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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>
The <i>Y</i><sup>2</sup> Isochrones Getting an Extra Dimension
Yi, Sukyoung K.,Kim, Yong-Cheol,Demarque, Pierre,Lee, Young-Wook,Han, Sang-Il,Kim, Do Gyun Cambridge University Press 2008 Proceedings of the International Astronomical Unio Vol.4 No.suppl252
<B>Abstract</B><P>The Yonsei-Yale Isochrones have been widely used since its birth in 2001. We announce a major upgrade mainly making varieties of helium values available. The recent works on the globular clusters with extreme helium abundances have called for such a need. The new version of the <I>Y</I><SUP>2</SUP> Isochrones are available for [α/<I>Fe</I>] = 0 through 0.6, Δ<I>Y</I>/Δ<I>Z</I> = 1.5 through 3.0, and extreme helium abundances (<I>Y</I> = normal 0.05, 0.1, 0.15, 0.2), and for 11 metallicity grids, with full capability of interpolation. The database will be powerful for making population models. Besides, the accuracy of the models on the lower main sequence has been substantially improved. We illustrate the major upgrades and demonstrate the power of the new grids.</P>