The majority of clusters in the Universe have masses well below 10^5 Msun. Hence their integrated fluxes and colors can be affected by the random presence of a few bright stars introduced by stochastic sampling of the stellar mass function. Specific methods are being developed to extend the analysis of cluster SEDs into the low-mass regime. In this paper, we apply such a method to observations of star clusters, in the nearby spiral galaxy M83. We reassess ages and masses of a sample of 1242 objects for which UBVIHalpha fluxes were obtained with the HST/WFC3 images. Synthetic clusters with known properties are used to characterize the limitations of the method. The ensemble of color predictions of the discrete cluster models are in good agreement with the distribution of observed colors. We emphasize the important role of the Halpha data in the assessment of the fraction of young objects, particularly in breaking the age-extinction degeneracy that hampers an analysis based on UBVI only. We find the mass distribution of the cluster sample to follow a power-law of index -2.1 +/-0.2, and the distribution of ages a power-law of index -1.0 +/-0.2 for M > 10^3.5 Msun and ages between 10^7 and 10^9 yr. An extension of our main method, that makes full use of the probability distributions of age and mass of the individual clusters, is explored. It produces similar power-law slopes and will deserve further investigation. Although the properties derived for individual clusters significantly differ from those obtained with traditional, non-stochastic models in ~30% of the objects, the first order aspect of the age and mass distributions are similar to those obtained previously for this M83 sample in the range of overlap of the studies. We extend the power-law description to lower masses with better mass and age resolution and without most of the artifacts produced by the classical method.
Massive and Newly Dead: Discovery of a Significant Population of Galaxies with High Velocity Dispersions and Strong Balmer Lines at z~1.5 from Deep Keck Spectra and HST/WFC3 Imaging
我们提出了深Keck/LRIS光谱和HST/WFC 3成像在静止坐标系光学的8个星系的样品在z~1.5与高光度测定的恒星质量。这些数据与van de Sande等人（2011年，2012年待提交）的五个星系的VLT/XShooter光谱相结合。我们发现，这13个星系具有高速度色散，其中值为sigma=301 km s^{-1}。这个高值与它们相对较高的恒星质量和紧凑的尺寸相一致。我们研究了它们的恒星星族使用的巴耳末吸收线，这是不敏感的尘埃吸收的强度。我们发现巴尔末吸收强度的范围很大，许多星系显示出非常强烈的线条，表明年轻的年龄。直接确定或从H10线推断的中值Hdelta_A等效宽度为5.4埃，表明亮度加权年龄约为1 Gyr。虽然由于选择效应，这个值可能偏向于更高的值，但具有如此年轻年龄的高色散星系在本地宇宙中非常罕见。有趣的是，我们没有发现静止坐标系U-V色的简单相关性：一些最红的星系具有非常强的巴耳末吸收线。这些结果表明，许多z~1.5的高色散星系最近被淬灭。这意味着在z~2处一定存在着一群具有高速色散或线宽的恒星形成祖先，而这在CO/H α选择巡天中是众所周知的。
HST/WFC3 Near-Infrared spectroscopy of quenched galaxies at z~1.5 from the WISP Survey: Stellar population properties
我们将哈勃空间望远镜（HST）G102和G141近红外棱纹光谱与HST/WFC 3-UVIS、HST/WFC 3-IR和Spitzer/IRAC[3.6μm]测光相结合，组装了一个z~1.5的质量（log（M_star/M_sun）~ 11）淬灭星系样本。我们的41个星系的样本是最大的G102+G141近红外光谱淬灭源在这些红移。与局域宇宙相比，z~1.5淬灭星系在高质量范围内具有广泛的恒星星族性质。我们发现它们的SEDs很好地拟合了指数下降的SFH，和短的恒星形成时间尺度（τ<100 Myr）。淬火星系的年龄分布也很广，在1- 4 Gyr之间。在（u-r）_0-versus-mass空间中，淬火星系在给定质量下的静止坐标系颜色有很大的扩展。大多数猝灭星系都分布在z~1.5的红色序列（RS）中，但其中很大一部分（32%）的颜色更蓝。虽然有很大的扩展，我们发现RS上的淬灭星系的年龄中位数（3.1 Gyr）比RS外的淬灭星系（1.5 Gyr）要大。我们还表明，一个振兴的SED不能再现所观察到的堆叠光谱（蓝色）淬火星系关闭RS。我们推导出z~1.5时大质量星系在RS上所占比例的上限为<43%. We speculate that the young quenched galaxies OFF the RS are in a transition phase between vigorou…
