Molecular Gas Disk in the Flocculent Spiral Galaxy NGC 4414
Molecular Gas Disk in the Flocculent Spiral Galaxy NGC 4414
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絮状螺旋星系 NGC 4414 中的分子气盘
DOI：
10.1086/177961
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
K. Sakamoto
中科院分区：
文献类型：
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作者：
K. Sakamoto
利用Nobeyama毫米波阵在CO（J = 1-0）谱线上以4“（190 pc）的分辨率观测到絮状旋涡星系NGC 4414中心3×6 kpc。发现分子气体分布在一个相对光滑的轴对称圆盘中，圆盘中心有一个直径为0.5-1 kpc的孔。没有明显的螺旋或条形结构或明显的流动运动。几团分子气体（称为“盘GMAs”）被发现随机地位于气体盘中，质量为几个107 M，直径约为400 pc。分子气体的分布与Pogge的Hα分布相一致，包括团簇和中心空穴。Hα峰与盘状GMAs的对应关系表明盘状GMAs是这个絮状星系中大质量星星形成的主要场所。利用Toomre Q参数对气盘的稳定性进行了分析，结果表明，气盘是不稳定的，但除中心孔外，其余地方都接近中性稳定。由于气体表面密度和气体速度弥散的径向变化，气盘具有几乎恒定的径向Q分布。所观察到的气体团块的质量与最不稳定模式的预测质量吻合得很好。这些结果表明，盘GMAs是通过气体盘的引力不稳定性形成的。结果表明，两个圆盘巨磁链具有很大的速度色散，不可能被引力束缚。由于星星的形成，它们目前可能正在消散。这些观测结果与该星系中分子气盘通过反馈机制保持在中性稳定的假设是一致的。反馈的步骤是通过不稳定性形成团块，在团块处形成星星，以及通过星星形成而破坏团块。在NGC 4414中心，一个盘状GMA的形成破裂周期长达107年。
The central 3×6 kpc of the flocculent spiral galaxy NGC 4414 has been observed in the CO (J = 1–0) line with a 4'' (190 pc) resolution using the Nobeyama Millimeter Array. Molecular gas is found to be distributed in a relatively smooth axisymmetric disk with a central hole of 0.5–1 kpc diameter. There is no prominent spiral or bar structure or noticeable streaming motions. Several clumps of molecular gas (called “disk GMAs”) are found to be located randomly in the gas disk, with masses of a few 107 M☉ and diameters of ~ 400 pc. The molecular gas distribution agrees well with the Hα distribution imaged by Pogge, including the clumps and the central hole. The correspondence between the Hα peaks and the disk GMAs indicates that disk GMAs are the main sites of massive star formation in this flocculent galaxy. Stability analysis of the gas disk using the Toomre Q parameter shows that the gas disk is unstable but close to neutral stability everywhere except the central hole. The gas disk has a nearly constant radial Q profile owing to the radial variations of the gas surface density and the gas velocity dispersion. Masses of the observed gas clumps agree well with the predicted masses of the most unstable mode. These results suggest that the disk GMAs were formed through gravitational instability of the gas disk. It is found that two disk GMAs have too large a velocity dispersion to be gravitationally bound. They are probably dispersing at present as a result of star formation. These observational results are consistent with the scenario that the molecular gas disk in this galaxy has been maintained around neutral stability by a feedback mechanism. The steps involved in the feedback are clump formation through instability, star formation at the clumps, and clump disruption by star formation. The formation disruption cycle of a disk GMA takes several 107 yr in the center of NGC 4414.