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X-RAY BINARIES AND GLOBULAR CLUSTERS IN ELLIPTICAL GALAXIES

机译：椭圆星系中的X射线双星和球形团簇

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The X-ray emission from normal elliptical galaxies has two major components: soft (kT ≈ 0.2-1 keV) emission from diffuse gas and harder (kT ≈ 6 keV) emission from populations of accreting (low mass) stellar X-ray binaries. If the low-mass X-ray binary (LMXB) population is intimately tied to the field stellar population in a galaxy, its aggregate X-ray luminosity is expected to be simply proportional to the optical luminosity of the galaxy. However, recent ASCA and Chandra X-ray observations show that the global luminosities of LMXB components in elliptical galaxies exhibit significant scatter (a factor of ～4) at a given optical luminosity. This scatter may reflect a range of evolutionary stages among X-ray binary populations in elliptical galaxies of different ages. If so, the ratio of the global LMXB X-ray luminosity to the galactic optical luminosity, L_(LMXB)/L_(opt), may in principle be used to determine when the bulk of stars were formed in individual elliptical galaxies. To test this we compare variations in L_(LMXB)/L_(opt), for LMXB populations in elliptical galaxies to optically derived estimates of stellar ages in the same galaxies. We find no correlation, which suggests that variations in L_(LMXB)/L_(opt) are not a good age indicator for elliptical galaxies. Alternatively, LMXBs may be formed primarily in globular clusters (through stellar tidal interactions) rather than comprise a subset of the primordial binary star population in a galactic stellar field. Since elliptical galaxies exhibit a wide range of globular cluster populations for a given galactic luminosity, this may induce a dispersion in the LMXB populations of elliptical galaxies with similar optical luminosities. Indeed, we find that L_(LMXB)/L_(opt) ratios for LMXB populations are strongly correlated with the specific globular cluster frequencies in elliptical galaxies. This suggests that most LMXBs were formed in globular clusters. If so, Chandra observations of central dominant galaxies with unusually large globular cluster populations should find proportionally excessive numbers of LMXBs.
机译：正常椭圆星系的X射线发射有两个主要成分：弥散气体的软发射（kT≈0.2-1 keV）和吸积的（低质量）恒星X射线双星群的较硬（kT≈6 keV）发射。如果低质量X射线双星（LMXB）种群与星系中的场恒星种群紧密相关，则其总X射线光度将仅与该星系的光度成正比。但是，最近的ASCA和Chandra X射线观测表明，在给定的光度下，椭圆星系中LMXB组件的整体光度显示出显着的散射（约4倍）。这种散射可能反映了不同年龄的椭圆星系中X射线双星种群中一系列演化阶段。如果这样，则总体上LMXB X射线光度与银河系光学光度的比率L_（LMXB）/ L_（opt）原则上可以用来确定何时在单个椭圆形星系中形成大量恒星。为了测试这一点，我们比较了椭圆星系中LMXB群体的L_（LMXB）/ L_（opt）变化与同一星系中光学衍生的恒星年龄估计。我们没有发现相关性，这表明L_（LMXB）/ L_（opt）的变化并不是椭圆星系的良好年龄指标。可替代地，LMXB可以主要在球状星团中形成（通过恒星-潮汐相互作用），而不是在银河系恒星场中包括原始双星群的子集。由于椭圆星系在给定的银河光度下显示出广泛的球状星团种群，因此这可能会导致在具有相似光学光度的椭圆星系的LMXB种群中产生色散。实际上，我们发现LMXB群体的L_（LMXB）/ L_（opt）比与椭圆星系中的特定球状星团频率密切相关。这表明大多数LMXB是在球状星团中形成的。如果是这样，钱德拉（Chandra）对具有异常大球状星团种群的中央优势星系的观测应该发现成比例地过量存在LMXB。

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