A search for supernova remnants in the nearby spiral galaxy M74 (NGC 628)

An optical search was carried out for supernova remnants (SNRs) in the Sc type nearby spiral galaxy M 74, using ground-based observations at the TUBITAK National Observatory (TUG, Antalya/Turkey) and the Special Astrophysics Observatory (SAO, Russia). Observations were supplemented by the spectral analysis of archived X-ray data from XMM-Newton and Chandra. The survey of M74 covered similar to 9 arcmin(2) with [S II], H alpha, and their continuum filters. Interference filter images of M 74 were obtained the with the 1.5 m Russian Turkish Telescope (RTT150) at TUG and spectral data taken with the 6 m Bolsoi Azimuthal Telescope (BTA) at SAO. The emission nebulae with continuum-subtracted line ratio values of [S II]lambda lambda 6716,6731 /H alpha >= 0.4 are identified as SNRs. Followup spectroscopy confirmed optical SNR identifications. We have identified nine new SNR candidates in M 74 with [S II]/H alpha >= 0.4 as the basic criterion. The [S II]/H alpha ratio ranges from 0.40 to 0.91 and H alpha intensities from 2.8 x 10(-15) erg cm(-2) s(-1) to 1.7 x 10(-14) erg cm(-2) s(-1). We also present spectral follow-up observations of these SNR candidates, however, we are able to spectrally confirm only three of them (SNR2, SNR3, and SNR5). The lack of confirmation for the rest might come from contamination by the nearby H II emission regions, as well as from the inaccurate positioning of the long slit on these objects. In addition, we searched the XMM-Newton and 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 x 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 calculated an X-ray flux upper limit of 9.0 x 10(-15) erg cm(-2) s(-1) for the rest of the SNRs, including spectroscopically identified SNR3 and SNR5.