Gli assorbitori densi nel mezzo Intergalattico e l'idrogeno neutro nelle galassie

My PhD thesis focuses on the study of the connection between the densest absorbers in the Intergalactic Medium (IGM), in particular the Damped Lyman alpha systems (DLAs), and the atomic hydrogen (HI) in galaxies. As tools for my research I used different versions of the 'state-of-the-art' semi-analytic model Galaxy Evolution and Assembly (GAEA), coupled to cosmological N-body simulations. In the first part of this thesis I analyze the predictions of the GAEA model for the DLA observables in the redshift range $2<z<3$. I studied, in particular, the BR run presented in Xie et. al (2017), that adopts a partitioning scheme based on the Blitz-Rosolowsky relation, derived from observations of galaxies in the local Universe (Blitz \& Rosolowsky, 2006). I have chosen to investigate the redshift range $2<z<3$ for two reasons: first, this epoch is very important for galaxy evolution (at $z\sim 2$ we have the peak of the SFR density) and second in this redshift range we have the most complete DLA surveys and the most precise DLA measurements. I estimated the DLA column density distribution function (CDDF), the comoving HI density associated to DLAs ($\Omega^{DLA}_{\rm HI}$), their typical host halo mass, the average impact parameter and the DLA metallicity distribution. At $z<3$ our fiducial model predicts the correct shape of the CDDF, but its normalization falls short of the observations, with the discrepancy increasing at higher redshift. The agreement of the model with observations is significantly improved increasing both the HI masses and the disk radii of model galaxies by a factor 2. Haloes with $M_{200} \geq {10}^{11} M_{\odot}$ give the major contribution to $\Omega^{DLA}_{\rm HI}$, and the typical DLA host halo mass is $\sim{10}^{11} M_{\odot}$ . The simulated DLA metallicity distribution is in relatively good agreement with observations, but our model predicts an excess of DLAs at low metallicities. The results of this work suggest possible improvements for the adopted modelling of the angular momentum of galactic disks, of the filtering mass and metal ejection in low-mass haloes. In the second part of this thesis I focus on improving the modelling of HI in the GAEA model, to get a better agreement with DLA observations at $z<3$. In particular I work on a new version of the model (Xie et al. 2020), that includes a new treatment for the ram pressure stripping and implements a different modelling tracing the specific angular momentum of the gaseous disk in galaxies. We refer to this model version as RPS model. As a starting point I modified the assumed reionization history from an early to a late-reionization scenario. Then I tested different models for the filtering mass $M_F$(e.g. Macciò et al. 2010, Kim et al. 2015). The change of the reionization scenario does not affect in a significant way on the galaxy properties at the redshift of interest. The effect of the modified prescription for the filtering mass is larger and the alternative prescriptions considered bring very different results in terms of HI density evolution. Our new reference prescription for $M_F$ produces a better distribution of the HI in halos of different masses, increasing the relative contribution of intermediate/low mass haloes to the HI density, and consequently it leads to a shape of the CDDF that is more in agreement with observations. However, this new implementation does not solve the problem of the different normalization of the simulated CDDF with respect to the observed one at $z=2$. All the modifications considered change only up to $10\%$ the HI content in the model galaxies, in the redshift range of interest. Finally, I investigated the effects of adopting different prescriptions for the partitioning of cold gas into molecular and atomic. In particular I studied on three cold gas partitioning schemes that relates the HI content of a galaxy to galaxy properties.

My PhD thesis focuses on the study of the connection between the densest absorbers in the Intergalactic Medium (IGM), in particular the Damped Lyman alpha systems (DLAs), and the atomic hydrogen (HI) in galaxies. As tools for my research I used different versions of the 'state-of-the-art' semi-analytic model Galaxy Evolution and Assembly (GAEA), coupled to cosmological N-body simulations. In the first part of this thesis I analyze the predictions of the GAEA model for the DLA observables in the redshift range $2<z<3$. I studied, in particular, the BR run presented in Xie et. al (2017), that adopts a partitioning scheme based on the Blitz-Rosolowsky relation, derived from observations of galaxies in the local Universe (Blitz \& Rosolowsky, 2006). I have chosen to investigate the redshift range $2<z<3$ for two reasons: first, this epoch is very important for galaxy evolution (at $z\sim 2$ we have the peak of the SFR density) and second in this redshift range we have the most complete DLA surveys and the most precise DLA measurements. I estimated the DLA column density distribution function (CDDF), the comoving HI density associated to DLAs ($\Omega^{DLA}_{\rm HI}$), their typical host halo mass, the average impact parameter and the DLA metallicity distribution. At $z<3$ our fiducial model predicts the correct shape of the CDDF, but its normalization falls short of the observations, with the discrepancy increasing at higher redshift. The agreement of the model with observations is significantly improved increasing both the HI masses and the disk radii of model galaxies by a factor 2. Haloes with $M_{200} \geq {10}^{11} M_{\odot}$ give the major contribution to $\Omega^{DLA}_{\rm HI}$, and the typical DLA host halo mass is $\sim{10}^{11} M_{\odot}$ . The simulated DLA metallicity distribution is in relatively good agreement with observations, but our model predicts an excess of DLAs at low metallicities. The results of this work suggest possible improvements for the adopted modelling of the angular momentum of galactic disks, of the filtering mass and metal ejection in low-mass haloes. In the second part of this thesis I focus on improving the modelling of HI in the GAEA model, to get a better agreement with DLA observations at $z<3$. In particular I work on a new version of the model (Xie et al. 2020), that includes a new treatment for the ram pressure stripping and implements a different modelling tracing the specific angular momentum of the gaseous disk in galaxies. We refer to this model version as RPS model. As a starting point I modified the assumed reionization history from an early to a late-reionization scenario. Then I tested different models for the filtering mass $M_F$(e.g. Macciò et al. 2010, Kim et al. 2015). The change of the reionization scenario does not affect in a significant way on the galaxy properties at the redshift of interest. The effect of the modified prescription for the filtering mass is larger and the alternative prescriptions considered bring very different results in terms of HI density evolution. Our new reference prescription for $M_F$ produces a better distribution of the HI in halos of different masses, increasing the relative contribution of intermediate/low mass haloes to the HI density, and consequently it leads to a shape of the CDDF that is more in agreement with observations. However, this new implementation does not solve the problem of the different normalization of the simulated CDDF with respect to the observed one at $z=2$. All the modifications considered change only up to $10\%$ the HI content in the model galaxies, in the redshift range of interest. Finally, I investigated the effects of adopting different prescriptions for the partitioning of cold gas into molecular and atomic. In particular I studied three cold gas partitioning schemes that relates the HI content of a galaxy to galaxy properties.