And XVIII的恒星形成历史：一个孤立的矮球状星系
Star formation history of And XVIII: a dwarf spheroidal galaxy in isolation
摘要：
我们提出了一个光度研究仙女座XVIII矮球状星系与M31，并位于很好的外的维里半径的M31晕。这个星系被哈勃太空望远镜/高级巡天相机分解成恒星，揭示了古老的红巨星分支和红色团块。根据新的观测数据，我们用红巨星分支的尖端方法确定仙女座XVIII的距离为D = 1.33+-0.08 Mpc。因此，这颗矮星位于距离M31 579 kpc的地方。我们从恒星光度学和帕多瓦理论恒星等时线模拟了仙女座XVIII的恒星形成历史。一个古老的恒星形成爆发发生在12-14 Gyr前。在过去的1.5 Gyr中没有最近/正在形成的恒星的迹象。古老恒星和中等年龄恒星的质量分数分别为34%和66%，恒星总质量为4.2x10^6 Msun。这个星系很可能在过去没有经历过与M31的相互作用。我们还讨论了dSphs KKR 25，KKs 03，以及dTr KK 258的恒星形成过程。它们的恒星形成历史是由我们从HST/ACS观测中统一测量的。所有的星系都位于本星系群之外，两个dSphs KKR 25和KKs 03非常孤立。很明显，这些天体的演化没有受到邻近天体的影响。
英文摘要：
We present a photometric study of the Andromeda XVIII dwarf spheroidal galaxy associated with M31, and situated well outside of the virial radius of the M31 halo. The galaxy was resolved into stars with Hubble Space Telescope/Advanced Camera for Surveys revealing the old red giant branch and red clump. With the new observational data we determined the Andromeda XVIII distance to be D = 1.33+-0.08 Mpc using the tip of red giant branch method. Thus, the dwarf is situated at the distance of 579 kpc from M31. We model the star formation history of Andromeda XVIII from the stellar photometry and Padova theoretical stellar isochrones. An ancient burst of star formation occurred 12-14 Gyr ago. There is no sign of recent/ongoing star formation in the last 1.5 Gyr. The mass fractions of the ancient and intermediate age stars are 34 and 66 per cent, respectively, and the total stellar mass is 4.2x10^6 Msun. It is probable that the galaxy has not experienced an interaction with M31 in the past. We also discuss star formation processes of dSphs KKR 25, KKs 03, as well as dTr KK 258. Their star formation histories were uniformly measured by us from HST/ACS observations. All the galaxies are situated well beyond the Local Group and the two dSphs KKR 25 and KKs 03 are extremely isolated. Evidently, the evolution of these objects has proceeded without influence of neighbours.
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