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Rutkowski, Michael J.,Jeong, Hyunjin,Cohen, Seth H.,Kaviraj, Sugata,Windhorst, Rogier A.,Ryan Jr., Russell E.,Koekemoer, Anton,Yi, Sukyoung K.,Hathi, Nimish P.,Dopita, Michael A. IOP Publishing 2014 The Astrophysical journal Vol.796 No.2
<P>We present an analysis of the stellar populations of 102 visually selected early-type galaxies (ETGs) with spectroscopic redshifts (0.35 less than or similar to z less than or similar to 1.5) from observations in the Early Release Science program with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). We fit one-and two-component synthetic stellar models to the ETGs UV-optical-near-IR spectral energy distributions and find that a large fraction (similar to 40%) are likely to have experienced a minor (f(YC) less than or similar to 10% of stellar mass) burst of recent (t(YC) less than or similar to 1 Gyr) star formation. The measured age and mass fraction of the young stellar populations do not strongly trend with measurements of galaxy morphology. We note that massive (M > 10(10.5) M-circle dot) recent star-forming ETGs appear to have larger sizes. Furthermore, high-mass, quiescent ETGs identified with likely companions populate a distinct region in the size-mass parameter space, in comparison with the distribution of massive ETGs with evidence of recent star formation (RSF). We conclude that both mechanisms of quenching star formation in disk-like ETGs and (gas-rich, minor) merger activity contribute to the formation of young stars and the size-mass evolution of intermediate redshift ETGs. The number of ETGs for which we have both HST WFC3 panchromatic (especially UV) imaging and spectroscopically confirmed redshifts is relatively small, therefore, a conclusion about the relative roles of both of these mechanisms remains an open question.</P>
Mantha, Kameswara Bharadwaj,McIntosh, Daniel H,Brennan, Ryan,Ferguson, Henry C,Kodra, Dritan,Newman, Jeffrey A,Rafelski, Marc,Somerville, Rachel S,Conselice, Christopher J,Cook, Joshua S,Hathi, Nimish Oxford University Press 2018 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.475 No.2
<P>The rate of major galaxy-galaxy merging is theoretically predicted to steadily increase with redshift during the peak epoch of massive galaxy development (1 <= z <= 3). We use close-pair statistics to objectively study the incidence of massive galaxies (stellar M1 > 2 x 10(10)M(circle dot)) hosting major companions (1 <= M-1/M-2 <= 4; i.e. <4: 1) at six epochs spanning 0 < z < 3. We select companions from a nearly complete, mass-limited (>= 5 x 10(9)M(circle dot)) sample of 23 696 galaxies in the five Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey fields and the Sloan Digital Sky Survey. Using 5-50 kpc projected separation and close redshift proximity criteria, we find that the major companion fraction f(mc)(z) based on stellar mass-ratio (MR) selection increases from 6 per cent (z similar to 0) to 16 per cent (z similar to 0.8), then turns over at z similar to 1 and decreases to 7 per cent (z similar to 3). Instead, if we use a major F160W flux-ratio (FR) selection, we find that f(mc)(z) increases steadily until z similar to 3 owing to increasing contamination from minor (MR > 4: 1) companions at z > 1. We show that these evolutionary trends are statistically robust to changes in companion proximity. We find disagreements between published results are resolved when selection criteria are closely matched. If we compute merger rates using constant fraction-to-rate conversion factors (C-merg,C-pair = 0.6 and T-obs,T-pair = 0.65 Gyr), we find that MR rates disagree with theoretical predictions at z > 1.5. Instead, if we use an evolving T-obs,T-pair(z) alpha (1 + z)- 2 from Snyder et al., our MR-based rates agree with theory at 0 < z < 3. Our analysis underscores the need for detailed calibration of C-merg,C-pair and T-obs,T-pair as a function of redshift, mass, and companion selection criteria to better constrain the empirical major merger history.</P>