Deep optical CCD images of the supernova remnant G 17.4–2.3 were obtained and faint emission has been discovered .
The images , taken in the emission lines of H \alpha + [ N ii ] , [ S ii ] and [ O iii ] , reveal filamentary structures in the east , south–east area , while diffuse emission in the south and central regions of the remnant is also present .
The radio emission in the same area is found to be well correlated with the brightest optical filament .
The flux calibrated images suggest that the optical filamentary emission originates from shock-heated gas ( [ S ii ] /H \alpha > 0.4 ) , while the diffuse emission seems to originate from an H ii region ( [ S ii ] /H \alpha < 0.3 ) .
Furthermore , deep long–slit spectra were taken at the bright [ O iii ] filament and clearly show that the emission originates from shock heated gas .
The [ O iii ] flux suggests shock velocities into the interstellar `` clouds ” greater than 100 km s ^ { -1 } , while the [ S ii ] \lambda 6716 / 6731 ratio indicates electron densities \sim 240 cm ^ { -3 } .
Finally , the H \alpha emission has been measured to be between 7 to 20 \times 10 ^ { -17 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } .