Outflowing material in the compact steep-spectrum source quasar 3C 48: evidence of jet-cloud interaction?

Abstract

We report on the detection of a z_abs= 0.3654 associated absorption-line system in the ultraviolet spectrum of the compact steep-spectrum source (CSS) quasar 3C 48. The absorbing material is blueshifted with respect to the quasar emission-line redshift, z_em= 0.3700, suggesting an outflow velocity of ∼1000 km s^-1. We detect absorption lines over a range of ionization states from Lγβ, Lγγ, C IV, N IV, S VI to O VI and possibly O IV and Ne VIII. The kinematical properties of the absorption-line system are similar to the blueshifted emission-line gas seen in [O III]λ5007, which is believed to have interacted with the radio jet. We study the properties of the absorbing material using cloudy and find that photoionization models with solar abundance ratios (with overall metallicity in the range 0.1 ≤Z/Z_⊙≤ 1.3) are enough to explain the observed column densities of all the species except Ne viii, detection of which requires confirmation. Because the cooling and recombination time for the gas is ∼10⁵ yr, the consistency with the photoionization models suggests that any possible interaction of absorbing material with the jet must have taken place before ∼10⁵ yr. The abundance ratio of nitrogen to carbon is close to solar values, unlike in the case of most quasars, especially at high redshifts, which have supersolar values. We observed 3C 48 with the Giant Metrewave Radio Telescope (GMRT) to search for redshifted 21-cm H i absorption. We did not detect any significant features in our spectra and estimate the 3σ upper limit to the optical depth to be in the range 0.001-0.003. However, due to the diffuse nature of the radio source, optical depths as high as 0.1 towards individual knots or compact components cannot be ruled out.