arXiv 1601.055652016-01-28astro-ph.GA

星暴星系NGC253中扩展CO（6-5）辐射的SMA观测

SMA Observations of the Extended CO(6-5) Emission in the Starburst Galaxy NGC253

Melanie Krips, Sergio Martin, Alison Peck, Kazushi Sakamoto, Roberto Neri, Mark Gurwell, Glen Petitpas, Jun-Hui Zhao

备注：: accepted for publication in ApJ, 21 pages, 12 Figures, 6 Tables; acknowledgements updated in revised version
DOI：: 10.3847/0004-637X/821/2/112

摘要：

本文报道了用亚毫米波阵在~4arcsec的角分辨率下对NGC 253中的$^{12}$ CO（6-5）线和686 GHz连续谱的观测。$^{12}$ CO（6-5）辐射沿盘清晰地被探测到，并遵循较低的$^{12}$ CO谱线跃迁的分布，其中谱线比率变化很小。大速度梯度分析表明，盘中分子气体的双温模型，可能由低速激波和盘宽PDR的组合所主导。只有边缘$^{12}$ CO（6-5）的排放被检测到在附近的扩展壳在东部和西部边缘的磁盘。而东部的壳层含有的气体温度更高（T $_{\rm kin}$ >300~K）比热气体组分（T $_{\rm kin}$ = 300 K），西壳层周围的气体温度低得多（T $_{\rm kin}$ = 60 K），但比盘的冷气体成分稍热（对于相似的H $_2$和CO柱密度），指示不同的（或不同效率的）加热机制。在686 GHz的连续辐射在形状和大小上与较低（亚）毫米频率的连续辐射一致，表现出与热尘埃辐射一致的光谱指数。我们发现尘埃的温度约为10- 30 K，主要是光学薄发射。然而，我们的拟合表明，在较高温度（T $_{\rm d}$ > 60 K）下存在第二种（光学厚度更大）尘埃成分，类似于分子气体。我们估计全球尘埃质量约10 $^6$ Msun的磁盘转换成气体尘埃质量比为几百与附近的其他活动星系一致。

英文摘要：

We present observations of the $^{12}$CO(6-5) line and 686GHz continuum emission in NGC253 with the Submillimeter Array at an angular resolution of ~4arcsec. The $^{12}$CO(6-5) emission is clearly detected along the disk and follows the distribution of the lower $^{12}$CO line transitions with little variations of the line ratios in it. A large-velocity gradient analysis suggests a two-temperature model of the molecular gas in the disk, likely dominated by a combination of low-velocity shocks and the disk wide PDRs. Only marginal $^{12}$CO(6-5) emission is detected in the vicinity of the expanding shells at the eastern and western edges of the disk. While the eastern shell contains gas even warmer (T$_{\rm kin}$>300~K) than the hot gas component (T$_{\rm kin}$=300K) of the disk, the western shell is surrounded by gas much cooler (T$_{\rm kin}$=60K) than the eastern shell but somewhat hotter than the cold gas component of the disk (for similar H$_2$ and CO column densities), indicative of different (or differently efficient) heating mechansisms. The continuum emission at 686GHz in the disk agrees well in shape and size with that at lower (sub-)millimeter frequencies, exhibiting a spectral index consistent with thermal dust emission. We find dust temperatures of ~10-30K and largely optically thin emission. However, our fits suggest a second (more optically thick) dust component at higher temperatures (T$_{\rm d}$>60K), similar to the molecular gas. We estimate a global dust mass of ~10$^6$Msun for the disk translating into a gas-to-dust mass ratio of a few hundred consistent with other nearby active galaxies.

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