Optical spectroscopy of supernova remnants in m81 and m82

Abstract

We present spectroscopy of 28 supernova remnant (SNR) candidates as well as one H II region in M81 and two SNR candidates in M82. Twenty-six of the M81 candidates turn out to be genuine SNRs, and two in M82 may be hocked condensations in the galactic outflow or SNRs. The distribution of [N II]/H alpha ratios of M81 SNRs is bimodal. M81 SNRs are divided into two groups in the spectral line ratio diagrams: an [O III]-strong group and an [O III]-weak group. The latter are larger and may have faster shock velocities. [N II]/Ha. ratios of the SNRs show a strong correlation with [S II]/H alpha ratios. They show a clear radial gradient in their [N II]/H alpha and [S II]/H alpha ratios: dLog ([N II]/H alpha)/dLog R = -0.018 +/- 0.008 dex kpc(-1) and dLog ([S II]/H alpha)/dLog R = -0.016 +/- 0.008 dex kpc(-1), where R is the deprojected galactocentric distance. We estimate the nitrogen and oxygen abundances of the SNRs from comparison with shock-ionization models. We obtain a value for the nitrogen radial gradient of dLog(N/H)/dLog R= -0.023 +/- 0.009 dex kpc(-1), and find little evidence for an oxygen gradient. This nitrogen abundance shows a gradient that is a few times flatter than those of the planetary nebulae and H II regions. We find that five SNRs are matched with X-ray sources. Their X-ray hardness colors are consistent with thermal SNRs.