Formation and Evolution of Early-Type Galaxies: Spectro-Photometry from Cosmo-Chemo-Dynamical Simulations

One of the major challenges in modern astrophysiscs is to understand the origin and the evolution of galaxies, the bright Early-Type Galaxies (ETGs) in particular, in the context of a Universe dominated by Cold Dark Matter (CDM), with some kind of Dark Energy in form of the Cosmological Constant ?. These spheroidal systems are of interest in their own right as they contain more than half of the total stellar mass in the local Universe (Fukugita et al 1998). Giant elliptical galaxies are the most massive stellar systems and they appear to define a homogeneous class of objects which predominantly consist of uniformly old and red populations, which implies that they must have formed at high redshift, have negligible amounts of gas and very little star formation (Bressan et al. 1993). There is strong observational evidence that ellipticals are already in place at z ~ 2-3 and that formed most of their stars well before a redshift z=1 (Searle et al. 1973; Brinchmann & Ellis 2000; Treu et al. 2005; van der Wel et al. 2005). These galaxies are therefore likely to be good probes of galaxy evolution, star formation and metal enrichment in the early Universe. The main goal of this PhD thesis is the derivation of a tool that combines the results derived from the cosmo-chemo-dynamical models of elliptical galaxies obtained from N-body simulations, together with the spectro-photometric models computed from the stellar Evolutionary Population Synthesis (EPS) technique. The aim is to reproduce the observational integrated properties of early-type galaxies in any photometric bandpass, and in particular in the systems used by the modern imaging surveys of observational cosmology, that cover any spectral range. The EPS technique is based on Simple Stellar Populations (SSPs) due to their characteristics, these are suitable for purposes of population synthesis of more complex systems of stellar populations, such as simulated ETGs derived from numerical simulations, and so are suitable for the modelling of the integrated properties (light), and allow easy testing for different input prescriptions in the description of a galaxy (different masses, star formation, initial mass functions, physical processes, etc.) and reproduction of basic observational constraints. In the present work, the approach allows the computation of spectroscopic and photometric quantities by combining the EPS technique to three-dimensional self-consistent cosmo-chemo-dynamical Tree-SPH numerical simulations, carried on in the last years by the Padova group (Merlin & Chiosi 2006, 2007), that follow the evolution of ETGs from the epoch of their complex formation to the present. The method has been tested so far on three simulated galaxies: these models have different cosmological metrics, both cold dark matter (CDM) scenarios, one in the SCDM and two in the ?CDM cosmologies. In the first part of the thesis we consider the template galaxies, which have been dynamically simulated with a detailed chemical evolution, and recover their spectro-photometric evolution in the rest-frame and the integrated properties, such as magnitudes and colors, at the different epochs through the entire history of the Universe up to the formation of present-day ellipticals. This is done in particluar for two inportant photometric systems, the Bessell-Brett passbands and the Sloan Digital Sky Survey (SDSS). The advent of the modern giant telescope facilities has opened a new era in observational cosmology and galaxy evolution can be traced back to its early stages. In this sense, deep multicolor imaging surveys are established as a powerful tool to access the population of faint galaxies with relatively high efficiency. These surveys sample the whole spectral range from the UV to the near-IR bands, enabling galaxy evolution to be followed on a wider range of redshifts. Starting from the evolutionary synthesis results we compute the evolutionary and cosmological corrections, along with magnitudes and colors and their evolution at different redshifts for the simulated galaxies at our disposal. We consider the COSMOS (Giavalisco et al. 2004) and the GOODS (Scoville et al 2007) databases, which allow us to select a sample of galaxies that are catalogued as early-type and to make a qualitative and quantitative comparison between the theoretical results obtained from our model galaxies and the observational data. For the COSMOS database we find that the models follow the general trend for all data at high redshift and, in particular, are in good agreement with those galaxies selected as ellipticals. For the galaxies selected from the GOODS database, theoretical colors seem to match better with data than what found for the COSMOS data. Having a better morphological classificator, the selection is done by eye and by correlating a catalog of photometric and spectroscopic redshifts with a morphological one for GOODS in contrast to the selection derived from the automated pipelines used for COSMOS, is certainly discriminating in favour of the GOODS database. For both datasets our findings show that simulated colors for the different cosmological scenarios follow the general trend at lower redshifts and are in good agreement with the data up to z ~ 1, where the number of early-type galaxies observed falls abruptly. In conclusion, within the redshift range considered, all the simulated colors reproduce quite well the observational data. The dynamical and geometrical informations, derived from the numerical simulations, and the spectro-photometric properties, recovered from our tool, combined together, allow to tackle in some detail important physical issues that deal with the scaling relations governing the photometric and structural parameters of ETGs, and in particluar with the Kormendy relation that allows a comparison with observables in the luminosity-radius plane. The method we introduced for the derivation of the parameters that enter the scaling laws deals with the construction of artificial images in a bi-dimensional plane, starting from the three-dimensional structure of the simulated galaxies. By matching the population synthesis models with the three-dimensional geometric information of the galaxy's structure along with the chemical details, both provided by the N-body simulations, we create synthetic images of a galaxy in a given photometric system, from which we derive the structural and morphological parameters, such as the galaxy's effective radius and the luminosity within this, the shape indices through Fourier and Sersic analysis, color profiles, and radial profiles of most of the parameters that define the structure of galaxies. The most interesting aspect of these results is that the investigation of the simulated galaxies, via the photometric analysis of the artificial images, led us to recover properties that resemble those of observed galaxies. The results obtained in this way are studied and compared within the scaling laws, the Kormendy relation in particular, as it is the only one we can construct so far, due to the limited resolution of our simulations. The observational data with which we compare our simulated results have been selected form the SDSS database. We separate a subsample of elliptical galaxies, and our findings show that the values of luminosities and effective radii, the two parameters that compare in the Kormendy relation, measured for our model galaxies are consistent with the archivial data from the SDSS.