Understanding the vertical structure of galactic stellar disc and the role of interstellar gas and dark matter halo

Abstract

The aim of this thesis is to understand the dynamical effect of interstellar gas and dark matter halo on the vertical distribution of stars in our Galaxy as well as modeling the stellar distribution in a generalized and realistic set-up, consistent with recent observed data. To do this we have modeled the Galaxy as a gravitationally coupled multi-component system of stars, interstellar gas in the potential of dark matter halo. The self-consistent solution of the joint hydrostatic balance-Poisson equation for this system gives us the vertical distribution of stars as well as gas. We have shown that the gravitational force from gas and dark matter halo constrain the vertical distribution of stars in the inner and the outer Galaxy, respectively, compared to the self-gravitating stars-alone case. It raises the mid-plane density and reduces the scale height of the stellar disc. The outer disc is found to show a flared scale height, in consistence with recent observed data. We have predicted a flared stellar disc in a low surface brightness galaxy UGC 7321 using the above theoretical model. We have also shown that, interestingly, despite the constraining effect of gas on stars, the gravitational potential energy per unit area of the stellar disc remains unchanged in the gravitationally coupled stars plus gas system. Nevertheless, stars are more strongly bound to the mid-plane of the Galaxy due to the above constraining effect. When the vertical distribution of stars is modeled in a generalized fashion by taking account of the effect of various detailed kinematical features, such as tilt of the velocity ellipsoid, planar random motion, varying rotation curve etc., consistent with recent data, the distribution is found to be affected in the outer Galaxy, as high as 40%. Also, on exploring the effect of an increasing, i.e, a non-isothermal vertical velocity dispersion of stars, we found the total dynamical mid-plane density to be reduced by 16%, in the solar neighborhood. This can affect the determination of the local dark matter density.