NGC 5128中中等年龄球状星团的紫外探测
Probing the Intermediate-Age Globular Clusters in NGC 5128 from Ultraviolet Observations
摘要：
利用银河系演化探测器（GALEX）的紫外测光技术，以紫外光颜色作为年龄指标，研究了椭圆星系NGC 5128的球状星团（GC）系统的年龄分布。NGC 5128中的大部分GC在紫外-光学色图中遵循M31和银河系中GC的一般趋势，这表明NGC 5128中的大部分GC与M31和银河系中的老GC的年龄范围相似。在FUV通带中未检测到光谱鉴定的具有~ 3-8 Gyr的中间年龄GC（IAGC）候选者的大部分。考虑到在远紫外（FUV）通带中中年人群的性质是微弱的，我们认为，许多光谱鉴定的IAGCs可能是真正的中间年龄。这与M31的情况相反，M31中有很大一部分光谱学上建议的IAGCs是在FUV中检测到的，因此可能不是真正的IAGCs，而是更老的GC，具有发达的蓝色水平分支恒星。我们的紫外测光加强了先前提出的GC和恒星子群的存在与中间年龄的NGC 5128的结果。IAGCs的存在强烈表明，在高红移的恒星爆发之后，至少还发生了一次主要的恒星形成事件。
英文摘要：
We explore the age distribution of the globular cluster (GC) system of the nearby elliptical galaxy NGC 5128 using ultraviolet (UV) photometry from Galaxy Evolution Explorer (GALEX) observations, with UV - optical colors used as the age indicator. Most GCs in NGC 5128 follow the general trends of GCs in M31 and Milky Way in UV - optical color-color diagram, which indicates that the majority of GCs in NGC 5128 are old similar to the age range of old GCs in M31 and Milky Way. A large fraction of spectroscopically identified intermediate-age GC (IAGC) candidates with ~ 3-8 Gyr are not detected in the FUV passband. Considering the nature of intermediate-age populations being faint in the far-UV (FUV) passband, we suggest that many of the spectroscopically identified IAGCs may be truly intermediate in age. This is in contrast to the case of M31 where a large fraction of spectroscopically suggested IAGCs are detected in FUV and therefore may not be genuine IAGCs but rather older GCs with developed blue horizontal branch stars. Our UV photometry strengthens the results previously suggesting the presence of GC and stellar subpopulation with intermediate age in NGC 5128. The existence of IAGCs strongly indicates the occurrence of at least one more major star formation episode after a starburst at high redshift.
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