We examine, from a theoretical viewpoint, how the physical parameters of H II regions are controlled in both normal galaxies and in starburst environments. These parameters are the H II region luminosity function, the time-dependent size, the covering fraction of molecular clouds, the pressure in the ionized gas, and the ionization parameter. The factors that control them are the initial mass function (IMF) of the exciting stars, the cluster mass function, the metallicity, and the mean pressure in the surrounding interstellar medium. We investigate the sensitivity of the Hα luminosity to the IMF, and find that this can translate to more than a factor 2 variation in derived star formation rates. The molecular cloud dissipation timescale is estimated from a case study of M17 to be ~1 Myr for this object. Based on H II luminosity function fitting for nearby galaxies, we suggest that the H II region cluster mass function is fitted by a lognormal form peaking at ~100 M☉. The cluster mass function continues the stellar IMF to a higher mass regime. The pressure in the H II regions is controlled by the mechanical luminosity flux from the central cluster. Since this is closely related to the ionizing photon flux, we show that the ionization parameter is not a free variable, and that the diffuse ionized medium may be composed of many large, faint, and old H II regions. Finally, we derive theoretical probability distributions for the ionization parameter as a function of metallicity and compare these to those derived for SDSS galaxies.
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机译：从理论的角度，我们研究了在正常星系和星爆环境中如何控制H II区的物理参数。这些参数是H II区域的光度函数，随时间变化的大小，分子云的覆盖率，电离气体中的压力以及电离参数。控制它们的因素是恒星的初始质量函数（IMF），星团质量函数，金属性以及周围星际介质中的平均压力。我们研究了Hα光度对IMF的敏感性，发现这可以转化为派生恒星形成速率的2倍以上变化。根据M17的案例研究，该分子的云消散时间尺度约为1 Myr。基于对附近星系的H II发光度函数拟合，我们建议H II区域簇质量函数由在〜100M☉处达到峰值的对数正态拟合。簇质量函数将恒星IMF延续到更高质量状态。 H II区的压力由中央星团的机械光通量控制。由于这与电离光子通量密切相关，我们证明了电离参数不是自由变量，并且弥散的电离介质可能由许多大的，模糊的和旧的H II区组成。最后，我们推导了电离参数作为金属性函数的理论概率分布，并将其与为SDSS星系推导的分布进行了比较。
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