Populations of High-Luminosity Density-bounded H II Regions in Spiral Galaxies: Evidence and Implications
Populations of High-Luminosity Density-bounded H II Regions in Spiral Galaxies: Evidence and Implications
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螺旋星系中高光度密度 H II 区域的族群：证据与启示
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发表时间：
2000
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通讯作者：
J. Knapen
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作者：
J. E. Beckman;M. Rozas;A. Zurita;R. Watson;J. Knapen
我们提供的证据表明，盘状星系中高光度的 H II 区域可能受到密度限制，因此令人兴奋的 OB 恒星发射的电离光子的很大一部分会从这些区域逃逸。关键证据是 H II 区域群体的 Hα 光度函数中存在毛刺，即在明显不变的光度下出现局部尖锐峰值。明显不变的光度被定义为 Strömgren 光度 (LStr)，使得 LHα = LStr = 1038.6 ± 100.1 ergs s-1（在任何光度函数中均未发现其他峰值），同时 LHα > LStr 的斜率变陡。这种行为很容易用物理模型来解释，其中（1）LHα = LStr 处的转变标志着从低光度下本质上的电离边界到较高值下的密度边界的变化，以及（2）要发生这种变化，将大质量恒星形成云中的恒星质量与胎盘云质量相关的定律必须使得云内产生的电离光子通量是一个比云质量更急剧上升的函数。还提出了这种转变假设的支持证据：LHα < LStr 的 H II 区域中心表面亮度的测量与 L 成正比，正如电离边界所预期的那样，但 LHα > LStr 显示出更陡峭的依赖性的急剧趋势，并且如果这些区域是密度有界的，则在高光度下观察到的内部湍流速度参数 σ 和光度 L 之间的关系可以得到很好的解释。如果得到证实，密度界限假说将产生许多有趣的含义。这意味着密度限制区域是电离盘状星系中扩散气体的光子的主要来源。我们基于假设的估计表明，这些区域发射出足够的莱曼连续光子，不仅可以电离扩散介质，而且可以导致典型的螺旋向星系间介质发射大量电离通量。在 LStr 中观察到的低散射，在迄今为止测量的仍然相当小的样本中小于 0.1 mag rms，是扩大数据库并根据主要标准进行校准的邀请，以获得精确的 (∼105 L⊙) 广泛分布的标准烛光。
We present evidence that the H II regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escapes from the regions. The key piece of evidence is the presence of glitches, local sharp peaks at an apparently invariant luminosity, in the Hα luminosity functions of the populations of H II regions. The apparently invariant luminosity is defined as the Strömgren luminosity (LStr), such that LHα = LStr = 1038.6 ± 100.1 ergs s-1 (no other peaks are found in any of the luminosity functions) accompanying a steepening of slope for LHα > LStr. This behavior is readily explicable by a physical model in which (1) the transition at LHα = LStr marks a change from essentially ionization bounding at low luminosities to density bounding at higher values and (2) for this to occur the law relating stellar mass in massive star-forming clouds to the mass of the placental cloud must be such that the ionizing photon flux produced within the cloud is a function that rises more steeply than the mass of the cloud. Supporting evidence for the hypothesis of this transition is also presented: measurements of the central surface brightnesses of H II regions for LHα < LStr are proportional to L, as expected for ionization bounding, but show a sharp trend to a steeper dependence for LHα > LStr, and the observed relation between the internal turbulence velocity parameter, σ, and the luminosity, L, at high luminosities can be well explained if these regions are density bounded. If confirmed, the density-bounding hypothesis would have a number of interesting implications. It would imply that the density-bounded regions were the main sources of the photons that ionize the diffuse gas in disk galaxies. Our estimates, based on the hypothesis, indicate that these regions emit sufficient Lyman continuum photons not only to ionize the diffuse medium but to cause a typical spiral to emit significant ionizing flux into the intergalactic medium. The low scatter observed in LStr, less than 0.1 mag rms in the still quite small sample measured to date, is an invitation to widen the database and to calibrate against primary standards to obtain a precise (∼105 L⊙) widely distributed standard candle.