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Fabello, Silvia (2012): HI properties of massive galaxies from stacking: quenching mechanisms. Dissertation, LMU München: Faculty of Physics
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Abstract

Galaxies have been found to divide into two families: one dominated by late-type, star forming, blue objects, which are rich in cold gas and have a low stellar mass surface density (mu*); the other is made of early-type, red and passive galaxies with higher mu* and on average low gas content. The physical mechanisms responsible for the galaxy transition between the active and passive regime are still debated. In the high mass range, mechanisms proposed to quench the star formation (SF) through cold gas heating or depletion are not efficient enough to reproduce the correct red sequence of passive systems, when implemented in models of galaxy evolution. Input for a better understanding of the physics of quenching mechanisms, and of their relative importance and efficiency, can come from a comparison of the cold atomic neutral hydrogen (HI) content and SF for a statistically significant sample of massive systems where quenching is at work. However, existing surveys do not sample this high mass, gas poor regime well enough. In this work, we study the HI properties of a volume-limited sample of ~5000 nearby galaxies with stellar mass M*>10^10 Msun, selected from the state-of-the-art blind HI survey ALFALFA to have optical and ultraviolet data so that star formation and galaxy properties can be derived. As ALFALFA does not sample with sufficient sensitivity the high mass, gas poorest range, we developed a software tool to co-add its data, in order to obtain average gas properties of galaxy classes which individually may be largely undetected. Using this technique, we study three types of quenching processes, namely the presence of a bulge component, feedback from an active galactic nucleus (AGN), and environmental mechanisms acting on the interstellar medium. Simulations of early-type galaxies with non star-forming HI disks have suggested that the presence of a bulge can stabilize the gas, thus preventing star formation, but on average we do not observe this. We find that, once mu* and NUV-r colours are fixed, the HI gas fraction in massive bulge- and disk-dominated galaxies is the same. A similar negative result is obtained if we compare M_HI/M* of AGN hosts and control galaxies, despite simulations that invoke feedback from AGN to heat or deplete cold gas in massive systems. The relation we observe between the cold gas content and the accretion rate in the red population actually points towards a co-evolution of SF and AGN activity, both driven by the amount of gas available. The last class of quenching mechanisms studied in this work includes environmental processes, which are known to affect the SF properties of galaxies and, at least in rich clusters, their cold gas content. For the first time, though, we study the effect of the environment on the HI content as a continuous function of local density, comparing it with global and inner specific star formation rate. The gradual increase in the suppression of SF from the inner to the outer regions that we observe, and the even stronger HI deficiency as a function of increasing local density, can be explained by a mechanism acting on the disk from the outside-in, like ram-pressure stripping of the HI. A comparison with mock catalogs from models, which include only removal of the hot gas, shows how models underestimate environmental effects, especially on the cold gas component of galaxies. We therefore suggest that, in order to improve our understanding of the galaxy bimodality in the local Universe, observations and models should particularly focus on environmental mechanisms acting on the cold interstellar medium. These processes are efficient over a broader range of local densities than previously thought, and could solve parts of the puzzle in the formation of massive and passive systems.