Modelling the bar in the centre of the starburst galaxy M82

Abstract

We present VLA A-array 21-cm atomic hydrogen (Hi) absorption observed against the central region of the starburst galaxy M82 with an angular resolution of ~1.3arcsec (~=20pc). These observations, together with MERLIN Hi absorption measurements, are compared with the molecular (CO) and ionized ([Neii]) gas distributions and are used to constrain the dynamics and structure of the ionized, neutral and molecular gas in this starburst. A position-velocity diagram of the Hi distribution reveals an unusual `hole' feature which, when previously observed in CO, has been interpreted as an expanding superbubble contained within a ring of gas in solid body rotation. However, we interpret this feature as a signature of a nearly edge-on barred galaxy. In addition, we note that the CO, Hi and [Neii] position-velocity diagrams reveal two main velocity gradients, and we interpret these as gas moving on x1- and x2-orbits within a bar potential. We find the best fit to the data to be produced using a bar potential with a flat rotation curve velocity vb=140kms-1 and a total length of 1kpc, a non-axisymmetry parameter q=0.9, an angular velocity of the bar Ωb=217kms-1arcsec-1, a core radius Rc=25pc, an inclination angle i=80° and a projected angle between the bar and the major axis of the galaxy φ'=4°. We also discuss the orientation of the disc and bar in M82.