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arXiv 2009.11299astro-ph.GA
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旋涡星系中的HI标度高度

HI scale height in spiral galaxies

Narendra Nath Patra

摘要:

我们模拟了附近七个大旋涡星系的星系盘,将其作为由恒星、分子气体和原子气体组成的三分量系统,并在垂直流体静力学平衡下进行计算。我们建立了相应的联合Poisson-Boltzmann方程,并通过数值求解,估算了HI在这些星系中的三维分布。在求解Poisson-Boltzmann方程时,我们不考虑恒定的HI速度色散($σ_{\rm HI}$),而是利用观测到的HI光谱立方体的二阶矩轮廓,发展了一种迭代方法来自洽地估计星系中的$σ_{\rm HI}$轮廓。利用密度解,我们确定了我们星系的HI垂直尺度高度。我们发现,HI盘耀斑在一个线性的方式作为半径的函数。在我们的星系中,HI标度的高度在中心的几百秒差距到外围的$\sim 1-2$千秒差距之间变化。我们估计在我们的样本星系的HI盘的轴比,并找到一个中位数比为0.1,这是远远低于矮星系的发现,表明薄得多的HI盘在螺旋星系。我们的三个样本星系(NGC 5055,NGC 6946和NGC 7331)的轴比非常低,这表明它们是潜在的超薄星系。利用NGC 6946中的HI分布和HI洞的大小,我们发现该星系中的大多数HI洞在环星系介质中破裂,并且这种破裂在内半径比外半径更有效。

英文摘要:

We model the galactic discs of seven nearby large spiral galaxies as three-component systems consist of stars, molecular gas, and atomic gas in vertical hydrostatic equilibrium. We set up the corresponding joint Poisson-Boltzmann equation and solve it numerically to estimate the three-dimensional distribution of HI in these galaxies. While solving the Poisson-Boltzmann equation, we do not consider a constant HI velocity dispersion ($σ_{\rm HI}$); rather, we develop an iterative method to self-consistently estimate the $σ_{\rm HI}$ profile in a galaxy by using the observed second-moment profile of the HI spectral cube. Using the density solutions, we determine the HI vertical scale height in our galaxies. We find that the HI discs flare in a linear fashion as a function of radius. HI scale height in our galaxies is found to vary between a few hundred parsecs at the center to $\sim 1-2$ kpc at the outskirts. We estimate the axial ratio of the HI discs in our sample galaxies and find a median ratio of 0.1, which is much lower than what is found for dwarf galaxies, indicating much thinner HI discs in spiral galaxies. Very low axial ratios in three of our sample galaxies (NGC 5055, NGC 6946, and NGC 7331) suggest them to be potential superthin galaxies. Using the HI distribution and the HI hole sizes in NGC 6946, we find that most of the HI holes in this galaxy are broken out into the circumgalactic medium and this breaking out is more effective in the inner radii as compared to the outer radii.

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