Study of extragalactic supernova remnants

Image tube photographs of eight fields in the nearby spiral galaxy M31, using interference filters that isolate the emission lines of Hα + [N II] at lambda 6570 A and [S II] at lambda 6725 A, have revealed nebulae that are believed to be supernova remnants (SNRs) in this galaxy. Spectroscopic observations have been used to confirm this identification for twelve of these nebulae. An estimate of the pressure in the optical filaments and the measured diameter permit an estimate of the initial energy in each remnant; a mean value of E0 approx. = 3 x 1050 ergs is found. However, the energy calculated in this way appears to be correlated with the remnant's diameter, an effect which may be related to magnetic pressure in the filaments. Comparison of the SNR spectra to shock wave models has allowed estimates of abundances of nitrogen, oxygen, and sulfur in the interstellar gas of M31. Eleven H II regions in M31 have been observed for comparison to the SNRs. Element abundances for these have been derived by an empirical method and are in substantial agreement with the abundances as derived from the SNRs. The variation of abundances as a function of galactocentric distance in M31 are found to be similar to the abundance gradients in our own Galaxy, with nitrogen and oxygen both decreasing by a factor of four or five from 4 to 23 Kpc. An emission region in the irregular galaxy NGC 4449 which is believed to be a very luminous, young SNR similar to the galactic remnant Cassiopeia A has been investigated. The absence of hydrogen in the oxygen rich knots suggets that nucleosynthesis has been important in the star that created this remnant