Neutral hydrogen absorption against supernova remnants in M82

Abstract

We present MERLIN neutral hydrogen absorption measurements against supernova remnants in the central starburst region of M82 with an angular resolution of ∼ 0.4 arcsec. We detect H i absorption or set significant upper limits against 33 supernova remnants from which we have been able to deduce column densities. Hence, using these measurements, we are able to probe the neutral hydrogen distribution and dynamics of the interstellar medium in M82 along 33 lines of sight on linear scales of order 1 pc. Our results show column densities ranging from <1.6 to >30 × 1021 atom cm−2 with the highest values seen towards the edge of the 250-pc ‘ring’. The absorption velocities show a gradient of 7.3 ± 4 km s−1 arcsec−1, consistent with rotation parameters of this ‘ring’ inferred from other measurements. The absorption velocities against individual remnants show deviations of typically 30 km s−1 from simple solid body rotation, and a number show multiple velocity absorption features. Although some of these deviations may be the result of the remnants being embedded at different depths within the neutral gas, the velocities cannot be explained by a simple rotating ring.