In nature there are groups and clusters of galaxies that present a large difference in luminosity
between their two brightest members. These galaxy aggregations are called fossils and
they host the most luminous and massive galaxies in the Universe. How did fossil systems
form? Which are the mechanisms driving their evolution? Which are the observational
differences between fossil and non-fossil systems? To answer these and other questions,
a large observational program was performed. The project is called Fossil Group Origins
(FOGO) and this thesis has been developed within its framework.
The objective of this thesis is to clarify the observational differences between fossil and
non-fossil systems through the following key aspects: (i) the global properties and scaling
relations, (ii) the properties of the galaxy population, and (iii) the presence of substructures.
To reach these goals, we analyze a sample of 34 fossil group candidates selected from the
Sloan Digital Sky Survey and presented in Santos et al. (2007).
In Chapter 2, we confirm that 15 out of 34 candidates are actually fossil systems. This
is a lower limit, because for some systems the determination of the key parameter Deltam12
(the magnitude gap between their two brightest members) is a lower limit only. Moreover,
we find some clear correlations between Deltam12 and some global properties of the systems,
such as the absolute magnitude and fraction of light enclosed in the central galaxy, or the
mass of the host halo. Finally, we confirm the existence of fossil clusters and the possible
existance of a transitional fossil group, which is a system that was fossil in the recent past
but that it does not accomplish the definition of fossil anymore.
In Chapter 3, we focus our attention on the dependence of the galaxy luminosity function
on the magnitude gap. The obtained results show that for larger Deltam12 the galaxy
luminosity functions present
atter faint ends as well as less M* galaxies in the bright end.
The latter result can be explained with the merging of the M*galaxy population into the
central object, due to the strong dynamical friction that these galaxies experience while
moving into the dark matter halo of the system. However, the small number of dwarf
galaxies can not be explained by dynamical friction, since it is less effective at dwarf scale.
This result suggests that other processes { such as primordial disruption or different types
of orbits { could be responsible for the destruction of the dwarf galaxy population.
In Chapter 4, we analyze the presence of substructures in fossil systems. This point
gives us information about the dynamical status of these systems. Following the most
widely-accepted scenario for the formation of fossil systems, these objects are thought to
be old and, consequently, they had more time to reach a dynamically-relaxed status. For
this reason, no substructures are expected in fossil systems. Nevertheless, from the analysis
of 13 fossil systems with a battery of statistical tests we find that a significative fraction
of fossil systems shows clear hints of substructures. Although it is dificult to quantify this
fraction due to the low number of analyzed systems, it seems similar to that of non-fossil
systems.
In Chapter 5, we study the kinematics and stellar population in NGC 7556, which is the
central galaxy of the bona-fide fossil system RXC J2315.7-0222 . We find that NGC 7556
is a massive (sigma_v,0 = 280 km s-1) and slowly rotating galaxy. Its central stellar population
is characterized by an age of 7:6+-1:7 Gyr, metallicity of [Z/H]=0:46+-0:05 dex, and alpha/Fe
enhancement of [alpha/Fe]=0.29 +- 0.03 dex. Moreover, the gradients of these three quantities
are Nabla_age = 3:7 +- 2:8 Gyr, Nabla_[Fe=H] = -0:20 +- 0:09 dex, and Nabla_alpha7Fe = -0:01 +- 0:08 dex,
respectively. This indicates that NGC 7556 shows two different stellar populations: one
old and more radially extended and the other younger and more centrally concentrated.
These results are consistent with a scenario in which NGC 7556 formed at high redshift via
gas-rich major mergers which triggered star formation in the center of the galaxy. The last
stellar population was built in a short time (Deltat = 0:3 Gyr), being the responsible for the
high central value of the alpha/Fe enhancement.
Summarizing, all these observational properties confirm that fossil systems present massive
central galaxies hosting a large fraction of the baryons located in stars. These galaxies
grew via the merging of M* galaxies located in the central regions of the system at high
redshift. Moreover, the dwarf galaxy population of fossil systems shows differences with
respect to non-fossil ones. This can be connected to typical internal processes expected
in fossil systems like galaxies moving onto more radial orbits. We are also able to discard
the hypothesis that all fossil systems are dynamically old and relaxed. In fact, the fraction
of fossil and non-fossil systems with galaxy substructures is similar. This means that the
Deltam12 parameter alone is not a good indicator of the dynamical status of clusters of galaxies.
Future hydrodynamic and semianalytic simulations have to explain the observational
properties that we find.