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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>
A WFC3 study of globular clusters in NGC 4150: an early‐type minor merger
Kaviraj, Sugata,Crockett, R. Mark,Whitmore, Bradley C.,Silk, Joseph,O’Connell, Robert W.,Windhorst, Rogier A.,Mutchler, Max,Rejkuba, Marina,Yi, Sukyoung,Frogel, Jay A.,Calzetti, Daniela Blackwell Publishing Ltd 2012 Monthly notices of the Royal Astronomical Society. Vol.422 No.1
<P><B>ABSTRACT</B></P><P>We combine near‐ultraviolet (NUV; 2250 Å) and optical (<I>U</I>, <I>B</I>, <I>V</I>, <I>I</I>) imaging from the Wide Field Camera 3 (WFC3), on‐board the <I>Hubble Space Telescope</I> (<I>HST</I>), to study the globular cluster (GC) population in NGC 4150, a sub‐L* (<I>M</I><SUB><I>B</I></SUB>∼−18.48 mag) early‐type minor‐merger remnant in the Coma I cloud. We use broad‐band NUV–optical photometry from the WFC3 to estimate individual ages, metallicities, masses and line‐of‐sight extinctions [<I>E</I>(<I>B</I>−<I>V</I>)] for 63 bright (<I>M</I><SUB><I>V</I></SUB> < −5 mag) GCs in this galaxy. In addition to a small GC population with ages greater than 10 Gyr, we find a dominant population of clusters with ages centred around 6 Gyr, consistent with the expected peak of stellar mass assembly in faint early types residing in low‐density environments. The old and intermediate‐age GCs in NGC 4150 are metal poor, with metallicities less than 0.1 Z<SUB>⊙</SUB>, and reside in regions of low extinction [<I>E</I>(<I>B</I>−<I>V</I>) < 0.05 mag]. We also find a population of young, metal‐rich (<I>Z</I> > 0.3 Z<SUB>⊙</SUB>) clusters that have formed within the last Gyr and reside in relatively dusty [<I>E</I>(<I>B</I>−<I>V</I>) > 0.3 mag] regions that are coincident with the part of the galaxy core that hosts significant recent star formation. Cluster disruption models (in which ∼80–90 per cent of objects younger than a few ×10<SUP>8</SUP> yr dissolve every dex in time) suggest that the bulk of these young clusters are a transient population.</P>