A powerful ultra steep spectrum radio galaxy having an extremely low excitation emission line spectrum

Abstract

In the course of our optical follow up of the Ooty sample of ultra-steep spectrum radio sources (USSRS), we have discovered an exceptionally interesting case of a classical double radio source of size ~14" and spectral index α = - 1.2 associated with a red galaxy at a modest redshift of z = 0.477. The galaxy exhibits gaseous wisps broadly coincident with the radio lobes. The optical spectrum is dominated by an emission line showing a very large rest-frame equivalent width of 350 Å, a spatial extent of ~10" at the position angle of the radio structure, and a tilt with a maximum rest-frame velocity difference of ~760 km s^-1. These properties, as well as the presence of the optical wisps suggest that the system is a product of a merger involving a gas-rich disk and a massive elliptical. Even though the object is relatively bright (V= 21.3), additional spectral lines could only be detected after a persistent, deep integration. These very faint features, identified with Hβ, [O III] λ5007, and H, K and G-band, have lead to an unambiguous interpretation of the dominant emission line as being [O II] λ3727. The detection of faint lines may have considerable relevance to some cases of radio galaxies for which extremely large redshifts (z > 3) have been inferred recently. The red colour of the galaxy, and the equivalent widths of the stellar absorption features suggest that the continuum of 1411-192 is dominated by an old stellar population. The physical conditions of the gas are similar to those of LINERS, with shock-heating as a possible ionization mechanism. The association of an "ultra-soil" emission line spectrum with such a powerful radio galaxy is intriguing.