The morphology of the emission-line region of compact steep-spectrum radio sources

We present the results of HST narrowband imaging of 11 compact steep-spectrum (CSS) radio sources. Five of them (3C 48, 3C 147, 3C303.1, 3C 277.1, and 4C 12.50) were observed as part of a dedicated pointed program of deep line imaging, at the redshifted wavelength of the [O III] lambda 5007 emission line. For six additional sources (3C 49, 3C 93.1, 3C 138, 3C 268.3, 3C305.1, and 3C343.1) snapshot images ([O III] lambda 5007 or [O II] lambda 3727) were taken from the HST archive. In all but one of the targets (3C 49) line emission has been detected, and only in the case of 3C 138 is it unresolved at the HST resolution. Three distinct components are found in the CSS emission-line morphologies: (1) compact nuclear emission regions whose size is less than a few kpc, (2) bright emission spatially related to the radio structure, and (3) faint emission that extends well beyond the radio source. A large fraction of the line emission (between 30% and 90%) originates within less than similar to3 kpc from the nucleus, as in the case of Seyfert and extended radio galaxies. In four out five of the sources for which deep observations are available, the line emission extends well beyond the size of the radio source but along the radio axis. Only in the case of the largest radio source (3C 277.3) is no emission beyond the radio lobes detected. Structures of similar surface brightness would have not been seen in the snapshot images. These emission-line structures extend to scales of 10 to 30 kpc and cover a projected angle, when seen from the nucleus, of similar to 30 degrees to 110 degrees, and they indicate that the nuclear illumination is anisotropic. Photon-counting arguments also support this interpretation. In agreement with the AGN unified scheme, only the CSSs with broad line regions show a strong unresolved continuum source in the HST images. In six objects the radio emission extends over more than 1 and the HST resolution is such that a detailed comparison can be made between radio and optical morphologies. In these cases the line emission has an elongated structure, linking the nucleus to the radio lobes, possibly tracing the path of the invisible radio jets. Nevertheless, the emission-line morphologies do not show the bow shocks at the extremities of the radio lobes one would expect if they are sources whose expansion is frustrated by a dense external medium. Our data favor the alternative model in which CSSs are the young phase of the large-size radio sources. When pointed pure continuum images are available, there appears to be no alignment between radio and continuum emission, which contradicts previous suggestions based on broadband HST imaging. We suggest that these broadband images are in most cases heavily contaminated by line emission, producing a spurious apparent alignment.