非等温银盘中的垂直恒星密度分布
Vertical stellar density distribution in a non-isothermal galactic disc
摘要：
传统上，星系盘中恒星的垂直密度分布是通过假设恒星的垂直速度色散等温来获得的。最近从SDSS、LAMOST、RAVE、Gaia等的观测表明，这种色散随着距离中平面的高度而增加。在这里，我们研究这种非等温色散的动力学效应的自洽垂直密度分布的薄盘星在银河系中，通过求解泊松方程和流体静力平衡方程。我们发现，在非等温情况下，由于垂直压力较高，中面密度较低，而标度高度高于等温分布的相应值，因此分布在垂直方向上更加延伸。对于典型观测线性梯度为+6.7 km $s^{-1}kpc^{-1}$的恒星单独盘，在太阳半径处的变化约为35%，并且随着半径和梯度的增加而变得更高。该分布在高z处显示出一个翼，与观测结果一致，并且与双$sech^{2}$很好地拟合，这可能被误解为存在第二个更厚的盘，特别是在外部星系中。我们还考虑了一个更现实的磁盘组成的引力耦合的恒星和气体领域的暗物质晕。结果显示了相同的趋势，但由于气体和晕重力的相反约束作用，非等温弥散的影响减小。此外，非等温分散降低了总的中面密度，即奥尔特极限值的理论估计，由16%。
英文摘要：
The vertical density distribution of stars in a galactic disc is traditionally obtained by assuming an isothermal vertical velocity dispersion of stars. Recent observations from SDSS, LAMOST, RAVE, Gaia etc show that this dispersion increases with height from the mid-plane. Here we study the dynamical effect of such non-isothermal dispersion on the self-consistent vertical density distribution for the thin disc stars in the Galaxy, obtained by solving together the Poisson equation and the equation of hydrostatic equilibrium. We find that in the non-isothermal case the mid-plane density is lower, and the scale height is higher than the corresponding values for the isothermal distribution, due to higher vertical pressure, hence the distribution is vertically more extended. The change is ~35% at the solar radius for a stars-alone disc for the typical observed linear gradient of +6.7 km $s^{-1}kpc^{-1}$ and becomes even higher with increasing radii and increasing gradients explored. The distribution shows a wing at high z, in agreement with observations, and is fitted well by a double $sech^{2}$ , which could be mis-interpreted as the existence of a second, thicker disc, specially in external galaxies. We also consider a more realistic disc consisting of gravitationally coupled stars and gas in the field of dark matter halo. The results show the same trend but the effect of non-isothermal dispersion is reduced due to the opposite, constraining effect of the gas and halo gravity. Further, the non-isothermal dispersion lowers the theoretical estimate of the total mid-plane density i.e, Oort limit value, by 16%.
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