Formation and evolution of massive early-type galaxies at high redshift

In this thesis I present the results obtained during my PhD studies at SISSA in the field of galaxy formation, with focus on the problem of formation of Early Type Galaxies (ETGs) at high redshift. The interest to this issue has been increasing with time, since in the last decade observational evidence brought the idea that the formation and evolution of quasars is strictly related to that of ETGs. In order to tackle the galaxy formation most of the efforts has been put in developing semi-analytical and numerical codes based on the assumption that the hierarchy of galaxy Dark Matter halos traces the history also of baryons (stars and gas) through continuous merging. On the other hand, the group in SISSA and in Padua Observatory put forward the concept that the galaxy and quasar phenomenology at high redshift is related chiefly to physical processes (such as cooling, collapse, star formation, accretion onto the central Black Hole seed and feedback) occurring in the gas associated to the phase of fast collapse/merging of the central 30-40% of the mass of DM halos, while the later phase of slow accretion is little affecting the evolution of stars and quasars

The SCUBA-2 850 μm\mu m follow-up of WISE-selected, luminous dust-obscured quasars

Hot dust-obscured galaxies (Hot DOGs) are a new population recently discovered in the \wise All-Sky survey. Multiwavelength follow-up observations suggest that they are luminous, dust-obscured quasars at high redshift. Here we present the JCMT SCUBA-2 850 $\mu m$ follow-up observations of 10 Hot DOGs. Four out of ten Hot DOGs have been detected at $>3\sigma$ level. Based on the IR SED decomposition approach, we derive the IR luminosities of AGN torus and cold dust components. Hot DOGs in our sample are extremely luminous with most of them having $L_{\rm IR}^{\rm tot}>10^{14} L_\odot$. The torus emissions dominate the total IR energy output. However, the cold dust contribution is still non-negligible, with the fraction of the cold dust contribution to the total IR luminosity $(\sim 8-24\%)$ being dependent on the choice of torus model. The derived cold dust temperatures in Hot DOGs are comparable to those in UV bright quasars with similar IR luminosity, but much higher than those in SMGs. Higher dust temperatures in Hot DOGs may be due to the more intense radiation field caused by intense starburst and obscured AGN activities. Fourteen and five submillimeter serendipitous sources in the 10 SCUBA-2 fields around Hot DOGs have been detected at $>3\sigma$ and $>3.5\sigma$ levels, respectively. By estimating their cumulative number counts, we confirm the previous argument that Hot DOGs lie in dense environments. Our results support the scenario in which Hot DOGs are luminous, dust-obscured quasars lying in dense environments, and being in the transition phase between extreme starburst and UV-bright quasars.Comment: 26 pages, 7 figures, PASP accepte

From outside-in to inside-out: galaxy assembly mode depends on stellar mass

In this Letter, we investigate how galaxy mass assembly mode depends on stellar mass $M_{\ast}$, using a large sample of $\sim$10, 000 low redshift galaxies. Our galaxy sample is selected to have SDSS R_{90}>5\arcsec.0, which allows the measures of both the integrated and the central NUV$-r$ color indices. We find that: in the $M_{\ast}-($ NUV$-r$) green valley, the M_{\ast}<10^{10}~M_{\sun} galaxies mostly have positive or flat color gradients, while most of the M_{\ast}>10^{10.5}~M_{\sun} galaxies have negative color gradients. When their central $D_{n}4000$ index values exceed 1.6, the M_{\ast}<10^{10.0}~M_{\sun} galaxies have moved to the UV red sequence, whereas a large fraction of the M_{\ast}>10^{10.5}~M_{\sun} galaxies still lie on the UV blue cloud or the green valley region. We conclude that the main galaxy assembly mode is transiting from "the outside-in" mode to "the inside-out" mode at M_{\ast} 10^{10.5}~M_{\sun}. We argue that the physical origin of this is the compromise between the internal and the external process that driving the star formation quenching in galaxies. These results can be checked with the upcoming large data produced by the on-going IFS survey projects, such as CALIFA, MaNGA and SAMI in the near future.Comment: Accepted for publication in ApJL,6 pages, 5 figure

Structure and morphology of X-ray selected AGN hosts at 1<z<3 in CANDELS-COSMOS field

We analyze morphologies of the host galaxies of 35 X-ray selected active galactic nucleus (AGNs) at $z\sim2$ in the Cosmic Evolution Survey (COSMOS) field using Hubble Space Telescope/WFC3 imaging taken from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We build a control sample of 350 galaxies in total, by selecting ten non-active galaxies drawn from the same field with the similar stellar mass and redshift for each AGN host. By performing two dimensional fitting with GALFIT on the surface brightness profile, we find that the distribution of S$\`e$rsic index (n) of AGN hosts does not show a statistical difference from that of the control sample. We measure the nonparametric morphological parameters (the asymmetry index A, the Gini coefficient G, the concentration index C and the M20 index) based on point source subtracted images. All the distributions of these morphological parameters of AGN hosts are consistent with those of the control sample. We finally investigate the fraction of distorted morphologies in both samples by visual classification. Only $\sim$15% of the AGN hosts have highly distorted morphologies, possibly due to a major merger or interaction. We find there is no significant difference in the distortion fractions between the AGN host sample and control sample. We conclude that the morphologies of X-ray selected AGN hosts are similar to those of nonactive galaxies and most AGN activity is not triggered by major merger.Comment: 5 pages, 3 figures, accepted for publication in The Astrophysical Journal Letter