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 H \alpha + [ 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 H \alpha + [ 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 ] /H \beta ratios .
This remnant ’ s [ O iii ] flux suggests shock velocities into the interstellar “ clouds ” between 100 and 120 km s ^ { -1 } , while the [ O iii ] absence in the other two remnants indicates slower shock velocities .
For all remnants , the [ S ii ] \lambda \lambda 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 H \alpha emission has been measured to be between 3.0 to 15.2 \times 10 ^ { -17 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } , 3.2 \times 10 ^ { -17 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } and between 6.5 to 16.8 \times 10 ^ { -17 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } for G 54.4 - 0.3 , G 59.8 + 1.2 and G 126.2 + 1.6 , respectively .