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

R. Cuellar, P. Noterdaeme, S. Balashev, S. López, V. D'Odorico, J. -K. Krogager

First listed 2025-01-22 | Last updated 2025-01-22

Abstract

We investigate the presence and kinematics of NV absorption proximate to high redshift quasars selected upon the presence of strong $H_{2}$ and HI absorption at the quasar redshift. Our spectroscopic observations with X-shooter at the VLT reveal a 70% detection rate of NV (9 of 13 quasars with 2.5 < z < 3.3), remarkably higher than the 10% detection rate in intervening DLA systems and the 30% rate observed within a few thousand km/s of the source in the general quasar population. While many NV 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 towards bluer velocities. We also observe significant variations in the NV/SiIV, which we attribute to varying ionization conditions, with a velocity-dependent trend: blueshifted NV 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 NV absorption. The results suggest that proximate $H_{2}$ absorption systems select critical stages of quasar evolution, during which the quasar remains embedded in a rich molecular environment. Redshifted systems trace infalling gas, potentially associated with mergers, preceding the onset of outflows. Such outflows may reach or even carry out neutral and molecular gas.This latter stage would correspond to proximate $H_{2}$ systems located around or blueshifted relative to the quasar's systemic z. Finally, the only case in our sample featuring highly blueshifted neutral gas 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.

Short digest

X-shooter spectroscopy of 13 z≈2.5–3.3 quasars with proximate H2+HI absorbers finds NV in 9/13 (70%), far exceeding intervening DLA (~10%) and generic proximate (~30%) rates. High-ionization lines show blue-extended, broader kinematics decoupled from the neutral/low-ionized profiles, and NV/SiIV varies with velocity such that blueshifted components require higher ionization parameters. Redshifted systems lack NV and are interpreted as infall pre-feedback, while blueshifted proximate H2 systems trace outflow phases that can reach neutral/molecular gas. The authors argue that ionization state—not a fixed velocity threshold—should define “associated” versus “intervening” classifications.

Key figures to inspect

• Figure 1 (J0019−0137): Compare NV 1238 and SiIV 1393 AOD profiles to the low-ionization velocity window; note the extended blue wing and component-by-component NV/SiIV ratios versus the 5/50/95% low-ionization percentiles.
• Figure 2: Contrast KS19 background with AGN spectra at 100 kpc vs 1 Mpc and inspect how HI shielding at N_HI≈10^18–10^21 cm⁻² alters flux above the N→N4+ edge, setting conditions for strong NV.
• Figure 3: Cloudy grids of high-ion column densities versus ionization parameter for different stopping N_HI; identify the U range that reproduces observed NV while tracking sensitivity to the assumed median metallicity.
• Figure 4: NV/SiIV color map in the U–N_HI plane with observed component crosses; use it to locate blueshifted components at higher U and gauge ~0.2 dex column-density uncertainties in the inferred parameter space.