2000A&A...353..371M

Deep Hα+[NII], [NII], [SII] and [OIII] CCD images have been obtained from the high galactic latitude supernova remnant CTA 1. The filamentary morphology in the light of the [OIII] emission is now clearly seen in the full field of the remnant. Contrary to the [OIII] morphology, the remnant appears less sharply defined in the light of the Hα+[NII], [NII] and [SII]. The morphological differences between the [OIII] and Hα+[NII] emission lines and the presence of a number of filaments with projected lengths ranging from 0.5 to several pc suggest an inhomogeneous insterstellar medium. The presence of incomplete recombination zones is supported by the variations seen in the [OIII]/H_β ratio which ranges from 5-20. A low dispersion spectrum taken in the south of CTA 1 indicates surface brightness values of Hα, [NII] and [SII] of ∼ 13, 14 and 18 in units of 10^–17erg/s/cm²/arcsec² respectively, while the sulfur line ratio suggests an electron density of ∼200cm^–3. Rough upper limits to the preshock cloud density are of the order of ∼1nuclei/cm³. The strong [OIII] emission is indicative of shock velocities higher than 100km/s while in certain areas of the remnant where the Hα emission dominates lower velocities are possible. It is the first time that significant optical emission is detected in the northwest area of the remnant, where diffuse radio emission is mainly seen. The small scale filaments detected there and their flux suggest that we observe shocked material, most likely associated to CTA 1. Three more distinct filamentary structures are detected in the south-west area, outside of the main emission shell. Their lengths range from 1.5' to 6.5.' The spatial locations of the newly detected emission features suggest an angular shock radius of 59'.