外紫外辐射诱导球状星团的形成
Formation of globular clusters induced by external ultraviolet radiation
摘要：
我们提出了一种新的球状星团（GC）的形成方案，其中紫外（UV）背景辐射有效地工作，从而产生紧凑的星团。在这里，我们探讨了在紫外辐射场的GC的形成。为此，我们计算了球对称下的重子和暗物质（DM）动力学，并通过求解紫外辐射的辐射转移来考虑自屏蔽效应。此外，我们规定的恒星形成冷却气体成分和追求形成的恒星的动力学。结果表明，亚银河系天体在紫外背景辐射下的演化可分为三种类型：（1）快速恒星形成，即~10^{5-8} M太阳质量较小的星云迅速自屏蔽并形成恒星;（2）延迟恒星形成，即延迟恒星形成。其中光电离质量大于10^8 M_sun的云在高温压力下坍缩，并最终以延迟的方式自我屏蔽形成恒星，（3）超音速坠落，在那里，光电离的质量较小的云~10^{5-8} M_sun以超声速下降速度收缩，并在致密核形成时自屏蔽。特别是，类型（3）是在目前的模拟中发现的一种新类型，并最终产生一个非常紧凑的星团。由此产生的质光比，半质量半径，和速度色散的三种类型的GC，矮椭球（dSphs），超紧凑矮星（UCD）的观测进行了比较。结果表明，由超音速撞击产生的星团的性质与观测到的GC的性质很好地匹配，而另外两种类型的星团则与GC不同。因此，我们得出结论，在紫外线背景下的超音速入流是一个有前途的机制，形成GC。
英文摘要：
We present a novel scenario for globular cluster (GC) formation, where the ultraviolet (UV) background radiation effectively works so as to produce compact star clusters. Here, we explore the formation of GCs in UV radiation fields. For this purpose, we calculate baryon and dark matter (DM) dynamics in spherical symmetry, incorporating the self-shielding effects by solving the radiative transfer of UV radiation. In addition, we prescribe the star formation in cooled gas components and pursue the dynamics of formed stars. As a result, we find that the evolution of subgalactic objects in UV background radiation are separated into three types, that is, (1) prompt star formation, where less massive clouds ~10^{5-8} M_sun are promptly self-shielded and undergo star formation, (2) delayed star formation, where photoionized massive clouds >10^8 M_sun collapse despite high thermal pressure and are eventually self-shielded to form stars in a delayed fashion, and (3) supersonic infall, where photoionized less massive clouds ~10^{5-8} M_sun contract with supersonic infall velocity and are self-shielded when a compact core forms. In particular, the type (3) is a novel type found in the present simulations, and eventually produces a very compact star cluster. The resultant mass-to-light ratios, half-mass radii, and velocity dispersions for the three types are compared to the observations of GCs, dwarf spheroidals (dSphs), and ultra-compact dwarfs (UCDs). It turns out that the properties of star clusters resulting from supersonic infall match well with those of observed GCs, whereas the other two types are distinct from GCs. Hence, we conclude that supersonic infall in a UV background is a promising mechanism to form GCs.
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