Gas And Galaxy Formation In The Local Universe

Under the cosmology of cold dark matter, there is large amount of baryons inthe halo around galaxies. Aside from the much more violent and temporary processof merger between galaxies,the inducement and suppression of star formationare the most basic process that make the picture of formation and evolution ofgalaxies. Thus the existence, accretion and expelling of gas–the material for starformation– are directly determining in the picture. This thesis aims at knowledgeabout the evolution of galaxies in the local universe, by studying the relationbetween gas content and other properties of galaxies in the local universe.In the second chapter of this thesis we study the formation and evolution ofgalaxies that have unusually high fraction of atomic gas. We analyze a sample ofmassive (M*> 10¹⁰M⊙) HI-rich galaxies in the local universe (z <0.05). I findthat at a certain stellar mass and global star fomration rate, galaxies with larger HIfractions have steeper colour gradients, in other words, they have blue outer diskscompared to the inner part of the galaxy. Theys also have larger radii in blue lightcompared to in red light. After correcting for dust attenuation, they have higherstar formation rate per stellar mass on the outside disk than on the inside region.Our results strongly support the picture of disk galaxy accreting gas and formingfrom inside-out, which is wildly used as the basis of disk galaxy formation underthe∧CDM cosmology. The lack of any intrinsic connection between HI fractionand galaxy asymmetry suggests that gas is accreted smoothly onto the outer disk.In the thrid chapter we look into the subsequent disc evolution induced by gasinflow. Here we discuss one of the major mechanism, the effect of bar structurein galaxies. Only when the bar is strong, with ellipticity greater than 0.5, doI find clear evidence that C(SF) (sSFR concentration) is larger for the barredsample than a control sample with similar global SFR. The fraction of galaxies with strong bars peaks at M*greater than 3×10¹⁰M⊙,μ*less than 3×10⁸M⊙and R₉₀/R₅₀ less than 2.5. Moreover, the strong bars preferentially occur intwo well separated regions: a high C(SF) region and a lower-than-average C(SF)region. This suggests that bars may play a role in the eventual quenching of starformation in galaxies. Only 50% of galaxies with strongly concentrated SF havestrong bars, indicating that other processes such as galaxy interactions also inducecentral star-bursts. We also find that D(bar)/D(disk) depends most strongly onthe colour of the galaxy, suggesting that the growth of the bar may be regulatedby gas accretion.Then in the fourth chapter we study the galaxies with the poorest gas content.We analysis a sample BCGs in low redshift (z < 0:1), optically-selected clusters,and a smaller sample of BCGs in X-ray selected clusters. We find that opticallyselectedBCGs exhibit smaller scatter in optical colours and redder inner NUV ??r colours than the control galaxies which have similar stellar mass, indicatingthat they are a homogenous and ancient population. The BCGs in the X-rayselected cluster sample span a similar range in optical colours, but have bluerNUV ?? r colours. Among X-ray selected BCGs, those located in clusters withcentral cooling times of less than 1 Gyr are significantly bluer than those locatedin clusters where the central gas cooling times are long. Our main conclusion isthat the location of a galaxy at the centre of its halo is not sufficient to determinewhether or not it is currently forming stars. One must also have information aboutthe thermodynamic state of the gas in the core of the halo, the cooling time scaleof gas, for example.In the fifth chapter, we introduce an on-going project about identifying thegalaxies disturbed by neighbors. The interaction between galaxy and neighbors isone important source of gas accretion as well as important mechanism of inducinggas inflow in galaxies. We develop parameters which are more sensitive to thelow surface density out region of galaxies, so can find out merging galaxies withmass ratio up to 39:1, and the effective dynamic time in the mergering processis on average longer than 0.2 Gyr. As a result, hopefully we can obtain a more complete picture of galaxy formation and evolution under the effect of externalenvironment disturbance。In the sixth chapter, we explain the role that this thesis plays in the GASSproject. We develop a photometry pipeline, a SED fitting technique for derivingstar formation rate and attenuation and a photometric predictor of atomic gasmass fraction for the GASS project. Moreover, the work in described in thesecond chapter belongs to the GASS (GALEX Arecibo SDSS Survey) project,and serves as the basis for the following WSRT (Westerbork Synthesis RadioTelescope) project proposal.We summarize and discuss about future work in the seventh chapter. In thefuture, we will analysis the influence of bar, bulge, spiral structure and externaldisturbance on the evolution of galaxies, by decomposing the image of galaxies andmaking use of the new morphological parameters that we devolop and introducein the fifth chapter.