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Understanding Galaxy Formation and


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


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


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).


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


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/



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


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


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



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


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)


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


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