Optical Properties of Supernova Remnants in Nearby Galaxies

Abstract

Supernova remnants (SNRs) in extragalactic galaxies are useful targets to investigate the global properties of interstellar medium (ISM) of a galaxy, provide information on the interplay between massive star formation and the local ISM, and understand the evolution of SNRs. I present a survey of optically selected SNRs in three nearby galaxies: M31, M33, and M83. They are close (<5 Mpc) and have well-formed spiral arms, indicating active star formation so that it is possible to fi nd many SNRs in these galaxies and characterize them in detail. I find the SNR candidates in M31 and M33 based on H a and [S II] images in the Local Group Survey, and I identify the M83 SNR candidates using archive HST images. Using these images, I select objects with enhanced [S II]:H a and circular shapes in each galaxy. I identify 156, 199, and 141 SNR candidates in M31, M33, and M83, respectively. Of the SNR candidates, 76 and 79, respectively, in M31 and M33 are newly found. I classify these SNR candidates in each galaxy according to two criteria: the SNR progenitor type and the morphological type. I find that fractions of the Type Ia SNR candidates in each galaxy are 23% (41 of 156), 15% (29 of 199), and 30% (43 of 141) for M31, M33, and M83, respectively. The majority (~90%) of the SNRs in each galaxy have shell structure. Most of the CC SNR candidates are concentrated in the spiral arms, while the Type Ia SNR candidates are rather distributed over the entire galaxy. The CC SNR candidates in M31 and M33 are brighter in H a and [S II] than the Type Ia SNR candidates. The H a luminosity of the SNR candidates in M31 ranges from L(H ) = 10^34.8 erg s^-1 to 10^37.2 erg s^-1, and their [S II] luminosity ranges from L([S II]) = 10^34.6 erg s^-1 to 10^37 erg s^-1. SNR candidates in M33 are detected to nearly the same limit. However, SNR candidates in M83 have much brighter optical luminosities than those in M31 and M33. This may be indicative of signi ficantly higher mean ISM densities in M83 compared with those in M31 and M33. Most of the SNR candidates in M31 and M33 have sizes of 20 pc < D < 60 pc, while most of the SNR candidates in M83 have smaller sizes than those in these two galaxies, 10 pc < D < 35 pc. The cumulative size distribution of the SNR candidates in M31 with 17 pc < D < 50 pc is well fi tted by a power law with an index of a= 2.53 +0.04, and a power law index for M33 in the same range of SNR size is a = 2.38+0.05. This indicates that most of both M31 and M33 SNR candidates are probably in the Sedov-Taylor phase. On the other hand, the cumulative size distribution of the M83 SNR candidates is well represented by a power law form in the range from D = 15 pc to 36 pc. The power law index for the distribution is a = 1.61+0.05. The [S II]:H a distribution of the SNR candidates in M31 is bimodal, with peaks at [S II]:Ha ~ 0.4 and ~0.9. Similarly, the [S II]:H a distribution of all the SNR candidates in M33 is bimodal, with peaks at [S II]:Ha ~ 0.6 and ~ 0.85. On the other hand, most of the SNR candidates in M83 have [S II]:H a of 0.3-0.6, and their distribution has a peak at [S II]:H a ~ 0.45. A higher fraction of SNR candidates with relatively high surface brightnesses, small sizes, and complete shapes in each galaxy are detected in X-rays. I fi nd a good correlation between the optical properties and X-ray luminosities for the SNR candidates with complete shells. I fi nd strong correlations between radio luminosities and optical properties for the M33 SNR candidates that are center-bright. In cases of the SNR candidates with complete shapes, objects with higher luminosity or surface brightness at optical wavelengths tend to have higher X-ray luminosity and radio luminosity. The SNR candidates with X-ray counterparts have higher surface brightness at optical wavelengths and smaller sizes than those without such counterparts. I fi nd good correlation between optical and near-IR luminosities of the SNR candidates with complete shells.