火轮IV。环形星系AM0644-741的恒星形成和中性星际介质
Wheels of Fire IV. Star Formation and the Neutral Interstellar Medium in the Ring Galaxy AM0644-741
摘要：
我们结合来自ATNF和SEST的数据来研究AM 0644 -741中的中性ISM，AM 0644 -741是一个巨大而健壮的恒星形成环星系。星系的ISM集中在42 kpc直径的星暴环，但似乎主要由原子气体，与全球的分子分数（f_mol）只有7.9%。除了星暴峰外，气环相对于引力不稳定性的增长是稳定的（Qgas =2-7）。包括恒星会降低整体Q，但不足以使Q处处<1。环的整体恒星形成效率（SFE）似乎有所提高，但环周围的变化超过一个数量级，在恒星形成最激烈的地方达到峰值。AM 0644 -741的恒星形成定律很特殊：HI遵循施密特定律，而H2与SFR/面积无关。光解模型得出环中的体积密度较低，特别是在星爆象限（n~2 cm^-3），这意味着ISM中以温暖的中性介质为主。与此同时，环的压力和周围的远紫外辐射场导致一个占主导地位的分子ISM的预期。我们认为，环的独特的恒星形成规律，n，SFE，和fmol的结果从ISM的> 1亿年的约束时间在星暴环，这增强了嵌入的大质量恒星和超新星的破坏性影响。因此，环的分子ISM变得由恒星形成最强烈的小云团主导，导致H2被12 CO线通量低估：实际上X（CO）>> X（Gal），尽管环的太阳金属丰度。观察到的大HI组分主要是低密度光解离产物，即，是大质量恒星形成的示踪剂而不是前兆这样一个“煮过头”的ISM可能是演化的星暴环星系的一般特征。
英文摘要：
We combine data from the ATNF and the SEST to investigate the neutral ISM in AM0644-741, a large and robustly star-forming ring galaxy. The galaxy's ISM is concentrated in the 42-kpc diameter starburst ring, but appears dominated by atomic gas, with a global molecular fraction (f_mol) of only 7.9%. Apart from the starburst peak, the gas ring is stable against the growth of gravitational instabilities (Q_gas=2-7). Including stars lowers Q overall, but not enough to make Q<1 everywhere. The ring's global star formation efficiency (SFE) appears somewhat elevated, but varies around the ring by more than an order of magnitude, peaking where star formation is most intense. AM0644-741's star formation law is peculiar: HI follows a Schmidt law while H2 is uncorrelated with SFR/area. Photodissociation models yield low volume densities in the ring, particularly in the starburst quadrant (n~2 cm^-3), implying a warm neutral medium dominated ISM. At the same time, the ring's pressure and ambient far-ultraviolet radiation field lead to the expectation of a predominantly molecular ISM. We argue that the ring's peculiar star formation law, n, SFE, and f_mol result from the ISM's >100 Myr confinement time in the starburst ring, which enhances the destructive effects of embedded massive stars and supernovae. As a result, the ring's molecular ISM becomes dominated by small clouds where star formation is most intense, causing H2 to be underestimated by 12CO line fluxes: in effect X(CO) >> X(Gal) despite the ring's solar metallicity. The observed large HI component is primarily a low density photodissociation product, i.e., a tracer rather than a precursor of massive star formation. Such an "over-cooked" ISM may be a general characteristic of evolved starburst ring galaxies.
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