Near-infrared [Fe II] emission from supernova remnants and the supernova rate of starburst galaxies

Abstract

In an effort to better calibrate the supernova rate of starburst galaxies as deter- mined from near-infrared [Fe II] features, we report on a [Fe II] λ1.644 µm line-imaging survey of a sample of 42 optically-selected supernova remnants (SNRs) in M33. A wide range of [Fe II] luminosities are observed within our sample (from less than 6 to 695 L⊙). Our data suggest that the bright [Fe II] SNRs are entering the radiative phase and that the density of the local interstellar medium (ISM) largely controls the amount of [Fe II] emission. We derive the following relation between the [Fe II] λ1.644 µm line luminosity of radiative SNRs and the electronic density of the postshock gas, ne: L[Fe II] (L⊙) ≈ 1.1 ne (cm−3). We also find a correlation in our data between L[Fe II] and the metallicity of the shock-heated gas, but the physical interpretation of this re- sult remains inconclusive, as our data also show a correlation between the metallicity and ne. The dramatically higher level of [Fe II] emission from SNRs in the central regions of starburst galaxies is most likely due to their dense environments, although metallicity effects might also be important. The typical [Fe II]-emitting lifetime of a SNR in the central regions of starburst galaxies is found to be of the order of 104 yr. On the basis of these results, we provide a new empirical relation allowing the de- termination of the current supernova rate of starburst galaxies from their integrated near-infrared [Fe II] luminosity.