We present deep Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) optical observations obtained as part of the ACS Nearby Galaxy Survey Treasury (ANGST) as well as early release Wide Field Camera 3 (WFC3) ultra-violet and infrared observations of the nearby dwarf starbursting galaxy NGC 4214. Our data provide a detailed example of how covering such a broad range in wavelength provides a powerful tool for constraining the physical properties of stellar populations. The deepest data reach the ancient red clump at M_F814W -0.2. All of the optical data reach the main sequence turnoff for stars younger than ~300 Myr, and the blue He burning sequence for stars younger than 500 Myr. The full CMD-fitting analysis shows that all three fields in our data set are consistent with ~75% of the stellar mass being older than 8 Gyr, in spite of showing a wide range in star formation rates at the present day. Thus, our results suggest that the scale length of NGC 4214 has remained relatively constant for many Gyr. As previously noted by others, we also find the galaxy has recently ramped up production, consistent with its bright UV luminosity and its population of UV-bright massive stars. In the central field we find UV point sources with F336W magnitudes as bright as -9.9. These are as bright as stars with masses of at least 52-56 M_sun and ages near 4 Myr in stellar evolution models. Assuming a standard IMF, our CMD is well-fitted by an increase in star formation rate beginning 100 Myr ago. The stellar populations of this late-type dwarf are compared with those of NGC 404, an early-type dwarf that is also the most massive galaxy in its local environment. The late-type dwarf appears to have a similar high fraction of ancient stars, suggesting that these dominant galaxies may form at early epochs even if they have low total mass and very different present-day morphologies.
The ACS Fornax Cluster Survey. VIII. The Luminosity Function of Globular Clusters in Virgo and Fornax Early-Type Galaxies and its Use as a Distance Indicator
Direct Observation of Cosmic Strings via their Strong Gravitational Lensing Effect: II. Results from the HST/ACS Image Archive
我们已经搜索了4.5平方度的HST/ACS档案图像，寻找宇宙弦，识别出相似的、微弱的星系对，并选择了那些排列与引力透镜一致的星系群。我们在五次大面积的HST财政调查（覆盖总面积为2.22平方度）中没有发现宇宙弦的证据，或者在346张多过滤器的客座观察者图像（1.18平方度）中没有发现宇宙弦的证据。假设模拟精确地预测了宇宙中宇宙弦的数量，这种未探测到的情况使我们能够在95%的置信水平下（在每种情况下，相对于另一个参数，边缘化）将无单位的普适宇宙弦张力的上限定为G μ/c^2 < 2.3 x 10^-6，宇宙弦密度的上限定为Omega_s < 2.1 x 10^-5。我们在318张单过滤器访客观测者图像（1.08平方度）中发现了四个可疑的宇宙弦候选者，我们无法用现有数据最终消除它们。这些候选者中的任何一个被确认为宇宙弦都意味着G μ/c^2 ~ 10^-6和Ω_s ~ 10^-5。然而，我们估计至少有92%的机会，这些字符串候选人是随机排列的星系。如果我们假设这些候选者确实是错误的检测，那么我们对G μ/c^2和Ω_s的最终限制将降到6.5 x 10^-7和7.3 x 10^-6。由于HST/ACS图像存档的天空覆盖范围很广，因此上述限制是通用的。它们对搜索的字段数量非常敏感，并且可以使用即将到来的HST数据进一步减少两倍以上。
全色哈勃仙女座宝库（PHAT）是一个正在进行的哈勃望远镜多光谱宝库计划，用6个滤光片拍摄M31恒星形成盘的1/3，从紫外到近红外。完整的调查将解决银河系超过1亿颗恒星，投影半径从0-20千秒差距在一个连续的0.5平方度的区域在828轨道，产生成像的F275 W和F336 W过滤器与WFC 3/UVIS，F475 W和F814 W与ACS/WFC，F110 W和F160 W与WFC 3/IR。由此产生的波长覆盖范围提供了良好的约束恒星温度，辐射光度，和消光大多数光谱类型。测光在F275 W =25.1、F336 W =24.9、F475 W =27.9、F814 W =27.1、F110 W =25.5和F160 W =24.6处达到SNR=4;然而，光学和近红外数据是有限的，最深的可靠拥挤度在内部隆起处高达5等。所有点都是抖动的，并在F475 W、F814 W和F160 W中产生奈奎斯特采样图像。我们描述的观测策略，测光，天体测量，和数据产品，以及广泛的测试光度稳定性，拥挤误差，空间依赖的光度偏差，和望远镜指向控制。我们报告的初始适合M31的磁盘的结构，来自RGB恒星的密度，在某种程度上是独立的假设M/L和强大的尘埃消光的变化。这些拟合还表明，10 kpc环不仅仅是一个增强的近期恒星形成区域，而是一个包含年龄>1 Gyr的恒星的显着过密度的动力学结构。（节录）
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。
