest amounts of energy
known in astronomical objects. This phenomenon of Active Galactic Nuclei
(AGN) was much more frequent in the past than in the present, being the
high–redshift quasars (QSO’s) the most powerful incarnation of the AGN
phenomenon. But the most astonishing surprise of galaxies comes from the
fact that luminous matter (stars, gas, AGN’s, etc.) is only a tiny fraction
(∼ 1 − 5%) of all the mass measured in galaxies and the giant halos around
them. What this dark component of galaxies is made of? This is one of the
most acute enigmas of modern science.
Thus, exploring and understanding galaxies is of paramount interest to cos-
mology, high–energy and particle physics, gravitation theories, and, of course,
astronomy and astrophysics. As astronomical objects, among other questions,
we would like to know how do they take shape and evolve, what is the origin of
their diversity and scaling laws, why they cluster in space as observed, follow-
ing a sponge–like structure, what is the dark component that predominates
in their masses. By answering to these questions we would able also to use
galaxies as a true link between the observed universe and the properties of the
early universe, and as physical laboratories for testing fundamental theories.
The content of these notes is as follows. In §2 a review on main galaxy
properties and correlations is given. By following an analogy with biology,
the taxonomical, anatomical, ecological and genetical study of galaxies is pre-
sented. The observational inference of dark matter existence, and the baryon
budget in galaxies and in the Universe is highlighted. Section 3 is dedicated
to a pedagogical presentation of the basis of cosmic structure formation the-
ory in the context of the Λ Cold Dark Matter (ΛCDM) paradigm. The main
questions to be answered are: why CDM is invoked to explain the formation of
galaxies? How is explained the origin of the seeds of present–day cosmic struc-
tures? How these seeds evolve?. In §4 an updated review of the main results on
properties and evolution of CDM halos is given, with emphasis on the aspects
that influence the propertied of the galaxies expected to be formed inside the
halos. A short discussion on dark matter candidates is also presented (§§4.2).
The main ingredients of disk and spheroid galaxy formation are reviewed and
discussed in §5. An attempt to highlight the main drivers of the Hubble and
color sequences of galaxies is given in §§5.3. Finally, some selected issues and
open problems in the field are resumed and discussed in §6.
2 Galaxy properties and correlations
During several decades galaxies were considered basically as self–gravitating
stellar systems so that the study of their physics was a domain of Galactic
Understanding Galaxy Formation and Evolution
3
Dynamics. Galaxies in the local Universe are indeed mainly conglomerates of
hundreds of millions to trillions of stars supported against gravity either by
rotation or by random motions. In the former case, the system has the shape
of a flattened disk, where most of the material is on circular orbits at radii that are the minimal ones allowed by the specific angular momentum of the material. Besides, disks are dynamically fragile systems, unstable to perturbations.
Thus, the mass distribution along the disks is the result of the specific angular
momentum distribution of the material from which the disks form, and of the
posterior dynamical (internal and external) processes. In the latter case, the
shape of the galactic system is a concentrated spheroid/ellipsoid, with mostly
(disordered) radial orbits. The spheroid is dynamically hot, stable to pertur-
bations. Are the properties of the stellar populations in the disk and spheroid
systems different?
Stellar populations
Already in the 40’s, W. Baade discovered that according to the ages, metal-
licities, kinematics and spatial distribution of the stars in our Galaxy, they
separate in two groups: 1) Population I stars, which populate the plane of the
disk; their ages do not go beyond 10 Gyr –a fraction of them in fact are young
( < 106 yr) luminous O,B stars mostly in the spiral arms, and their metallicites
∼
are close to the solar one, Z ≈ 2%; 2) Population II stars, which are located
in the spheroidal component of the Galaxy (stellar halo and partially in the
bulge), where velocity dispersion (random motion) is higher than rotation
velocity (ordered motion); they are old stars (> 10 Gyr) with very low metal-
licities, on the average lower by two orders of magnitude than Population I
stars. In between Pop’s I and II there are several stellar subsystems. 1.
Stellar populations are true fossils of the galaxy assembling process. The
differences between them evidence differences in the formation and evolution
of the galaxy components. The Pop II stars, being old, of low metallicity, and
dominated by random motions (dynamically hot), had to form early in the
assembling history of galaxies and through violent processes. In the meantime,
the large range of ages of Pop I stars, but on average younger than the Pop
II stars, indicates a slow star formation process that continues even today
in the disk plane. Thus, the common wisdom says that spheroids form early
in a violent collapse (monolithic or major merger), while disks assemble by
continuous infall of gas rich in angular momentum, keeping a self–regulated
SF process.
1 Astronomers suspect also the existence of non–observable Population III of pris-
tine stars with zero metallicities, formed in the first molecular clouds ∼ 4 108
yrs (z ∼ 20) after the Big Bang. These stars are thought to be very massive,
so that in scaletimes of 1Myr they exploded, injected a big amount of energy to
the primordial gas and started to reionize it through expanding cosmological HII
regions (see e.g., [20, 27] for recent reviews on the subject).
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Vladimir Avila-Reese
Interstellar Medium (ISM)
Galaxies are not only conglomerates of stars. The study of galaxies is incom-
plete if it does not take into account the ISM, which for late–type galaxies
accounts for more mass than that of stars. Besides, it is expected that in
the deep past, galaxies were gas–dominated and with the passing of time
the cold gas was being transformed into stars. The ISM is a turbulent, non–
isothermal, multi–phase flow. Most of the gas mass is contained in neutral
instable HI clouds (102 < T < 104K) and in dense, cold molecular clouds
(T < 102K), where stars form. Most of the volume of the ISM is occuppied by
diffuse (n ≈ 0.1cm−3), warm–hot (T ≈ 104 − 105K) turbulent gas that con-
fines clouds by pressure. The complex structure of the ISM is related to (i)
its peculiar thermodynamical properties (in particular the heating and cool-
ing processes), (ii) its hydrodynamical and magnetic properties which imply
development of turbulence, and (iii) the different energy input sources. The
star formation unities (molecular clouds) appear to form during large–scale
compression of the diffuse ISM driven by supernovae (SN), magnetorotational
instability, or disk gravitational instability (e.g., [7]). At the same time, the energy input by stars influences the hydrodynamical conditions of the ISM: the
star formation results self–regulated by a delicate energy (turbulent) balance.
Galaxies are true “ecosystems” where stars form, evolve and collapse in
constant interaction with the complex ISM. Following a pedagogical analogy
with biological sciences, we may say that the study of galaxies proceeded
through taxonomical, anatomical, ecological and genetical approaches.
2.1 Taxonomy
As it happens in any science, as soon as galaxies were discovered, the next step
was to attempt to classify these news objects. This endeavor was taken on by
E. Hubble. The showiest characteristics of galaxies are the bright shapes pro-
duced by their stars, in particular those most luminous. Hubble noticed that
by their external look (morphology), galaxies can be divided into three prin-
cipal types: Ellipticals (E, from round to flattened elliptical shapes), Spirals
(S, characterized by spiral arms emanating from their central regions where
an spheroidal structure called bulge is present), and Irregulars (Irr, clumpy
without any defined shape). In fact, the last two classes of galaxies are disk–
dominated, rotating structures. Spirals are subdivided into Sa, Sb, Sc types
according to the size of the bulge in relation to the disk, the openness of the
winding of the spiral arms, and the degree of resolution of the arms into stars
(in between the arms there are also stars but less luminous than in the arms).
Roughly 40% of S galaxies present an extended rectangular structure (called
bar) further from the bulge; these are the barred Spirals (SB), where the bar
is evidence of disk gravitational instability.
From the physical point of view, the most remarkable aspect of the mor-
phological Hubble sequence is the ratio of spheroid (bulge) to total luminosity.
Understanding Galaxy Formation and Evolution
5
This ratio decreases from 1 for the Es, to ∼ 0.5 for the so–called lenticulars
(S0), to ∼ 0.5 − 0.1 for the Ss, to almost 0 for the Irrs. What is the origin of
this sequence? Is it given by nature or nurture? Can the morphological types
change from one to another and how frequently they do it? It is interesting
enough that roughly half of the stars at present are in galaxy spheroids (Es
and the bulges of S0s and Ss), while the other half is in disks (e.g., [11]), where some fraction of stars is still forming.
2.2 Anatomy
The morphological classification of galaxies is based on their external aspect
and it implies somewhat subjective criteria. Besides, the “showy” features
that characterize this classification may change with the color band: in blue
bands, which trace young luminous stellar populations, the arms, bar and
other features may look different to what it is seen in infrared bands, which
trace less massive, older stellar populations. We would like to explore deeper
the internal physical properties of galaxies and see whether these properties
correlate along the Hubble sequence. Fortunately, this seems to be the case in
general so that, in spite of the complexity of galaxies, some clear and sequential
trends in their properties encourage us to think about regularity and the
possibility to find driving parameters and factors beyond this complexity.
Figure 1 below resumes the main trends of the “anatomical” properties of galaxies along the Hubble sequence.
The advent of extremely large galaxy surveys made possible massive and
uniform determinations of global galaxy properties. Among others, the Sloan
Digital Sky Survey (SDSS2) and the Two–degree Field Galaxy Redshift Sur-
vey (2dFGRS3) currently provide uniform data already for around 105 galaxies
in limited volumes. The numbers will continue growing in the coming years.
The results from these surveys confirmed the well known trends shown in
Fig. 1; moreover, it allowed to determine the distributions of different properties. Most of these properties present a bimodal distribution with two main
sequences: the red, passive galaxies and the blue, active galaxies, with a frac-
tion of intermediate types (see for recent results [68, 6, 114, 34, 127] and more references therein). The most distinct segregation in two peaks is for
the specific star formation rate ( ˙
Ms/Ms); there is a narrow and high peak
of passive galaxies, and a broad and low peak of star forming galaxies. The
two sequences are also segregated in the luminosity function: the faint end is
dominated by the blue, active sequence, while the bright end is dominated by
the red, passive sequence. It seems that the transition from one sequence to
the other happens at the galaxy stellar mass of ∼ 3 × 1010M⊙.
2 www.sdss.org/sdss.html
3 www.aao.gov.au/2df/
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Vladimir Avila-Reese
Fig. 1. Main trends of physical properties of galaxies along the Hubble morpholog-
ical sequence. The latter is basically a sequence of change of the spheroid–to–disk
ratio. Spheroids are supported against gravity by velocity dispersion, while disks by
rotation.
The hidden component
Under the assumption of Newtonian gravity, the observed dynamics of galax-
ies points out to the presence of enormous amounts of mass not seen as stars
or gas. Assuming that disks are in centrifugal equilibrium and that the orbits
are circular (both are reasonable assumptions for non–central regions), the
measured rotation curves are good tracers of the total (dynamical) mass dis-
tribution (Fig. 2). The mass distribution associated with the luminous galaxy (stars+gas) can be inferred directly from the surface brightness (density) profiles. For an exponential disk of scalelength Rd (=3 kpc for our Galaxy), the
rotation curve beyond the optical radius (Ropt ≈ 3.2Rd) decreases as in the
Keplerian case. The observed HI rotation curves at radii around and beyond
Ropt are far from the Keplerian fall–off, implying the existence of hidden mass
called dark matter (DM) [99, 18]. The fraction of DM increases with radius.
It is important to remark the following observational facts:
Understanding Galaxy Formation and Evolution
7
Fig. 2. Under the assumption of circular orbits, the observed rotation curve of disk
galaxies traces the dynamical (total) mass distribution. The outer rotation curve of
a nearly exponential disk decreases as in the Keplerian case. The observed rotation
curves are nearly flat, suggesting the existence of massive dark halos.
• the outer rotation curves are not universally flat as it is as-
sumed in hundreds of papers. Following, Salucci & Gentile [101], let us define the average value of the rotation curve logarithmic slope,
▽ ≡ (dlogV/dlogR) between two and three Rd. A flat curve means
▽ = 0; for an exponential disk without DM, ▽ = −0.27 at 3Rs. Ob-
servations show a large range of values for the slope: −0.2 ≤ ▽ ≤ 1
• the rotation curve shape (▽) correlates with the luminosity and
surface brightness of galaxies [95, 123, 132]: it increases according the galaxy is fainter and of lower surface brightness
• at the optical radius Ropt, the DM–to–baryon ratio varies from
≈ 1 to 7 for luminous high–surface brightness to faint low–surface
brightness galaxies, respectively
• the roughly smooth shape of the rotation curves implies a fine
coupling between disk and DM halo mass distributions [24]
The HI rotation curves extend typically to 2 − 5Ropt. The dynamics at
larger radii can be traced with satellite galaxies if the satellite statistics allows
for that. More recently, the technique of (statistical) weak lensing around
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Vladimir Avila-Reese
galaxies began to emerge as the most direct way to trace the masses of galaxy
halos. The results show that a typical L∗ galaxy (early or late) with a stellar
mass of Ms ≈ 6 × 1010M⊙ is surrounded by a halo of ≈ 2 × 1012M⊙ ([80] and more references therein). The extension of the halo is typically ≈ 200−250kpc.
These numbers are very close to the determinations for our own Galaxy.
The picture has been confirmed definitively: luminous galaxies are just
the top of the iceberg (Fig. 3). The baryonic mass of (normal) galaxies is only
∼ 3 − 5% of the DM mass in the halo! This fraction could be even lower for
dwarf galaxies (because of feedback) and for very luminous galaxies (because
the gas cooling time > Hubble time). On the other hand, the universal baryon–
to–DM fraction (ΩB/ΩDM ≈ 0.04/0.022, see below) is fB,Un ≈ 18%. Thus,
galaxies are not only dominated by DM, but are much more so than the
average in the Universe! This begs the next question: if the majority of baryons
is not in galaxies, where it is? Recent observations, based on highly ionized
absorption lines towards low redshfit luminous AGNs, seem to have found a
fraction of the missing baryons in the interfilamentary warm–hot intergalactic
medium at T < 105 − 107 K [89].
∼
Fig. 3. Galaxies are just the top of the iceberg. They are surrounded by enormous
DM halos extending 10–20 times their sizes, where baryon matter is only less than
5% of the total mass. Moreover, galaxies are much more DM–dominated than the
average content of the Universe. The corresponding typical baryon–to–DM mass
ratios are given in the inset.
Understanding Galaxy Formation and Evolution
9
Global baryon inventory: The different probes of baryon abundance in the
Universe (primordial nucleosynthesis of light elements, the ratios of odd and
even CMBR acoustic peaks heights, absorption lines in the Lyα forest) have
been converging in the last years towards the same value of the baryon density:
Ωb ≈ 0.042 ± 0.005. In Table 1 below, the densities (Ω′s) of different baryon
components at low redshfits and at z > 2 are given (from [48] and [89]).
Table 1. Abundances of the different baryon components (h = 0.7)
Component
Contribution to Ω
Low redshifts
Galaxies: stars
0.0027 ± 0.0005
Galaxies: HI
(4.2 ± 0.7)×10−4
Galaxies: H2
(1.6 ± 0.6)×10−4
Galaxies: others
(≈ 2.0)×10−4
Intracluster gas
0.0018 ± 0.0007
IGM: (cold-warm)
0.013 ± 0.0023
IGM: (warm-hot)
≈ 0.016
z > 2
Lyα forest clouds
> 0.035
The present–day abundance of baryons in virialized objects (normal stars,
gas, white dwarfs, black holes, etc. in galaxies, and hot gas in clusters) is
therefore ΩB ≈ 0.0037, which accounts for ≈ 9% of all the baryons at low
redshifts. The gas in not virialized structures in the Intergalactic Medium
(cold-warm Lyα/β gas clouds and the warm–hot phase) accounts for ≈ 73%
of all baryons. Instead, at z > 2 more than 88% of the universal baryonic
fraction is in the Lyα forest composed of cold HI clouds. The baryonic budget’s
outstanding questions: Why only ≈ 9% of baryons are in virialized structures at the present epoch?
2.3 Ecology
The properties of galaxies vary systematically as a function of environment.
The environment can be relatively local (measured through the number of
nearest neighborhoods) or of large scale (measured through counting in de-
fined volumes around the galaxy). The morphological type of galaxies is earlier
in the locally denser regions (morphology–density relation),the fraction of el-
lipticals being maximal in cluster cores [40] and enhanced in rich [96] and poor groups. The extension of the morphology–density relation to low local–density
environment (cluster outskirts, low mass groups, field) has been a matter of
debate. From an analysis of SDSS data, [54] have found that (i) in the sparsest regions both relations flatten out, (ii) in the intermediate density regions (e.g.,
cluster outskirts) the intermediate–type galaxy (mostly S0s) fraction increases
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Vladimir Avila-Reese
towards denser regions whereas the late–type galaxy fraction decreases, and
(iii) in the densest regions intermediate–type fraction decreases radically and
early–type fraction increases. In a similar way, a study based on 2dFGRS
data of the luminosity functions in clusters and voids shows that the popu-
lation of faint late–type galaxies dominates in the latter, while, in contrast,
very bright early–late galaxies are relatively overabundant in the former [34].
This and other studies suggest that the origin of the morphology–density (or
morphology-radius) relation could be a combination of (i) initial (cosmologi-
cal) conditions and (ii) of