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During several decades galaxies were considered basically as self-gravitating stellar systems so that the study of their physics was a domain of Galactic 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 perturbations. 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, metallicities, 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 (ltapprox 106 yr) luminous O,B stars mostly in the spiral arms, and their metallicites are close to the solar one, Z approx 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 metallicities, 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.

Interstellar Medium (ISM)

Galaxies are not only conglomerates of stars. The study of galaxies is incomplete 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 < 104 K) and in dense, cold molecular clouds (T <102 K), where stars form. Most of the volume of the ISM is occuppied by diffuse (n approx 0.1 cm-3), warm-hot (T approx 104 - 105 K) turbulent gas that confines clouds by pressure. The complex structure of the ISM is related to (i) its peculiar thermodynamical properties (in particular the heating and cooling 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 produced by their stars, in particular those most luminous. Hubble noticed that by their external look (morphology), galaxies can be divided into three principal 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 morphological Hubble sequence is the ratio of spheroid (bulge) to total luminosity. 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.

Figure 1

Figure 1. Main trends of physical properties of galaxies along the Hubble morphological 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 advent of extremely large galaxy surveys made possible massive and uniform determinations of global galaxy properties. Among others, the Sloan Digital Sky Survey (SDSS 2) and the Two-degree Field Galaxy Redshift Survey (2dFGRS 3) 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 fraction 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 (dot{M}s / 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 × 1010 Modot.

The hidden component

Under the assumption of Newtonian gravity, the observed dynamics of galaxies 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 distribution (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 approx 3.2 Rd) 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.

Figure 2

Figure 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.

It is important to remark the following observational facts:

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 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 approx 6 × 1010 Modot is surrounded by a halo of approx 2 × 1012 Modot ([80] and more references therein). The extension of the halo is typically approx 200-250 kpc. 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 (OmegaB / OmegaDM approx 0.04 / 0.022, see below) is fB,Un approx 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 ltapprox 105-107 K [89].

Figure 3

Figure 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.

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 Lyalpha forest) have been converging in the last years towards the same value of the baryon density: Omegab approx 0.042 ± 0.005. In Table 1 below, the densities (Omega'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 Omega

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 (approx 2.0) × 10-4
Intracluster gas 0.0018 ± 0.0007
IGM: (cold-warm) 0.013 ± 0.0023
IGM: (warm-hot) approx 0.016
z > 2  
Lyalpha 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 OmegaB approx 0.0037, which accounts for approx 9% of all the baryons at low redshifts. The gas in not virialized structures in the Intergalactic Medium (cold-warm Lyalpha / beta gas clouds and the warm-hot phase) accounts for approx 73% of all baryons. Instead, at z > 2 more than 88% of the universal baryonic fraction is in the Lyalpha forest composed of cold HI clouds. The baryonic budget's outstanding questions: Why only approx 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 defined volumes around the galaxy). The morphological type of galaxies is earlier in the locally denser regions (morphology-density relation),the fraction of ellipticals 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 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 population 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 (cosmological) conditions and (ii) of external mechanisms (ram-pressure and tidal stripping, thermal evaporation of the disk gas, strangulation, galaxy harassment, truncated star formation, etc.) that operate mostly in dense environments, where precisely the relation steepens significantly.

The morphology-environment relation evolves. It systematically flattens with z in the sense that the grow of the early-type (E+S0) galaxy fraction with density becomes less rapid ([97] and more references therein) the main change being in the high-density population fraction. Postman et al. conclude that the observed flattening of the relation up to z ~ 1 is due mainly to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population; the E fraction-density relation does not appear to evolve over the range 0 < z < 1.3! Observational studies show that other properties besides morphology vary with environment. The galaxy properties most sensitive to environment are the integral color and specific star formation rate (e.g. [68, 114, 127]. The dependences of both properties on environment extend typically to lower densities than the dependence for morphology. These properties are tightly related to the galaxy star formation history, which in turn depends on internal formation/evolution processes related directly to initial cosmological conditions as well as to external astrophysical mechanisms able to inhibit or induce star formation activity.

2.4. Genetics

Galaxies definitively evolve. We can reconstruct the past of a given galaxy by matching the observational properties of its stellar populations and ISM with (parametric) spectro-photo-chemical models (inductive approach). These are well-established models specialized in following the spectral, photometrical and chemical evolution of stellar populations formed with different gas infall rates and star formation laws (e.g. [16] and the references therein). The inductive approach allowed to determine that spiral galaxies as our Galaxy can not be explained with closed-box models (a single burst of star formation); continuous infall of low-metallicity gas is required to reproduce the local and global colors, metal abundances, star formation rates, and gas fractions. On the other hand, the properties of massive ellipticals (specially their high alpha-elements/Fe ratios) are well explained by a single early fast burst of star formation and subsequent passive evolution.

A different approach to the genetical study of galaxies emerged after cosmology provided a reliable theoretical background. Within such a background it is possible to "handle" galaxies as physical objects that evolve according to the initial and boundary conditions given by cosmology. The deductive construction of galaxies can be confronted with observations corresponding to different stages of the proto-galaxy and galaxy evolution. The breakthrough for the deductive approach was the success of the inflationary theory and the consistency of the so-called Cold Dark Matter (CDM) scenario with particle physics and observational cosmology. The main goal of these notes is to describe the ingredients, predictions, and tests of this scenario.

Galaxy evolution in action

The dramatic development of observational astronomy in the last 15 years or so opened a new window for the study of galaxy genesis: the follow up of galaxy/protogalaxy populations and their environment at different redshifts. The Deep and Ultra Deep Fields of the Hubble Spatial Telescope and other facilities allowed to discover new populations of galaxies at high redshifts, as well as to measure the evolution of global (per unit of comoving volume) quantities associated with galaxies: the cosmic star formation rate density (SFRD), the cosmic density of neutral gas, the cosmic density of metals, etc. Overall, these global quantities change significantly with z, in particular the SFRD as traced by the UV-luminosity at rest of galaxies [79]: since z ~ 1.5-2 to the present it decreased by a factor close to ten (the Universe is literally lightening off), and for higher redshifts the SFRD remains roughly constant or slightly decreases ([51, 61] and the references therein). There exists indications that the SFRD at redshifts 2-4 could be approximately two times higher if considering Far Infrared/submmilimetric sources (SCUBA galaxies), where intense bursts of star formation take place in a dust-obscured phase.

Concerning populations of individual galaxies, the Deep Fields evidence a significant increase in the fraction of blue galaxies at z ~ 1 for the blue sequence that at these epochs look more distorted and with higher SFRs than their local counterparts. Instead, the changes observed in the red sequence are small; it seems that most red elliptical galaxies were in place long ago. At higher redshifts (z gtapprox 2), galaxy objects with high SFRs become more and more common. The most abundant populations are:

Lyman Break Galaxies (LBG), selected via the Lyman break at 912Å in the rest-frame. These are star-bursting galaxies (SFRs of 10 - 1000 Modot/yr) with stellar masses of 109 - 1011 Modot and moderately clustered.

Sub-millimeter (SCUBA) Galaxies, detected with sub-millimeter bolometer arrays. These are strongly star-bursting galaxies (SFRs of ~ 1000 Modot/yr) obscured by dust; they are strongly clustered and seem to be merging galaxies, probably precursors of ellipticals.

Lyman alpha emitters (LAEs), selected in narrow-band studies centered in the Lyman alpha line at rest at z > 3; strong emission Lyman alpha lines evidence phases of rapid star formation or strong gas cooling. LAEs could be young (disk?) galaxies in the early phases of rapid star formation or even before, when the gas in the halo was cooling and infalling to form the gaseous disk.

Quasars (QSOs), easily discovered by their powerful energetics; they are associated to intense activity in the nuclei of galaxies that apparently will end as spheroids; QSOs are strongly clustered and are observed up to z approx 6.5.

There are many other populations of galaxies and protogalaxies at high redshifts (Luminous Red Galaxies, Damped Lyalpha disks, Radiogalaxies, etc.). A major challenge now is to put together all the pieces of the high-redshift puzzle to come up with a coherent picture of galaxy formation and evolution.

1 Astronomers suspect also the existence of non-observable Population III of pristine 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). Back.

2 Back.

3 Back.

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