从H2和FIR谱研究NGC 891分子气体的能量学
The Energetics of Molecular Gas in NGC 891 from H2 and FIR Spectroscopy
摘要：
利用Spitzer红外光谱仪观测到的最低三条纯转动H $_2$线的34位映象，研究了边缘旋涡星系NGC 1，891的氢分子能量学. S（0）、S（1）和S（2）线是明亮的，消光校正后的总光度为$\sim2.8 \times 10^{7}$ L $_{\odot}$，或为NGC 1，891总红外光度的0.09%。H $_2$谱线强度比在星系平面上几乎是恒定的--我们没有观察到之前报道的在星系外部区域S（1）/S（0）谱线强度比的强烈下降，所以我们没有发现用来解释过去观测结果的非常大的冷的无CO分子云的证据。H $_2$能级激发温度单调增加，表明发射气体的组分多于一种。超过99%的质量在最低激发（T $_{ex}$ $\sim$ 125 K）“温”组分中。在星系内部，温暖的H $_2$发射气体是CO（1-0）示踪的冷分子气体的$\sim$ 15%，而在外部区域，该比例是其两倍。这大质量的温暖气体被来自光离解区（PDR）的恒星的远紫外光子和湍流动能的耗散相结合而加热。将观察到的远红外[OI]和[CII]精细结构线发射和远红外连续谱发射以自洽的方式约束PDR模型，我们发现基本上所有的S（0）和大部分（70%）S（1）线来自低激发PDR，而大部分（80%）的S（2）线和其余的S（1）线发射来自低速微湍流耗散。
英文摘要：
We have studied the molecular hydrogen energetics of the edge-on spiral galaxy NGC\,891, using a 34-position map in the lowest three pure rotational H$_2$ lines observed with the Spitzer Infrared Spectrograph. The S(0), S(1), and S(2) lines are bright with an extinction corrected total luminosity of $\sim2.8 \times 10^{7}$ L$_{\odot}$, or 0.09\% of the total-infrared luminosity of NGC\,891. The H$_2$ line ratios are nearly constant along the plane of the galaxy -- we do not observe the previously reported strong drop-off in the S(1)/S(0) line intensity ratio in the outer regions of the galaxy, so we find no evidence for the very massive cold CO-free molecular clouds invoked to explain the past observations. The H$_2$ level excitation temperatures increase monotonically indicating more than one component to the emitting gas. More than 99\% of the mass is in the lowest excitation (T$_{ex}$ $\sim$125 K) ``warm'' component. In the inner galaxy, the warm H$_2$ emitting gas is $\sim$15\% of the CO(1-0)-traced cool molecular gas, while in the outer regions the fraction is twice as high. This large mass of warm gas is heated by a combination of the far-UV photons from stars in photo-dissociation regions (PDRs) and the dissipation of turbulent kinetic energy. Including the observed far-infrared [OI] and [CII] fine-structure line emission and far-infrared continuum emission in a self-consistent manner to constrain the PDR models, we find essentially all of the S(0) and most (70\%) of the S(1) line arises from low excitation PDRs, while most (80\%) of the S(2) and the remainder of the S(1) line emission arises from low velocity microturbulent dissipation.
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