Deep optical observations of the supernova remnants G 126.2+1.6, G 59.8+1.2 and G 54.4-0.3

Optical CCD imaging and spectroscopic observations of three supernova remnants are presented. Optical emission from G 54.4-0.3 and G 59.8+1.2 is detected for the first time, while the first flux calibrated CCD images of the supernova remnant G 126.2+1.6 were performed in the optical emission lines of Halpha+[N II], [O III] and [S II]. A mixture of filamentary and diffuse structures is observed in G 54.4-0.3 and G 59.8+1.2, mainly in Halpha+[N II], while the deep optical images of G 126.2+1.6 reveal several new filamentary and diffuse structures inside the extent of the remnant as defined by its known radio emission. In all cases, the radio emission is found to be well correlated with the optical filaments. [O III] emission was not detected at G 54.4-0.3 and G 59.8+1.2 while in G 126.2+1.6, significant morphological differences between the low and medium ionization images are present suggesting incomplete shock structures. Deep long-slit spectra were taken at different positions of the remnants. Both the flux calibrated images and the long-slit spectra clearly show that the emission originates from shock-heated gas, while some spectra of G 126.2+1.6 are characterized by large [O III]/Hbeta ratios. This remnants [O III] flux suggests shock velocities into the interstellar clouds between 100 and 120 km/s, while the [O III] absence in the other two remnants indicates slower shock velocities. For all remnants, the [S II]6716/6731 ratio indicates electron densities below 600 cm^{-3} with particularly low densities for G 54.4-0.3 (below 50 cm^{-3}). Finally, the Halpha emission has been measured to be between 3.0 to 15.2x10^{-17} erg/s/cm^2/ arcsec^2, 3.2x10^{-17} erg/s/cm^2/ arcsec^2 and between 6.5 to 16.8x10^{-17} erg/s/cm^2/ arcsec^2 for G 54.4-0.3, G 59.8+1.2 and G 126.2+1.6, respectively.