Discovery of optical candidate supernova remnants in Sagittarius

During an [O III] survey of planetary nebulae, we identified a region in Sagittarius containing several candidate supernova remants (SNRs) and obtained deep optical narrow-band images and spectra to explore their nature. We obtained images of the area of interest by acquiring observations in the emission lines of H alpha + [N II], [S II] and [O III]. The resulting mosaic covers an area of 1.4 degrees x 1.0 degrees, where both filamentary and diffuse emission was discovered, suggesting that there is more than one SNR in the area. Deep long-slit spectra were also taken of eight different regions. Both the flux-calibrated images and the spectra show that the emission from the filamentary structures originates from shock-heated gas, while the photo-ionization mechanism is responsible for the diffuse emission. Part of the optical emission is found to be correlated with the radio at 4850 MHz suggesting that they are related, while the infrared emission found in the area at 12 mu m and 22 mu m marginally correlates with the optical. The presence of the [O III] emission line in one of the candidate SNRs implies that the shock velocities in the interstellar "clouds" are between 120 km s(-1) and 200 km s(-1), while its absence in the other candidate SNRs indicates that the shock velocities there are slower. For all candidate remnants, the [S II] lambda lambda 6716/6731 ratio indicates that the electron densities are below 240 cm(-3), while the H alpha emission is measured to be between 0.6 and 41 x 10(-17) erg s(-1) cm(-2) arcsec(-2). The existence of eight pulsars within 1.5 degrees of the center of the candidate SNRs also implies that there are many SNRs in the area as well as that the detected optical emission could be part of a number of supernovae explosions.