通过直接坍缩形成超大质量黑洞：维里温度T_vir >~ 10^4 K的暗物质晕中保持原星系气体H_2-游离
Supermassive Black Hole Formation by Direct Collapse: Keeping Protogalactic Gas H_2--Free in Dark Matter Halos with Virial Temperatures T_vir >~ 10^4 K
摘要：
在不存在H_2分子的情况下，早期暗物质晕中的原始气体（维里温度略高于T_vir >~ 10^4 K）通过H原子的碰撞激发而冷却。虽然它有效地冷却，但这种气体仍然相对较热，温度接近T ~ 8000 K，因此可能能够避免碎裂并直接坍缩成超大质量黑洞（SMBH）。为了有效抑制H_2-的生成和冷却，必须用足够强的紫外光照射气体。我们对T_vir >~ 10^4 K的三种不同的原星系晕在不同强度和光谱的紫外光照射下的气体坍缩进行了三维流体动力学自适应网格加密（AMR）模拟。我们确定了抑制H_2冷却所需的临界比强度Jcrit。对于代表无金属恒星的硬光谱，我们发现（以10^{-21}尔格·s^{-1} Hz^{-1} sr^{-1} cm^{-2}为单位）10 ^4 <Jcrit<10 ^5，而对于一个较软的光谱，这是正常恒星星族的特征，H^{-} -解离对它很重要，我们发现30<Jcrit<300。这些数值比以前的估计数低3- 10倍。我们将这种差异归因于我们采用了更高、更精确的H_2碰撞解离速率。Jcrit的减少指数地增加了暴露在超临界辐射下的稀有晕的数量。当H_2冷却被抑制时，气体坍缩开始有一个延迟，但最终进行得更快。当H_2冷却有效时，质量为太阳质量M ~ 10^2的恒星会形成，而当H_2冷却有效时，质量为太阳质量M ~ 10^5的恒星会形成。
英文摘要：
In the absence of H_2 molecules, the primordial gas in early dark matter halos with virial temperatures just above T_vir >~ 10^4 K cools by collisional excitation of atomic H. Although it cools efficiently, this gas remains relatively hot, at a temperature near T ~ 8000 K, and consequently might be able to avoid fragmentation and collapse directly into a supermassive black hole (SMBH). In order for H_2--formation and cooling to be strongly suppressed, the gas must be irradiated by a sufficiently intense ultraviolet (UV) flux. We performed a suite of three--dimensional hydrodynamical adaptive mesh refinement (AMR) simulations of gas collapse in three different protogalactic halos with T_vir >~ 10^4 K, irradiated by a UV flux with various intensities and spectra. We determined the critical specific intensity, Jcrit, required to suppress H_2 cooling in each of the three halos. For a hard spectrum representative of metal--free stars, we find (in units of 10^{-21} erg s^{-1} Hz^{-1} sr^{-1} cm^{-2}) 10^4<Jcrit<10^5, while for a softer spectrum, which is characteristic of a normal stellar population, and for which H^{-} --dissociation is important, we find 30<Jcrit<300. These values are a factor of 3--10 lower than previous estimates. We attribute the difference to the higher, more accurate H_2 collisional dissociation rate we adopted. The reduction in Jcrit exponentially increases the number of rare halos exposed to super--critical radiation. When H_2 cooling is suppressed, gas collapse starts with a delay, but it ultimately proceeds more rapidly. The infall velocity is near the increased sound speed, and an object as massive as M ~ 10^5 solar mass may form at the center of these halos, compared to the M ~ 10^2 solar mass stars forming when H_2--cooling is efficient.
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