We examine the star formation properties of group and field galaxies in two surveys, the Sloan Digital Sky Survey (SDSS; at z ~ 0.08) and the Group Environment and Evolution Collaboration (GEEC; at z ~ 0.4). Using UV imaging from the GALEX space telescope, along with optical and, for GEEC, near infrared photometry, we compare the observed spectral energy distributions to large suites of stellar population synthesis models. This allows us to accurately determine star formation rates and stellar masses. We find that star forming galaxies of all environments undergo a systematic lowering of their star formation rate between z=0.4 and z=0.08 regardless of mass. Nonetheless, the fraction of passive galaxies is higher in groups than the field at both redshifts. Moreover, the difference between the group and field grows with time and is mass-dependent, in the sense the the difference is larger at low masses. However, the star formation properties of star forming galaxies, as measured by their average specific star formation rates, are consistent within the errors in the group and field environment at fixed redshift. The evolution of passive fraction in groups between z=0.4 and z=0 is consistent with a simple accretion model, in which galaxies are environmentally affected 3 Gyrs after falling into a ~ 10E13 Msun group. This long timescale appears to be inconsistent with the need to transform galaxies quickly enough to ensure that star forming galaxies appear similar in both the group and field, as observed.
The Rise and Fall of Acoustic Oscillations at Cosmic Dawn
New York University（纽约大学）；Johns Hopkins University（约翰斯·霍普金斯大学）；University of Texas at Austin（德克萨斯大学奥斯汀分校）；Ben-Gurion University of the Negev（内盖夫本-古里安大学）
On the Last 10 Billion Years of Stellar Mass Growth in Star-Forming Galaxies
恒星形成率-恒星质量关系（SFR-M*）及其演化（即，SFR主序）描述了给定恒星质量和给定红移的星系的增长率。假设现在的恒星形成星系在过去总是恒星形成的，这些增长率观测可以被整合来计算平均恒星形成历史（SFH）。使用这种主序列积分（MSI）方法，我们可以追溯到现在的大质量恒星形成星系，当它们是当前恒星质量的10-20%时。在这些时期的整合是强大的：支持我们的计算的SFR数据是一致的恒星质量密度在这个政权的演变。这些SFH的分析近似。基于积分的结果重申了以前的建议，即目前的恒星形成星系形成几乎所有的恒星质量在z <2. It follows that massive galaxies observed at z>2不是典型的祖先恒星形成星系今天。我们还检查基于MSI的SFH对那些从化石记录的分析推断-从光谱能量分布（SED）的恒星形成星系在斯隆数字巡天，和色星等图（CMD）的矮不规则星系解决恒星。一旦考虑到恒星星族年龄的不确定性，主序列与基于SED的SFH（来自VESPA）非常一致。外推SFR主序星观测到矮星系，我们发现的ACS附近的星系巡天金库和本星系群星系的CMD分析的MSI结果和SFH之间的差异。解析出的矮星系似乎比主序列趋势所暗示的要慢得多，也比质量稍高的SED分析星系慢。这种差异可能标志着SFH测定的问题，但它也可能标志着在最低恒星质量下恒星形成趋势的转变。
