Modelling the bar in the centre of the starburst galaxy M82

We present VLA A-array 21-cm atomic hydrogen (H i) absorption observed against the central region of the starburst galaxy M82 with an angular resolution of ∼1.3 arcsec (≃20 pc). These observations, together with MERLIN H i absorption measurements, are compared with the molecular (CO) and ionized ([Ne ii]) 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 H i 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, H i and [Ne ii] 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=140 km s−1 and a total length of 1 kpc, a non-axisymmetry parameter q=0.9, an angular velocity of the bar Ωb=217 km s−1 arcsec−1, a core radius Rc=25 pc, 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.