A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES
A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES
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DOI：
10.1088/0004-6256/142/2/37
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发表时间：
2011-05
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通讯作者：
A. Schruba;A. Leroy;F. Walter;F. Bigiel;E. Brinks;W. D. Blok;G. Dumas;C. Kramer;E. Rosolowsky;K. Sandstrom;K. Schuster;A. Usero;A. Weiss;H. Wiesemeyer
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作者：
A. Schruba;A. Leroy;F. Walter;F. Bigiel;E. Brinks;W. D. Blok;G. Dumas;C. Kramer;E. Rosolowsky;K. Sandstrom;K. Schuster;A. Usero;A. Weiss;H. Wiesemeyer
我们使用IRAM HERACLES调查来研究33个附近螺旋星系的CO排放，直到非常低的强度。利用21 cm线原子氢（H i）数据，主要来自于THINGS，我们基于平均H i速度预测了局部平均CO速度。通过重新归一化CO速度轴，使零对应于局部平均H i速度，我们能够在大范围内相干地叠加光谱。这使我们能够测量CO强度，具有很高的意义，低至ICO≈0.3 K km s−1 (M☉pc−2)，比以前的研究提高了大约一个数量级。我们探测到CO的星系中心半径为rgal ~ r25，并发现CO的径向分布遵循一个非常均匀的指数下降，尺度长度为~ 0.2 r25。在这里，我们重点研究了堆积作为半径的函数，比较了我们的敏感CO剖面与在24 μm和70 μm下的H i， Hα，远紫外（FUV）和红外（IR）发射的匹配剖面。我们观察到CO和IR强度之间存在紧密的、大致的线性关系，在由分子气体(\Sigma _{\rm H\,\mathsc{i}}$？>)和以原子气体为主的（）。我们使用FUV+24 μm和Hα+24 μm的组合来估计最近的恒星形成速率（SFR）表面密度ΣSFR，并发现ΣSFR和。我们将此解释为恒星在分子气体中形成的证据，与局部总气体表面密度几乎没有关系。虽然星系在sfr - h2比中显示出很小的内部变化，但我们确实观察到星系间的系统性变化。这些星系间的变化主导了大尺度测量的CO和SFR示踪剂之间关系的分散。这些变化的意义在于，质量较小的星系比大质量星系表现出更大的sfr - co比率。与srr - co比不同，原子气体和分子气体之间的平衡在很大程度上取决于气体的总表面密度和星系中心半径。它还必须依赖于其他参数。我们的研究结果加强并扩展到较低表面密度的情况，在这种情况下，星系中的恒星形成可以分为两个过程：恒星形成分子云的组装和H2形成恒星。这些过程之间的相互作用产生了随斜率变化的总气体- sfr关系，这在之前已经被观察到并确定为恒星形成的阈值。
We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using 21 cm line atomic hydrogen (H i) data, mostly from THINGS, we predict the local mean CO velocity based on the mean H i velocity. By re-normalizing the CO velocity axis so that zero corresponds to the local mean H i velocity we are able to stack spectra coherently over large regions. This enables us to measure CO intensities with high significance as low as ICO ≈ 0.3 K km s−1 ( M☉ pc−2), an improvement of about one order of magnitude over previous studies. We detect CO out to galactocentric radii rgal ∼ r25 and find the CO radial profile to follow a remarkably uniform exponential decline with a scale length of ∼0.2 r25. Here we focus on stacking as a function of radius, comparing our sensitive CO profiles to matched profiles of H i, Hα, far-UV (FUV), and Infrared (IR) emission at 24 μm and 70 μm. We observe a tight, roughly linear relationship between CO and IR intensity that does not show any notable break between regions that are dominated by molecular gas ( \Sigma _{\rm H\,\mathsc{i}}$?>) and those dominated by atomic gas (). We use combinations of FUV+24 μm and Hα+24 μm to estimate the recent star formation rate (SFR) surface density, ΣSFR, and find approximately linear relations between ΣSFR and . We interpret this as evidence of stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H2 ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relationships between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between atomic and molecular gas depends strongly on the total gas surface density and galactocentric radius. It must also depend on additional parameters. Our results reinforce and extend to lower surface densities, a picture in which star formation in galaxies can be separated into two processes: the assembly of star-forming molecular clouds and the formation of stars from H2. The interplay between these processes yields a total gas–SFR relation with a changing slope, which has previously been observed and identified as a star formation threshold.