Using star formation histories derived from optically resolved stellar populations in 19 nearby starburst dwarf galaxies observed with the Hubble Space Telescope, we measure the stellar mass surface densities of stars newly formed in the bursts. By assuming a star formation efficiency (SFE), we then calculate the inferred gas surface densities present at the onset of the starbursts. Assuming an SFE of 1%, as is often assumed in normal star-forming galaxies, and assuming that the gas was purely atomic, translates to very high H I surface densities (~102-103 M ☉?pc–2), which are much higher than have been observed in dwarf galaxies. This implies either higher values of SFE in these dwarf starburst galaxies or the presence of significant amounts of H2 in dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with observations of more massive starbursts associated with merging galaxies), still results in H I surface densities higher than observed in 10 galaxies. Thus, these observations appear to require that a significant fraction of the gas in these dwarf starbursts galaxies was in the molecular form at the onset of the bursts. Our results imply molecular gas column densities in the range 1019-1021?cm–2 for the sample. In the galaxies where CO observations have been made, these densities correspond to values of the CO–H2 conversion factor (X CO) in the range (3-80) × 1020?cm–2 (K?km?s–1)–1, or up to 40 × greater than Galactic X CO values.
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机译：使用哈勃太空望远镜观测到的来自附近19个星暴矮星系中光学分辨的恒星群体的恒星形成历史，我们测量了爆发中新形成的恒星的恒星质量表面密度。通过假设恒星形成效率（SFE），我们可以计算出在爆发时出现的推断气体表面密度。像正常的恒星系通常假定的那样，假设SFE为1％，并且假设气体是纯原子的，则转化为非常高的HI表面密度（〜102-103 M ☉pc–2），高于矮星系中的观测值。这意味着在这些矮星爆星系中SFE值较高，或者在矮星中存在大量H2（或同时存在）。将假定的SFE提高到10％或更高（与观察到的与合并星系相关的更大质量的星爆相一致），仍然会导致H I表面密度高于10个星系中观测到的。因此，这些观察似乎要求这些矮星爆星系中的大部分气体在爆发时以分子形式存在。我们的结果表明样品的分子气相色谱柱密度在1019-1021？cm–2范围内。在进行了CO观测的星系中，这些密度对应于CO-H2转换因子（X CO）的值，范围>（3-80）×1020？cm–2（K？km？s–1） –1，或比银河系X CO值大40倍。
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