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Schawinski, Kevin,Treister, Ezequiel,Urry, C. Megan,Cardamone, Carolin N.,Simmons, Brooke,Yi, Sukyoung K. IOP Publishing 2011 ASTROPHYSICAL JOURNAL LETTERS - Vol.727 No.2
<P>We present the rest-frame optical morphologies of active galactic nucleus (AGN) host galaxies at 1.5 < z < 3, using near-infrared imaging from the Hubble Space Telescope Wide Field Camera 3, the first such study of AGN host galaxies at these redshifts. The AGNs are X-ray-selected from the Chandra Deep Field South and have typical luminosities of 10(42) erg s(-1)< L-X < 10(44) erg s(-1). Accreting black holes in this luminosity and redshift range account for a substantial fraction of the total space density and black hole mass growth over cosmic time; they thus represent an important mode of black hole growth in the universe. We find that the majority (similar to 80%) of the host galaxies of these AGNs have low Sersic indices indicative of disk-dominated light profiles, suggesting that secular processes govern a significant fraction of the cosmic growth of black holes. That is, many black holes in the present-day universe grew much of their mass in disk-dominated galaxies and not in early-type galaxies or major mergers. The properties of the AGN host galaxies are furthermore indistinguishable from their parent galaxy population and we find no strong evolution in either effective radii or morphological mix between z similar to 2 and z similar to 0.05.</P>
RECENT GALAXY MERGERS AND RESIDUAL STAR FORMATION OF RED SEQUENCE GALAXIES IN GALAXY CLUSTERS
Sheen, Yun-Kyeong,Yi, Sukyoung K.,Ree, Chang H.,Jaffé,, Yara,Demarco, Ricardo,Treister, Ezequiel American Astronomical Society 2016 The Astrophysical journal Vol.827 No.1
<P>This study explored the Galaxy Evolution Explorer ultraviolet (UV) properties of optical red sequence galaxies in four rich Abell clusters at z <= 0.1. In particular, we tried to find a hint of merger-induced recent star formation (RSF) in red sequence galaxies. Using the NUV - r' colors of the galaxies, RSF fractions were derived based on various criteria for post-merger galaxies and normal galaxies. Following k-correction, about 36% of the post-merger galaxies were classified as RSF galaxies with a conservative criterion (NUV - r' <= 5), and that number was doubled (similar to 72%) when using a generous criterion (NUV - r' <= 5.4). The trend was the same when we restricted the sample to galaxies within 0.5 x R-200. Post-merger galaxies with strong UV emission showed more violent, asymmetric features in the deep optical images. The RSF fractions did not show any trend along the clustocentric distance within R-200. We performed a Dressler-Shectman test to check whether the RSF galaxies had any correlation with the substructures in the galaxy clusters. Within R-200 of each cluster, the RSF galaxies did not appear to be preferentially related to the clusters' substructures. Our results suggested that only 30% of RSF red sequence galaxies show morphological hints of recent galaxy mergers. This implies that internal processes (e.g., stellar mass loss or hot gas cooling) for the supply of cold gas to early-type galaxies may play a significant role in the residual star formation of early-type galaxies at a recent epoch.</P>