A Pressure Anomaly for H II Regions in Irregular Galaxies
A Pressure Anomaly for H II Regions in Irregular Galaxies
复制标题
不规则星系中H II区的压力异常
DOI：
10.1086/309382
复制
发表时间：
2000
期刊：
影响因子：
--
通讯作者：
D. Hunter
中科院分区：
文献类型：
--
作者：
B. Elmegreen;D. Hunter
在六个矮不规则星系的巨大的H II区域的压力被发现是一个比相应的星系盘，从恒星和气体柱密度获得的平均压力大~10倍。这与螺旋星系的情况不同，在螺旋星系中，这两种压力近似相等。要么这些矮星中的H II区域都非常年轻，以至于它们仍在膨胀，要么存在意想不到的盘自引力源，增加了背景压力。我们首先考虑是否有任何额外的自引力可能来自磁盘暗物质，无论是冷的H2气体在弥漫或自引力云弱CO排放，或从旋转曲线推断的晕暗物质是相同的材料。H2的解决方案是可能的，因为冷分子云将在现有的调查中几乎是不可见的，如果他们也从这些星系中的低金属丰度的CO弱。宇宙暗物质也可能存在，因为盘在整个星系势中占据了相对较大的体积分数。然而，这两种解决方案都存在一个问题：对于不规则星系推断的垂直尺度高度仅与发光物质一致。需要大量的盘状暗物质来解释H II区的高压力，这将使气体和恒星尺度的高度太小。恒星形成区域的异常压力更可能是来自大气体浓度的引力场局部峰值的结果。这些峰值也解释了早期在不规则星系中发现的星星形成的极低的平均柱密度阈值，并且它们允许冷的H I相的存在作为迈向致密分子核心的第一步。H I的浓度在区域的星星形成的证据进行了总结，峰值柱密度被证明是一致的局部压力平衡的H II区域。强烈的自引力恒星形成区也应该限制金属向星系际介质的扩散。第三种可能性是，这些矮星系中所有可见的H II区域都处于强烈的超压状态，并且仍在膨胀。达到压力平衡的平均时间约为它们当前年龄的15倍，这意味着如果它们活得那么长，观测到的种群只占总数的7%;其余的可能太微弱而看不见。膨胀模型还暗示体积填充因子可以达到当前因子的100倍，在这种情况下，暗淡和老化的H II区域应该合并并占据几乎整个矮星系体积。这将解释在这些星系中看到的巨大H I壳层的起源，这些壳层是以前由OB协会驱动的旧的、扩展的H II区域的结果。令人兴奋的星团现在已经非常古老和分散，以至于它们不容易被识别出来。由于缺乏剪切力，炮弹仍然是圆的。
The pressures of giant H II regions in six dwarf irregular galaxies are found to be a factor of ~10 larger than the average pressures of the corresponding galaxy disks, obtained from the stellar and gaseous column densities. This is unlike the situation for spiral galaxies, where these two pressures are approximately equal. Either the H II regions in these dwarfs are all so young that they are still expanding, or there is an unexpected source of disk self-gravity that increases the background pressure. We consider first whether any additional self-gravity might come from disk dark matter that either is cold H2 gas in diffuse or self-gravitating clouds with weak CO emission, or is the same material as the halo dark matter inferred from rotation curves. The H2 solution is possible because cold molecular clouds would be virtually invisible in existing surveys if they were also CO-weak from the low metal abundances in these galaxies. Cosmological dark matter might be possible too because of the relatively large volume fraction occupied by the disk within the overall galaxy potential. There is a problem with both of these solutions, however: the vertical scale heights inferred for irregular galaxies are consistent with the luminous matter alone. The amount of disk dark matter that is required to explain the high H II region pressures would give gas and stellar scale heights that are too small. The anomalous pressures in star-forming regions are more likely the result of local peaks in the gravitational field that come from large gas concentrations. These peaks also explain the anomalously low average column density thresholds for star formation that were found earlier for irregular galaxies, and they permit the existence of a cool H I phase as the first step toward dense molecular cores. The evidence for concentrations of H I in regions of star formation is summarized; the peak column densities are shown to be consistent with local pressure equilibrium for the H II regions. Strongly self-gravitating star-forming regions should also limit the dispersal of metals into the intergalactic medium. The third possibility is that all of the visible H II regions in these dwarf galaxies are strongly overpressured and still expanding. The mean time to pressure equilibrium is ~15 times their current age, which implies that the observed population is only 7% of the total if they live that long; the rest are presumably too faint to see. The expansion model also implies that the volume-filling factor can reach ~100 times the current factor, in which case faint and aging H II regions should merge and occupy nearly the entire dwarf galaxy volume. This would explain the origin of the giant H I shells seen in these galaxies as the result of old, expanded H II regions that were formerly driven by OB associations. The exciting clusters would now be so old and dispersed that they would not be recognized easily. The shells are still round because of a lack of shear.