Diffuse and Gravitationally Stable Molecular Gas in the Post-Starburst Galaxy NGC 5195
Diffuse and Gravitationally Stable Molecular Gas in the Post-Starburst Galaxy NGC 5195
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星暴后星系 NGC 5195 中的扩散且重力稳定的分子气体
DOI：
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
R. Kawabe
中科院分区：
文献类型：
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作者：
K. Kohno;T. Tosaki;S. Matsushita;B. Vila;T. Shibatsuka;R. Kawabe
野边山毫米阵列（NMA）已经被用来对星暴后的星系NGC 5195进行孔径合成CO（1 −0）观测。本文还介绍了用Nobeyama 45 m望远镜对NGC 5195进行的CO（1 - 0）和HCN（1 - 0）观测。高分辨率（1.�� 九乘一。�� NMA图显示，尽管在NGC 5195中心的几× 100 pc区域中，目前的大质量星星形成受到抑制，但CO辐射在该区域的浓度很高。如果采用银河系NH 2/ICO转换因子，则在r< 2 ″或90 pc区域内的面气体密度（NH3气体）达到3.7 × 103 Mpc −2。中心CO峰的长度约为5 μ s，即230 pc，沿东西向沿着伸长，具有典型的棒盘星系的双臂螺旋状结构.亮温标上HCN与CO的积分强度比RHCN/CO在中心r< 400 pc区域内约为0.02。这个RHCN/CO比星暴区的小5-15倍。这些分子气体的性质可以解释为什么NGC 5195处于恒星暴增后的阶段;大多数致密的分子核心（即，大质量星星形成的地点）已经被过去的恒星爆发事件消耗掉了，因此大质量星星形成的爆发不再能持续，尽管大量的低密度气体仍然存在。我们发现NGC 5195的自转速度向中心方向急剧上升。因此，气体盘局部引力不稳定性的临界气体表面密度变得非常高（临界气体表面密度为6.9 × 103 Mpc −2），这表明NGC 5195中心区域的分子气体是引力稳定的，与星暴星系相反。我们认为，致密分子气体不能形成从剩余的扩散分子气体，因为在NGC 5195的中心的分子气体是太稳定，形成致密的核心，通过重力不稳定性的扩散分子气体。推断出的非常高的阈值密度似乎是由于NGC 5195中的高质量浓度。如果早型星系具有高阈值密度的特征，那么早型星系核星星形成的发生和光度的已知趋势就可以自然地理解。
The Nobeyama Millimeter Array (NMA) has been used to make aperture synthesis CO(1 −0) observations of the post-starburst galaxy NGC 5195. CO(1 −0) and HCN(1 −0) observations of NGC 5195 using the Nobeyama 45 m telescope are also presented. High-resolution (1. �� 9 ×1. �� 8o r 86pc×81pc resolution at D =9 .3Mpc) NMA maps show a strong concentration of CO emission toward the central a few ×100pc region of NGC 5195, despite the fact that the current massive star formation is suppressed there. The face-on gas surface density, Σgas, within the r< 2 �� or 90pc region reaches 3.7 × 10 3 Mpc −2 if a Galactic NH2 /ICO conversion factor is applied. The extent of the central CO peak is about 5 �� , or 230pc, and is elongated along the E-W direction with two-armed spiral-like structures, which are typical for barred disk galaxies. The HCN-to-CO integrated intensity ratio on the brightness temperature scale, RHCN/CO, is about 0.02 within the central r< 400 pc region. This RHCN/CO is smaller than those in starburst regions by a factor of 5-15. These molecular-gas properties would explain why NGC 5195 is in a post-starburst phase; most of the dense molecular cores (i.e., the very sites of massive star formation) have been consumed away by a past starburst event, and therefore a burst of massive star formation can no longer last, although a large amount of low density gas still exists. We find a steep rise of the rotation velocity toward the center of NGC 5195. As a consequence, the critical gas surface density for a local gravitational instability of the gas disk becomes very high (Σcrit ∼6.9 ×10 3 Mpc −2 ), suggesting that the molecular gas in the central region of NGC 5195 is gravitationally stable, in contrast to that of starburst galaxies. We propose that dense molecular gas can not be formed from remaining diffuse molecular gas because the molecular gas in the center of NGC 5195 is too stable to form dense cores via gravitational instabilities of diffuse molecular gas. The deduced very high threshold density seems to be due to a high mass concentration in NGC 5195. The known trends on the occurrence and luminosity of nuclear star formation in early-type galaxies can be understood naturally if the high threshold density is characteristic for early-type galaxies.