Secondary evolution of galaxies investigated by N-body simulations
Coppola, Giuseppina (2008) Secondary evolution of galaxies investigated by N-body simulations. [Tesi di dottorato] (Inedito)
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In the ΛCDM cosmology, merging is one of the most important physical processes that drives the formation and evolution of galaxies. In the present work, we use N-body techniques to investigate some of currently open issues related to the formation and evolution of galaxies along the Hubble sequence (see Chapter 1). In particular, we address (i) the role of dissipation-less merging on the scaling relations and internal color gradients of early-type galaxies, by modeling these systems with two-component Sérsic models; (ii) the formation and survival of cold disks in the merging of late-type gas-rich systems, and (iii) the different merging history of galaxy types through cosmological simulations. We present new spherical, isotropic, non rotating, two-component (dark + stellar matter) models of early-type galaxies (see Chapter 2). In order to realistically describe the observed light profile of early-type systems and the shape of the mass profile of galaxy dark matter haloes predicted by recent numerical simulation results, both components of these models are described by a deprojected Sérsic law. We perform a detailed analysis of structural properties and distribution function of these models, proving that they represent physically admissible and stable systems. The free parameters of the models are derived from observational properties of early-type galaxies. We perform discrete realizations of the two-component Sérsic models, analyzing in detail how to derive an optimal softening length for the gravitational potential of these discrete systems. The models are then used to simulate dry mergers of early-type galaxy systems, by means of the N-body simulation code Gadget-2 (Springel 2005, see Chapter 3). The mergers are performed with progenitors spanning a wide range of galaxy luminosities and with a variety of initial orbital parameters. We find that dissipation-less merging preserves the Fundamental Plane relation of early-type galaxies, in agreement with previous works. However, in contrast to previous findings, we find that dissipationless merging also moves galaxies along other observed correlations, such as the Kormendy, the Faber-Jackson, and the luminosity–size relations. Hence, we conclude that all the above correlations are preserved after dissipationless encounters of early-type galaxies. For the first time, we are also able to perform a detailed analysis of how dissipationless merging affects internal stellar population gradients of ETGs. We find that the metallicity profiles, initially assigned to the merging progenitors, can be significantly flattened after the encounters. The amount of flattening is larger for low mass-ratio mergers (down to a minor-merger ratio of 1:4), and also becomes larger as the mass of the progenitors decreases. Remarkably, this allows the existence of shallow stellar population gradients in ETGs to be explained as a result of galaxy-galaxy merging. The second issue we have addressed is the possibility of rebuilding late-type systems starting from merging of disk galaxies. Recent pioneering works have shown that, in merger simulations with a significant stellar feedback, even a major merger can produce a disk-dominated remnant. These works show that a combination of strong stellar feedback (in very peculiar conditions) and a large gas content are essential ingredients to the survival of disks after a merging process. However, merger remnants result to have a large bulge component, which can likely describe only early spiral galaxy types. In contrast, our simulations show that disk formation through merging of gas rich systems might be an important ingredient of galaxy formation theories in more general conditions. Using realistic galaxy models (M33-like) whose main novelty is that of having a hot gaseous halo component (in addition to cold gas in the disk), we performed a set of hydrodynamical merger simulations. We show that mergers between these progenitors, whose baryonic component mainly consists of gas, produce late-type galaxies rather than elliptical/S0 systems. We interpret this result by the fact that gas cooling from the halo has a crucial role in producing the disk-dominated remnants. In fact, we find that gas particles in the halo have a temperature very close to the peak of the gas cooling function. Hence, the hot gas cools very rapidly after merging, acquiring angular momentum from the orbit, and settles on the final disk. Finally, we have studied the formation and evolution of different galaxy types, with the main focus of studying the properties of S0 galaxies, in N-body cosmological simulations. From a large cosmological simulation, we have selected a cluster of galaxies with size and velocity dispersion similar to the Virgo cluster, developing an original scheme to identify elliptical, S0, and spiral galaxy candidates. With this scheme, we have derived the morphology-radius relation and the velocity distribution of galaxies in the simulated cluster at redshift z = 0, and compared them to observational results. The first results presented in this work show a relatively good match of the simulated and observed morphology-radius relation, but unfortunately, we also find that our simulations suffers of the low resolution problem. The number of substructures that we included in the analysis is lower than that expected from the total abundance of Virgo cluster members. As a future work, we plan to re-do the analysis on different simulated clusters with improved mass and spatial resolution. Conclusions for each part of the present work are reported at the end of each chapter of the thesis.
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