渐近巨星分支和红巨星分支的尖端作为恒星形成历史的探针：附近的矮不规则星系KKH 98
The Asymptotic Giant Branch and the Tip of the Red Giant Branch as Probes of Star Formation History: The Nearby Dwarf Irregular Galaxy KKH 98
摘要：
我们研究了渐近巨星分支（AGB）和红巨星分支（RGB）作为探测附近（D=2.5 Mpc）矮不规则星系KKH 98的恒星形成历史（SFH）的实用性。凯克激光导星自适应光学（AO）的近红外（IR）图像分辨出592颗红外明亮恒星，亮度超过红巨星分支尖端以下的1等。哈勃太空望远镜在同一视场拍摄的更深的光学图像（F475 W和F814 W）包含了超过2500颗恒星，达到了0.5 Gyr年龄的红球和主序星。与光学色星等图（CMD）相比，近红外CMD显示出显着更紧密的AGB序列，提供了一个很好的探针的中间年龄（0.5 - 5 Gyr）人口。我们将观测到的CMD与恒星演化模型相匹配，恢复了KKH 98的SFH。平均而言，在宇宙的大部分时间里，该星系经历了相对恒定的低水平恒星形成（5 x 10^-4 Mo/年）。除了最年轻的主序星群（年龄< 0.1 Gyr），它们通常比AO数据通量极限更暗，从592颗红外亮星估计的SFH与从更大的光学数据集得出的SFH是合理的匹配。0.1 - 1陀螺人口的光学和红外衍生SFH之间的差异表明，目前的恒星演化模型可能会过度生产AGB多达三个因素在这个星系。在AO数据的深度，红外发光星并不拥挤。因此，这些技术可以潜在地用于确定星系的恒星星族在显着更远的距离。
英文摘要：
We investigate the utility of the asymptotic giant branch (AGB) and the red giant branch (RGB) as probes of the star formation history (SFH) of the nearby (D=2.5 Mpc) dwarf irregular galaxy, KKH 98. Near-infrared (IR) Keck Laser Guide Star Adaptive Optics (AO) images resolve 592 IR bright stars reaching over 1 magnitude below the Tip of the Red Giant Branch. Significantly deeper optical (F475W and F814W) Hubble Space Telescope images of the same field contain over 2500 stars, reaching to the Red Clump and the Main Sequence turn-off for 0.5 Gyr old populations. Compared to the optical color magnitude diagram (CMD), the near-IR CMD shows significantly tighter AGB sequences, providing a good probe of the intermediate age (0.5 - 5 Gyr) populations. We match observed CMDs with stellar evolution models to recover the SFH of KKH 98. On average, the galaxy has experienced relatively constant low-level star formation (5 x 10^-4 Mo yr^-1) for much of cosmic time. Except for the youngest main sequence populations (age < 0.1 Gyr), which are typically fainter than the AO data flux limit, the SFH estimated from the the 592 IR bright stars is a reasonable match to that derived from the much larger optical data set. Differences between the optical and IR derived SFHs for 0.1 - 1 Gyr populations suggest that current stellar evolution models may be over-producing the AGB by as much as a factor of three in this galaxy. At the depth of the AO data, the IR luminous stars are not crowded. Therefore these techniques can potentially be used to determine the stellar populations of galaxies at significantly further distances.
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