Optical‐Radio Alignment in Compact Steep‐Spectrum Radio Sources

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Abstract
We discuss Hubble Space Telescope (HST) WFPC2 observations through the broad red filter F702Wof 30 3CR sources from the Compact Steep Spectrum (CSS) radio source sample, and present 11 new HST/WFPC2 images through linear ramp filters (LRF), isolating either rest-frame [O II] 3727 Å or[O III] 5007 Å radiation. In nearly all the CSS galaxies, we find high surface brightness emission that is aligned with the radio axis. The strong nuclear PSF prevents us from detecting such aligned light at similar levels in most of the CSS quasars. However, a comparison between CSS galaxies and quasars with PSF signatures removed reveals no inconsistency with the viewing angle unification scheme. The alignment effect in CSS sources is not a strong function of redshift, and is seen over the entire redshift range of the sample (0.2 z 1.5). Our analysis of the LRF images and complementary KPNO spectroscopy reveals that the aligned light is predominantly emission-line gas. These observations demonstrate the existence of dense gas in the host galaxies of CSS sources, strongly interacting with the expanding radio sources. Assuming such a shock interaction, cooling-time arguments suggest that lobe expansion speeds of 1000 km s-1 and ambient densities of ~1 cm-3 are consistent with the observed offset between the leading edge of the radio lobe and the optical line emission.