Optical CCD imaging of the supernova remnant CTA 1

Deep H alpha + [NII], [NII], [SII] and [OIII] CCD images have been obtained from the high galactic latitude supernova remnant CTA 1. The filamentary m orphology in the light of the [OIII] emission is pow clearly seen in the fu ll field of the remnant. Contrary to the [OIII] morphology, the remnant app ears less sharply defined in the light of the H alpha + [NII], [NII] and [S II]. The morphological differences between the [OIII] and H alpha + [NII] e mission lines and the presence of a number of filaments with projected leng ths ranging from 0.5 to several pc suggest an inhomogeneous insterstellar m edium. The presence of incomplete recombination zones is supported by the v ariations seen in the [OIII]/H-beta ratio which ranges from 5-20. A low dis persion spectrum taken in the south of CTA 1 indicates surface brightness v alues of H alpha, [NII] and [SII] of similar to 13, 14 and 18 in units of 1 0(-17) erg s(-1) cm(-2) arcsec(-2) respectively, while the sulfur line rati o suggests an electron density of similar to 200 cm(-3) Rough upper limits to the preshock cloud density are of the order of similar to 1 nuclei cm(-3 ). The strong [OIII] emission is indicative of shock velocities higher than 100kms(-1) while in certain areas of the remnant where the H alpha emissio n dominates lower velocities are possible. It is the first time that signif icant optical emission is detected in the northwest area of the remnant, wh ere diffuse radio emission is mainly seen. The small scale filaments detect ed there and their flux suggest that we observe shocked material, most like ly associated to CTA 1. Three more distinct filamentary structures are dete cted in the south-west area, outside of the main emission shell. Their leng ths range from 1'.5 to 6'.5. The spatial locations of the newly detected em ission features suggest an angular shock radius of 59'.