Exploring quasar evolution with proximate molecular absorbers: Insights from the kinematics of highly ionized nitrogen⋆

Abstract

We investigate the presence and kinematics of N V absorption proximate to high redshift quasars with both strong H_{2 and H I absorption at the quasar redshift. Our spectroscopic observations with X-shooter at the VLT reveal a 70% detection rate of N V (in 9 out of 13 quasars with 2.5 < z < 3.3), remarkably higher than the ∼10% detection rate in intervening damped Lyman-α systems and the ∼30% rate observed within a few thousand km s^{−1 of the source in the general quasar population. While many N V components lie within the velocity range of the neutral gas, the kinematic profiles of high-ionization species appear decoupled from those of low-ionization species, with the former extending over much larger velocity ranges, particularly toward bluer velocities (up to several thousand km s−1). We also observe significant variations in the N V to Si IV ratio, which we attribute to varying ionization conditions, with a clear velocity-dependent trend: blueshifted N V components systematically exhibit higher ionization parameters compared to those near the quasar’s systemic redshift. Furthermore, the most redshifted systems relative to the quasar show no evidence of N V absorption. The results suggest that proximate H2 absorption systems are in critical stages of quasar evolution, during which the quasar remains embedded in a rich molecular environment. Redshifted systems likely trace infalling gas, potentially associated with mergers, prior to the onset of outflows. Such outflows, as traced by N V, may eventually reach or even carry out neutral and molecular gas. This stage would correspond to proximate H2 systems located around or blueshifted relative to the quasar’s systemic redshift. Finally, the only case in our sample featuring highly blueshifted neutral gas (−2000 km s−1) shows no evidence of an association with the quasar. Our findings highlight the need to account for the ionization state when defining a velocity threshold to distinguish quasar-associated systems from intervening ones.}}