The [Fe II] 1.644 Micron Emission in M82 and NGC 253: Is It a Measure of the Supernova Rate?^*

We present Hubble Space Telescope NICMOS [Fe II] 1.644 μm, Paα (1.87 μm), and continuum images of the starburst galaxies M82 and NGC 253 at an unprecedented spatial resolution. In both galaxies we detect [Fe II] compact sources superposed on a diffuse background in the disk of the galaxies, together with a component above and below the plane of the galaxy. The radio and [Fe II] emissions perpendicular to the disk of M82 show a remarkable similarity to each other, suggesting that both emissions originate in shocks from supernova explosions. We find a spatial correspondence between bright compact [Fe II]–emitting regions and the location of radio supernova remnants (SNRs) for approximately 30%–50% of radio SNRs in M82 and NGC 253. This lack of a one-to-one correspondence, more than being indicative of a different origin for the radio and [Fe II] emission in starbursts, suggests two populations of SNRs: an older population (≲10⁴ yr), traced by the [Fe II] emission, and a younger population (a few hundred years old), traced by the radio SNRs. We therefore conclude that the [Fe II] emission in starburst galaxies provides a good estimate of the supernova activity. Using our newly determined [Fe II] luminosities (corrected for extinction) of M82 and NGC 253, we reevaluate the calibration of the supernova rate in terms of the [Fe II] luminosity for starburst galaxies.