Kinematics of molecular gas in the nucleus of barred galaxies : NGC 253 and M82

In more than half of all observed spiral galaxies, the central molecular gas is distributed in a bar. Since gas is a dissipative fluid, it will tend to settle along closed orbits in the plane of a galaxy. Simulations of bar orbits have shown that there are mainly two types of closed orbits in a bar potential the x1 orbits lying along the length of the bar and the x2 orbits lying perpendicular to the length of the bar. We have used a simple form of a bar potential to model the bar in two nearby starburst galaxies, NGC 253 and M82. Using parameters taken from the observed rotation curves, we have constructed the position velocity diagram and velocity contour plot for the bar in these galaxies. These plots help us understand the observed distribution and velocity field of the gas in these galaxies. We find that in NGC 253, the nuclear star formation is concentrated mainly along the x2 orbits and the nuclear velocity field shows signs of a past merger event in the galaxy. In M82, we find that the supernova remnants and HI gas lies along the x1 orbits whereas the ionized gas is mainly along the inner x2 orbits.