Resumen de Observational properties of fossil galaxy systems from the fogo project
Stefano Zarattini
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.
