On the incidence of weak and strong Mg II absorbers towards the Flat and Steep Spectrum Radio quasars

The incidence rate of Mg II absorbers per unit redshift path (dN/dz) towards quasar sightlines has been used to probe the interaction between quasar jets and surrounding gas clouds. Studies using core- and lobe-dominated samples found a higher dN/dz for strong Mg II absorbers (rest equivalent width, W_r(2796) >= 1.0 A) in velocity offsets ranging from 5000 km/s < beta * c < 60000 km/s (with beta = v/c), toward core-dominated sources. In this study, we applied a stringent spectral index criterion: alpha_radio < -0.7 for steep-spectrum radio quasars (SSRQs) and alpha_radio > -0.3 for flat-spectrum radio quasars (FSRQs). Using this, we assembled the largest sample to date: 441 FSRQs and 464 SSRQs with suitable optical spectra to study both strong absorbers and weak (0.3 A < W_r(2796) < 1.0 A) Mg II absorbers. We conducted a detailed comparison of absorbers' incidence rate and offset velocity distributions. Our main findings are: (i) For both weak and strong absorbers, we found no significant excess in dN/dz towards FSRQ compared to SSRQ sightlines. (ii) The dN/dbeta distribution of Mg II absorbers along FSRQs and SSRQs are statistically similar. (iii) The cumulative distribution of weak Mg II absorbers is slightly lower for beta < 0.3, but shows an excess at higher beta. This suggests that, while intrinsic Mg II absorber abundance is comparable along both sightlines, FSRQs' more aligned relativistic jets cluster weak absorbers at high beta, consistent with the scenario of jet-driven acceleration of smaller gas clumps.