A Wide Field Planetary Camera 2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

The stellar population of the Pegasus dwarf irregular galaxy is investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. With WFPC2 the Pegasus dwarf is highly resolved into individual stars to limiting magnitudes of about 25.5 in B and V and 25 in I. These and ground-based data are combined to produce color-magnitude diagrams that show the complex nature of the stellar population in this small galaxy. A young (<0.5 Gyr) main-sequence stellar component is present and clustered in two centrally located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of AV = 0.47 mag. The mean color of the well-populated red giant branch (RGB) is relatively blue, consistent with a moderate-metallicity young, or older metal-poor, stellar population. The RGB also has significant width in color, implying a range of stellar ages and/or metallicities. A small number of extended asymptotic giant branch stars are found beyond the RGB tip. Near the faint limits of our data is a populous red clump superposed on the RGB. Efforts to fit self-consistent stellar population models based on the Geneva stellar evolutionary tracks yield a revised distance of 760 kpc. Quantitative fits to the stellar population are explored as a means of constraining the star formation history. The numbers of main-sequence and core helium burning blue-loop stars require that the star formation rate was higher in the recent past, by a factor of 3–4 about 1 Gyr ago. Unique results cannot be obtained for the star formation history over longer time baselines without better information on stellar metallicities and deeper photometry. The youngest model consistent with the data contains stars with constant metallicity of Z = 0.001 that mainly formed 2–4 Gyr ago. If stellar metallicity declines with increasing stellar age, then older ages are allowed of up to ≈8 Gyr. However, even at its peak of star-forming activity, the intermediate-age–dominated model for the Pegasus dwarf most likely remained relatively dim, with MV ≈ -14.

利用哈勃空间望远镜上的第二代广域行星相机（WFPC2）在F439W（B）、F555W（V）和F814W（I）波段拍摄的图像，对飞马座不规则矮星系的恒星族进行了研究。借助WFPC2，飞马座矮星系在B和V波段可高度分辨出单个恒星，极限星等约为25.5，在I波段约为25。将这些数据与地面观测数据相结合，生成了色 - 星等图，该图显示了这个小星系中恒星族的复杂性质。存在一个年轻（<0.5亿年）的主序星成分，聚集在位于中心的两个星团中，而较老的恒星则形成一个更广泛的盘状或晕状结构。主序星的颜色表明消光相对较大，AV = 0.47等。人口众多的红巨星支（RGB）的平均颜色相对较蓝，这与中等金属丰度的年轻恒星族或更老的贫金属恒星族相符。RGB在颜色上也有明显的宽度，这意味着恒星年龄和/或金属丰度存在一定范围。在RGB顶端之外发现了少量延伸的渐近巨星支恒星。在我们的数据接近暗弱极限处，有一个大量的红团叠加在RGB上。基于日内瓦恒星演化轨迹拟合自洽恒星族模型的努力得出了760千秒差距的修正距离。探索对恒星族进行定量拟合，作为约束恒星形成历史的一种手段。主序星和核心氦燃烧蓝环星的数量表明，在近期恒星形成率较高，大约10亿年前高出3 - 4倍。如果没有关于恒星金属丰度的更好信息和更深的测光数据，就无法在更长的时间基线上获得恒星形成历史的独特结果。与数据相符的最年轻模型包含金属丰度Z = 0.001恒定的恒星，它们主要形成于2 - 40亿年前。如果恒星金属丰度随着恒星年龄的增加而降低，那么允许的年龄可高达约80亿年前。然而，即使在恒星形成活动的高峰期，以中等年龄为主的飞马座矮星系模型很可能仍然相对较暗，MV≈ -14。