The Relationship Between Molecular Gas Tracers and Kennicutt-Schmidt Laws
Mark R. Krumholz, Todd A. Thompson
备注：
Accepted to ApJ. 11 pages, 4 figures, emulateapj format. This version has some additional models exploring the effects of varying metallicity and temperature. The conclusions are unchanged
We provide a model for how Kennicutt-Schmidt (KS) laws, which describe the correlation between star formation rate and gas surface or volume density, depend on the molecular line chosen to trace the gas. We show that, for lines that can be excited at low temperatures, the KS law depends on how the line critical density compares to the median density in a galaxy's star-forming molecular clouds. High critical density lines trace regions with similar physical properties across galaxy types, and this produces a linear correlation between line luminosity and star formation rate. Low critical density lines probe regions whose properties vary across galaxies, leading to a star formation rate that varies superlinearly with line luminosity. We show that a simple model in which molecular clouds are treated as isothermal and homogenous can quantitatively reproduce the observed correlations between galactic luminosities in far infrared and in the CO(1->0) and HCN(1->0) lines, and naturally explains why these correlations have different slopes. We predict that IR-line luminosity correlations should change slope for galaxies in which the median density is close to the line critical density. This prediction may be tested by observations of lines such as HCO^+(1->0) with intermediate critical densities, or by HCN(1->0) observations of intensely star-forming high redshift galaxies with very high densities. Recent observations by Gao et al. hint at just such a change in slope. We argue that deviations from linearity in the HCN(1->0)-IR correlation at high luminosity are consistent with the assumption of a constant star formation efficiency.
The Star Formation Rate - Dense Gas Relation in Galaxies as Measured by HCN (3-2) Emission
我们用10米的海因里希赫兹亚毫米望远镜观测了30个邻近星系的HCN（3-2）辐射，这些星系的红外光度范围为10^10 - 10^12.5 L太阳，HCN（3-2）光度范围为10^6 - 10^9 K km s^-1 pc ^2。我们检查了红外光度和HCN（3-2）光度之间的相关性，发现最佳拟合线性回归的斜率（在对数-对数空间中）为0.74+/-0.12。包括Gracia-Carpio等人最近发表的数据，将最佳拟合斜率的约束收紧至0.79+/-0.09。这个低于1的斜率表明HCN（3-2）分子线的光度并不是线性地追踪致密气体的数量。我们的研究结果是一致的预测，从最近的理论模型，发现斜率低于统一时，线光度取决于平均气体密度的幂律指数大于肯尼卡特-施密特指数为1.5。
恒星复合体是星系中最大的球状恒星形成区。如果有螺旋密度波、核环、潮汐臂或其他定义明确的恒星结构，那么气体成分的引力不稳定性就会产生巨大的云复合体，其间距大约是宽度的三倍。这些气体复合物形成恒星复合物，沿着螺旋臂形成一串熟悉的珠子，或者在环的情况下形成核热点。湍流压缩、超新星和巨云内部的自引力收缩产生了几乎无标度的结构，包括形成OB协会的巨分子云和形成星团的分子云核心。如果没有恒星密度波或类似的结构，随机的引力不稳定性会形成絮状的螺旋，这些螺旋以同样的方式分裂成恒星复合体，OB协会和星团。最大的相干恒星形成区域是絮状臂本身。在这个层次的核心是非常密集的团块，其中形成了单个恒星和双星。星系中恒星的整体形成率似乎在大尺度上受到引力坍缩的调节，这就给出了肯尼库特/施密特定律与密度的比例关系，但是在这个比例关系之前的效率因子取决于处于致密形式的气体的比例。湍流压缩可能有助于这一部分。最近由Heyer et al.（2004）推导出的施密特定律的CO版本，也是从湍流等级中得出的，OB协和星团中恒星形成的局部效率也是如此。恒星形成的效率随着云密度的增加而增加，这就是为什么大多数恒星形成于最初自我约束的星团中。
