| In this thesis I present results obtained during my PhD studies at Nanjing University and Harvard-Smithsonian Center for Astrophysics (CfA) in the field of galaxy formation and evolution. Two of the most important findings in galaxy formation and evolution during the last decade are that:(1) galaxies form a bimodal distribution in the color-magnitude diagram (CMD);(2) there is a tight correlation between the mass of central black holes and that of galaxy bulges. These findings lead to two of the most important questions:(1) which processes turn the blue galaxies red?(2)which processes trigger active galactic nuclei (AGN) and how do they affect their host galaxies? To answer these questions, we have to trace back to high redshifts, e.g.,1.5<z<2.5, as it was then that both the cosmic star formation history and the activity of luminous AGN peaked. During the last two and a half years, I focused on the study of physical (e.g. stellar mass, star-formation rate, colors) and morphological properties of massive galaxies at1.5<z<2.5, aiming to provide direct constraints on these processes.My main results are as follows:i) We construct a sample of massive galaxies at z-2using the same criteria as those of IRAC-selected Extremely Red Objects, which leads to a virtually complete for galaxies with M>10" M(?) and1.5<z<2.5. They can be well separated into quiescent and star forming galaxies using an infrared color [3.61-[241. With the new near-infrared imaging from HST/WFC3, we show a tight correlation between star-formation status and morphologies is already formed at z-2, with most quiescent galaxies tending to be bulge-dominated while star-forming galaxies tending to be disk-dominated. A significant fraction of quiescent galaxies also show disk components, but are more concentrated than their star-forming counterparts. We thus suggest that galaxy concentration serves as a good indicator for quiescent versus star-forming galaxies. We also find that most massive star-forming galaxies are actually disks with no clear signs of interactions. ⅱ) We study color gradients of a mass-selected (M*>1010.8M⊙) galaxy sample at1.5<z<2.5. All quiescent and most star-forming galaxies present a strong color gradient:the inner regions have redder rest-frame UV-optical colors than the outer parts. iii)We present a study on AGN and their host galaxies at0.5<z<2.5in the Chandra Deep Field-South. The deepest available X-ray data from the4Ms CDF-S survey, in combination with the new HST/WFC3imaging from CANDELS, allows us to extend such study to lower X-ray luminosities and higher redshifts compared to previous studies. We reveal a significant population of high-redshift moderate-luminosity AGN that are hosted by quiescent galaxies with no significant star-formation, as confirmed by both their rest-frame colors and lack of mid-IR emission. Moreover, we find that nearly all of AGN hosted by quiescent galaxies have low radiative efficiency, parameterized by Eddington ratio with10-3<REdd<10-2. In contrast, the Eddington ratio for AGN in star-forming galaxies spans over4orders of magnitude from REdd～10-3to REdd>1, most of which have REdd～10-2.This indicates the coupling between (intense) star-formation and AGN activity is present only at the highest accretion rates. Based on their relatively low luminosities, evolved stellar populations, concentrated profiles, lack of IR emission, and low accretion rates, we argue that AGN in quiescent hosts are likely to be analogs of today's low-luminosity AGNs prevalence in early-type galaxies.Throughout this thesis, we assume Omega_Lambda=0.7, Omega_M=0.3, and Ho=70km s-1Mpc-1. All magnitudes are in the AB system unless specified otherwise, and the notation "[3.6]" means the AB magnitude at wavelength3.6um.
|
PDF Full Text Request