Probability distribution functions of gas in M31 and M51
Probability distribution functions of gas in M31 and M51
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DOI：
10.1093/mnras/stv132
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发表时间：
2015-01
影响因子：
4.8
通讯作者：
E. M. Berkhuijsen;A. Fletcher
中科院分区：
文献类型：
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作者：
E. M. Berkhuijsen;A. Fletcher
给出了附近旋涡星系M31和M51中电离气体和中性气体表面密度的概率分布函数，以及M31中尘埃发射和消光Av的概率分布函数。PDF接近对数正态，而M31的HI和Av的PDF几乎相同。然而，H2的PDF比HI PDF更宽，M51的PDF比M31的PDF具有更大的分散度。我们使用一个简单的模型来确定PDF是如何随着穿过气体的视线(LOS)路径长度L、望远镜分辨率和气体的体积填充因子f_v的变化而变化的。在这些情况下，对数正态PDF的色散σ依赖于负幂规律的变量。我们还得到了M31和M51气体组分的平均LOS体积密度的PDF。结合银河系ISM不同分量的体密度函数，我们发现Sigma随长度的增加而减小，指数为-0.76+/-0.06，比预期的陡度大。我们指出，这种差异是由于fv的变化造成的，因为fv在M31、M51和Mw中是相似的，所以这些星系的气体中的密度结构一定是相似的。最后，我们证明了f_v随距银面距离的增加而增加，解释了微波辐射测量和FUV发射的pdf随纬度的减小。
We present probability distribution functions (PDFs) of the surface densities of ionized and neutral gas in the nearby spiral galaxies M31 and M51, as well as of dust emission and extinction Av in M31. The PDFs are close to lognormal and those for HI and Av in M31 are nearly identical. However, the PDFs for H2 are wider than the HI PDFs and the M51 PDFs have larger dispersions than those for M31. We use a simple model to determine how the PDFs are changed by variations in the line-of-sight (LOS) pathlength L through the gas, telescope resolution and the volume filling factor of the gas, f_v. In each of these cases the dispersion sigma of the lognormal PDF depends on the variable with a negative power law. We also derive PDFs of mean LOS volume densities of gas components in M31 and M51. Combining these with the volume density PDFs for different components of the ISM in the Milky Way (MW), we find that sigma decreases with increasing length L with an exponent of -0.76 +/- 0.06, which is steeper than expected. We show that the difference is due to variations in f_v. As f_v is similar in M31, M51 and the MW, the density structure in the gas in these galaxies must be similar. Finally, we demonstrate that an increase in f_v with increasing distance to the Galactic plane explains the decrease in sigma with latitude of the PDFs of emission measure and FUV emission observed for the MW.