局域宇宙中扩展紫外线（XUV）的空间密度及其对星系气体吸积的意义
The Space Density of Extended Ultraviolet (XUV) Disks in the Local Universe and Implications for Gas Accretion on to Galaxies
摘要：
我们提出的第一个公正的搜索扩展紫外（XUV）盘星系进行，以确定这样的星系的空间密度的结果。我们的样本包含561个本地（0.001 < z < 0.05）星系，它们位于可用的GALEX深度成像（曝光时间> 1.5 x 10^4 s）和SDSS DR 7足迹的交叉点。我们探索修改我们的样本，包括磁盘和膨胀为主的星系的标准分类方案。对样本中每个星系的视觉分类揭示了我们样本中最接近的部分的XUV盘频率高达20%。在整个样品上平均（直到z=0.05），频率范围从4%到14%的硬极限。GALEX成像允许我们检测超过100 Mpc的XUV磁盘。XUV盘星系周围的XUV区域始终比主体更蓝。我们发现在明亮的红（NUV-r > 5）和绿谷（3 < NUV-r < 5）星系周围有令人惊讶的高频率的XUV辐射。局域宇宙中的XUV盘空间密度大于1.5-4.2 x 10^-3 Mpc^-3。利用XUV辐射作为近期气体吸积的指标，我们估计这些星系的冷气体吸积速率大于1.7-4.6 x 10^-3 Msun Mpc^-3 yr^-1。绿谷中的XUV盘的数量和对这些星系的吸积率的估计指出了一种有趣的可能性，即这个群体中7%-18%的星系正在从红色序列过渡。
英文摘要：
We present results of the first unbiased search for extended UV (XUV)-disk galaxies undertaken to determine the space density of such galaxies. Our sample contains 561 local (0.001 < z < 0.05) galaxies that lie in the intersection of available GALEX deep imaging (exposure time > 1.5 x 10^4 s) and SDSS DR7 footprints. We explore modifications to the standard classification scheme for our sample that includes both disk- and bulge-dominated galaxies. Visual classification of each galaxy in the sample reveals an XUV-disk frequency of up to 20% for the most nearby portion of our sample. On average over the entire sample (out to z=0.05) the frequency ranges from a hard limit of 4% to 14%. The GALEX imaging allows us to detect XUV-disks beyond 100 Mpc. The XUV regions around XUV-disk galaxies are consistently bluer than the main bodies. We find a surprisingly high frequency of XUV emission around luminous red (NUV-r > 5) and green valley (3 < NUV-r < 5) galaxies. The XUV-disk space density in the local universe is > 1.5-4.2 x 10^-3 Mpc^-3. Using the XUV emission as an indicator of recent gas accretion, we estimate that the cold gas accretion rate onto these galaxies is > 1.7-4.6 x 10^-3 Msun Mpc^-3 yr^-1. The number of XUV-disks in the green valley and the estimated accretion rate onto such galaxies points to the intriguing possibility that 7%-18% of galaxies in this population are transitioning away from the red sequence.
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