Hunting for Metals Using XQ-100 Legacy Survey Composite Spectra

TL;DR

This study uses composite spectra from the XQ-100 survey to analyze NV absorption signals near quasars, confirming NV as a reliable indicator of intrinsic absorption systems and revealing ionization effects close to quasars.

Contribution

It demonstrates that NV absorption is a robust statistical tool for identifying intrinsic quasar absorption lines and explores the ionization state differences near quasars using multiple ion stacks.

Findings

NV absorption signal is significant within 5000 km/s of quasar redshift.

Gas near quasars shows a dearth of low-ionization species like MgII.

Ionization models explain the observed ionization state differences.

Abstract

We investigate the NV absorption signal along the line of sight of background quasars, in order to test the robustness of the use of this ion as criterion to select intrinsic (i.e. physically related to the quasar host galaxy) narrow absorption lines (NALs). We build composite spectra from a sample of $\sim$ 1000 CIV absorbers, covering the redshift range 2.55 < z < 4.73, identified in 100 individual sight lines from the XQ-100 Legacy Survey. We detect a statistical significant NV absorption signal only within 5000 km s$^{-1}$ of the systemic redshift, z$\rm_{em}$. This absorption trough is $\sim$ 15$\sigma$ when only CIV systems with N(CIV) > 10$^{14}$ cm$^{-2}$ are included in the composite spectrum. This result confirms that NV offers an excellent statistical tool to identify intrinsic systems. We exploit the stacks of 11 different ions to show that the gas in proximity to a quasar exhibits a considerably different ionization state with respect to gas in the transverse direction and intervening gas at large velocity separations from the continuum source. Indeed, we find a dearth of cool gas, as traced by low-ionization species and in particular by MgII, in the proximity of the quasar. We compare our findings with the predictions given by a range of Cloudy ionization models and find that they can be naturally explained by ionization effects of the quasar.