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

In an effort better to calibrate the supernova rate of starburst galaxies as determined from near infrared [Feii] features, we report on a [Feii] λ1.644μm line imaging survey of a sample of 42 optically selected supernova remnants (SNRs) in M33. A wide range of [Feii] luminosities are observed within our sample (from less than 6 to 695L
⊙
). Our data suggest that the bright [Feii] SNRs are entering the radiative phase and that the density of the local interstellar medium (ISM) largely controls the amount of [Feii] emission. We derive the following relation between the [Feii] λ1.644μm line luminosity of radiative SNRs and the electronic density of the post shock gas, n
e
: L
[Feii]
$(L_\odot ) \approx 1.1n_e$
(cm
⁻³
). We also find a correlation in our data between L
[Feii]
and the metallicity of the shock heated gas, but the physical interpretation of this result remains inconclusive, as our data also show a correlation between the metallicity and n
e
. The dramatically higher level of [Feii] 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 [Feii] emitting lifetime of a SNR in the central regions of starburst galaxies is found to be of the order of 10
⁴
yr. On the basis of these results, we provide a new empirical relation allowing the determination of the current supernova rate of starburst galaxies from their integrated near infrared [Feii] luminosity.