Are Compact High-Velocity Clouds Extragalactic Objects?
Are Compact High-Velocity Clouds Extragalactic Objects?
复制标题
致密高速云是河外天体吗？
DOI：
10.1086/374547
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发表时间：
2003
期刊：
影响因子：
--
通讯作者：
M. Putman
中科院分区：
文献类型：
--
作者：
P. Maloney;M. Putman
紧凑型高速云（CHVCs）是本群模式中距离最远的高速云，在d ~ 1 Mpc时，它们的H体积密度为~3 × 10-4 cm-3。具有这些体积密度和观测到的柱密度N ~ 1019 cm-2的云，即使只暴露在河外电离辐射场中，也会被大量电离。在这里，我们研究了这一过程对chvc模型的影响。我们已经模拟了球形云（有或没有暗物质晕）的电离结构，其密度和尺寸范围很大，适合于在建议距离范围内的chvc，暴露于河外电离光子通量（ϕi ~ 104光子cm-2 s-1）。恒定密度的云模型中，处于本群距离的chvc的总气体质量（电离加上中性气体）比中性气体质量大20-30倍，这意味着观测到的chvc族的气体质量仅为~4 × 1010 M☉。在现实的暗物质与气体质量比（10:1）下，这些chvc中的总质量是本星系群动态质量的重要组成部分，它们的线宽将大大超过观测到的ΔV。暗物质晕中自洽的气体模型表现更差；它们必须位于距离银河系大约200kpc的范围内，并且（对于给定的距离）比相应的均匀密度模型要大得多。我们还表明，指数中性氢柱密度分布是电离光子外部源的自然结果，并认为这些分布不能用于推导与模型无关的chvc距离。这些结果有力地证明，chvc不是宇宙天体，而是与银河晕有关。
Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Local Group model, and, at d ~ 1 Mpc, they have H I volume densities of ~3 × 10-4 cm-3. Clouds with these volume densities and the observed column densities N ~ 1019 cm-2 will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. Here we examine the implications of this process for models of CHVCs. We have modeled the ionization structure of spherical clouds (with and without dark matter halos) for a large range of densities and sizes, appropriate to CHVCs over the range of suggested distances, exposed to an extragalactic ionizing photon flux ϕi ~ 104 photons cm-2 s-1. Constant-density cloud models in which the CHVCs are at Local Group distances have total (ionized plus neutral) gas masses ~20-30 times larger than the neutral gas masses, implying that the gas mass alone of the observed population of CHVCs is ~4 × 1010 M☉. With a realistic (10 : 1) dark matter to gas mass ratio, the total mass in such CHVCs is a significant fraction of the dynamical mass of the Local Group, and their line widths would greatly exceed the observed ΔV. Self-consistent models of gas in dark matter halos fare even more poorly; they must lie within approximately 200 kpc of the Galaxy and (for a given distance) are much more massive than the corresponding uniform density models. We also show that exponential neutral hydrogen column density profiles are a natural consequence of an external source of ionizing photons and argue that these profiles cannot be used to derive model-independent distances to the CHVCs. These results argue strongly that the CHVCs are not cosmological objects and are instead associated with the Galactic halo.