超紧凑矮星的普遍性和双重性
The Ubiquity and Dual Nature of Ultra Compact Dwarfs
摘要：
我们提出了几个超紧凑型矮星（UCD）位于字段/组环境中的发现。这些对象的检查，加上文学对象，证实了两个不同的形成渠道的UCD的存在。我们发现，我们已经发现的UCD组椭圆NGC 3923（和UCD一般）的属性一致，它们是最明亮的成员的主星系的球状星团（GC）系统。我们将这种类型的UCD描述为巨型GC（GGC）。相比之下，我们发现的与孤立的S 0 NGC 4546相关的UCD显然是有核伴星系剥离的结果。UCD的年轻年龄（~3.4 Gyr），缺乏相应的年轻GC人口，UCD的动力学摩擦衰减时标明显较短（~0.5 Gyr），以及宿主星系中存在一个反向旋转的气体盘（与UCD共同旋转），这些都表明UCD是NGC 4546最近剥离伴星后剩余的释放核。我们提出了一个统一的GC，UCD和星系核的形成的一般计划。在这幅图中，“正常”的GC是一个复合的群体，由原位形成的GC，从吸积星系获得的GC和伪装成GC的低质量剥离矮核组成。在2 × 10^6 Msun（Mv ~ -10）的“标度起始质量”之上，UCD出现了质量-尺寸关系和可能的质量-金属丰度关系（“蓝色倾斜”）。在质量范围高达7 × 10^7 Msun（Mv ~ -13）的UCD中，包含GGC和剥离核的复合粒子群。在7 × 10^7 Msun以上，UCD必须几乎完全是剥离的核，因为不存在足够丰富的GC系统来填充GCLF的这种极端。
英文摘要：
We present the discovery of several Ultra Compact Dwarfs (UCDs) located in field/group environments. Examination of these objects, plus literature objects, confirms the existence of two distinct formation channels for UCDs. We find that the UCDs we have discovered around the group elliptical NGC3923 (and UCDs generally) have properties consistent with their being the most luminous members of the host galaxy's globular cluster (GC) system. We describe UCDs of this type as giant GCs (GGCs). In contrast, the UCD we have found associated with the isolated S0 NGC4546 is clearly the result of the stripping of a nucleated companion galaxy. The young age (~3.4 Gyr) of the UCD, the lack of a correspondingly young GC population, the apparently short dynamical friction decay timescale (~0.5 Gyr) of the UCD, and the presence of a counterrotating gas disc in the host galaxy (co-rotating with the UCD) together suggest that this UCD is the liberated nucleus remaining after the recent stripping of a companion by NGC4546. We suggest a general scheme that unifies the formation of GCs, UCDs, and galaxy nuclei. In this picture "normal" GCs are a composite population, composed of GCs formed in situ, GCs acquired from accreted galaxies, and a population of lower mass stripped dwarf nuclei masquerading as GCs. Above a "scaling onset mass" of 2x10^6 Msun (Mv ~ -10), UCDs emerge together with a mass-size relation and a likely mass-metallicity relation (the "blue tilt"). In the mass range up to 7x10^7 Msun (Mv ~ -13) UCDs comprise a composite population of GGCs and stripped nuclei. Above 7x10^7 Msun, UCDs must be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist to populate such an extreme of the GCLF.
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