Though supernova explosions have a profound effect on the morphology, kinematics, and ionization balance of galaxies, our census of supernova remnants (SNRs) even in our own Galaxy is incomplete. Based on statistical studies of predicted supernova (SN) rates, there should be many more SNRs in our Galaxy (about 1000) than are currently known. The missing remnants are likely concentrated toward the inner Galaxy where the diffuse synchrotron emission and thermal HII regions near the Galactic plane cause the most confusion. Thus, more-sensitive, high-resolution surveys of the inner Galaxy at low radio frequencies are the key to determining whether the "missing" remnants exist or if our understanding of SN rates is significantly flawed. We recently imaged the Galactic plane at 90 cm wavelength from l=+4.5 to +22 degrees and |b| <1.25 degrees using the VLA in the B, C, and D configurations. In total we have identified 35 new SNR candidates that meet the following criteria: (1) the source must be resolved in our 42" resolution, 90 cm wavelength image and show a shell-like morphology; (2) the radio continuum spectral index α (Sν ≈ ν^α) computed from the integrated flux densities must be negative, indicative of non-thermal emission; and (3) the source morphology must be distinct from bright mid-infrared 8 μm emission. Generally, the newly discovered SNRs are smaller and fainter than those previously known in this region. This 90 cm survey of only 42.5 deg^2 has increased the number of identified remnants within the survey boundaries by nearly a factor of 3 (from 19 to 54) and produced a 15 percent increase in the total number of known Galactic SNRs. Estimates of the level of completeness of SNR surveys in other parts of the plane suggest that a similar 90 cm survey of the inner |l| <50 degrees would approximately double the number of known remnants. Overall, given that this technique remains insensitive to very small (<2'), very large (> 50'), and Crab-like remnants (without distinct radio shells), these results suggest that the "missing SNRs" problem can be attributed to selection effects and not our understanding of SN rates.
|Original language||English (US)|
|Journal||NRAO Newsletter, No. 108|
|State||Published - Jul 1 2006|