HST/NICMOS observations of M82

The irregular galaxy M82 is known as the archetypal starburst galaxy. Its p roximity (3.5 Mpc) makes this galaxy an ideal laboratory for studying the p roperties of its starburst. The detailed morphology of the [FeII] 1.644 mum and emission Pa-alpha (at 1.87 mum) is revealed by the NICMOS images. The peak of the 2.2 mum continuum brightness (evolved population) lies very clo se to the dynamical centre. Most of the Pa-alpha emission (which traces the young population) is distributed in a ring of star formation (with a `hole ' lacking line emission at the centre of the galaxy). These observations su pport the scenario in which the starburst in M82 is propagating outwards. I t has long been suggested that the [FeII] emission in starburst galaxies ca n be used as a measure of supernova (SN) activity. M82 shows a large number of radio supernova remnants (SNRs), approximately 50, lying in the plane o f the galaxy. The comparison of the positions of the bright compact [FeII] emitting regions with the location of the radio SNRs shows that there is no one-to-one spatial correspondence between the two emissions, suggesting th at the radio and [FeII] emissions trace two populations of SNRs with differ ent ages. Young (a few hundred years) SNRs are best traced by their radio e mission, whereas the [FeII] stage lasts for at least a few 10(4) yr. The co mpact [FeII] sources contribute only some 20 % of the total [FeII] emission observed in M82. However, much of the remaining unresolved [FeII] emission in the plane of the galaxy may arise from SNRs that expanded and merged in to a general interstellar medium within a few 10(4) yr. Presumably, as much as 70% of the total extinction-corrected [FeII] 1.644 mum in M82 is associ ated with SNRs. The extended and diffuse [FeII] component in M82 seems to b e related with the superwind above and below the disc of the galaxy.