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Understanding Galaxy Formation and Evolution

2
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
(15%) 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
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