A search for supernova remnants in the nearby spiral galaxy M 74 (NGC 628)

Abstract

We have identified nine new SNR candidates in M74 with [S II]/H$\alpha$ $\geq$ 0.4 as the basic criterion. We obtain [S II]/H$\alpha$ ratio in the range from 0.40 to 0.91 and H$\alpha$ intensities from 2.8 $\times$ $10^{-15}$ erg cm$^{-2}$ s$^{-1}$ to 1.7 $\times$ $10^{-14}$ erg cm$^{-2}$ s$^{-1}$. We also present spectral follow-up observations of the SNR candidates and can confirm only three of them (SNR2, SNR3, and SNR5). The lack of confirmation for the rest might be due to the contamination by the nearby H II emission regions as well as due to the inaccurate positioning of the long slit on these objects. In addition, we search the $Chandra$ Observatory archival data for the X-ray counterparts to the optically identified candidates. We find positional coincidence with only three SNR candidates, SNR1, SNR2, and SNR8. The spectrum of SNR2 yields a shock temperature of 10.8 keV with an ionization timescale of 1.6 $\times$ 10$^{10}$ s cm$^{-3}$ indicating a relatively young remnant in an early Sedov phase which is not supported by our optical wavelength analysis. Given the high luminosity of 10$^{39}$ erg s$^{-1}$ and the characteristics of the X-ray spectrum, we favor an Ultra Luminous X-ray Source interpretation for this source associated with an SNR. We calculate an X-ray flux upper limit of 9.0 $\times$ $10^{-15}$ erg cm$^{-2}$ s$^{-1}$ for the rest of the SNRs including spectroscopically identified SNR3 and SNR5.