解决远红外线缺陷：NGC 1097和NGC 4559中的光电加热和远红外线冷却
Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559
摘要：
星际气体和尘埃的物理状态取决于加热和冷却这种介质的过程。为了探测ISM在大范围红外表面亮度上的加热和冷却，在亚千秒差距尺度上，我们使用NGC 1097和NGC 4559中[C \ii ] 158 $μ$ m，[O \one ] 63 $μ$ m和[N \ii ] 122 $μ$ m的线图，这些线图是用机载的PACS光谱仪{ \it Herschel}获得的。我们将新的观测结果与现有的Spitzer-IRS数据相匹配，这些数据追踪了多环芳烃（PAH）的总排放量。我们确认在这些星系的小尺度光电加热效率的规范措施，（[C \ii ] + [O \one ]）/TIR，减少作为远红外颜色，$νf_ν$（70 $μ$ m）/ $νf_ν$（100 $μ$ m），增加。相比之下，远红外（远IR）冷却与总PAH排放的比率（[C \ii ] + [O \one ]）/PAH在远红外颜色的宽范围内为接近恒定的$\sim$ 6%，0.5 \textless \ $νf_ν$（70 $μ$ m）/ $νf_ν$（100 $μ$ m）$\lesssim$ 0.95。在最温暖的地区，$νf_ν$（70 $μ$ m）/ $νf_ν$（100 $μ$ m）$\gtrsim$ 0.95，（[C \ii ] + [O \one ]）/PAH的比值迅速下降至4%。我们通过比较我们的观测结果与光解区域的模型，得出了局部紫外辐射密度$G_0$和气体密度$n_H$的代表值。比率$G_0/n_H$，来自精细结构线，被发现与平均灰尘加权星光强度，$<U>$来自IR SED模型。从表现出线赤字的区域发射的特征在于强烈的辐射场，这表明小颗粒容易受到电离效应的影响。我们注意到，在（[C \ii ] + [O \one ]）/PAH不足的区域，$μ$ m PAH比值为7.7 / 11.3，这表明小颗粒在这些环境中被电离。
英文摘要：
The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the ISM over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C \ii] 158 $μ$m, [O \one] 63 $μ$m, and [N \ii] 122 $μ$m in NGC 1097 and NGC 4559, obtained with the PACS spectrometer onboard {\it Herschel}. We matched new observations to existing Spitzer-IRS data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C \ii] + [O \one])/TIR, decreases as the far-infrared color, $νf_ν$(70 $μ$m)/$νf_ν$(100 $μ$m), increases. In contrast, the ratio of far-infrared (far-IR) cooling to total PAH emission, ([C \ii] + [O \one])/PAH, is a near constant $\sim$6% over a wide range of far-infrared color, 0.5 \textless\ $νf_ν$(70 $μ$m)/$νf_ν$(100 $μ$m) $\lesssim$ 0.95. In the warmest regions, where $νf_ν$(70 $μ$m)/$νf_ν$(100 $μ$m) $\gtrsim$ 0.95, the ratio ([C \ii] + [O \one])/PAH drops rapidly to 4%. We derived representative values of the local UV radiation density, $G_0$, and the gas density, $n_H$, by comparing our observations to models of photodissociation regions. The ratio $G_0/n_H$, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, $<U>$ derived from models of the IR SED. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7 / 11.3 $μ$m PAH ratio in regions that exhibit a deficit in ([C \ii] + [O \one])/PAH, suggesting that small grains are ionized in these environments.
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