A Mini X-Ray Survey of Sub-DLAs; Searching for AGNs Formed in Protogalaxies

TL;DR

This study uses Chandra X-ray observations of 21 quasars with sub-DLAs to investigate the presence of AGNs, finding potential X-ray sources near some sub-DLAs that could indicate active galactic nuclei in protogalaxies.

Contribution

First X-ray survey targeting sub-DLAs to identify potential AGNs, providing evidence for AGN activity in early galaxy stages.

Findings

Possible X-ray emission near 6 sub-DLAs suggests AGNs.

Estimated X-ray luminosities are consistent with galactic nuclei.

Further observations needed to confirm AGN presence.

Abstract

A significant fraction of the sub-damped Lyman-alpha (sub-DLA) absorption systems in quasar spectra appear to be metal-rich, many with even super-solar element abundances. This raises the question whether some sub-DLAs may harbor active galactic nuclei (AGN) since supersolar metallicities are observed in AGN. Here we investigate this question based on a mini-survey of 21 quasars known to contain sub-DLAs in their spectra. The X-ray observations were performed with the Chandra X-ray Observatory. In cases of no detection we estimated upper limits of the X-ray luminosities of possible AGNs at the redshifts of the sub-DLAs. In six cases we find possible X-ray emission within ~ 1 arcsec of the background quasar consistent with the presence of a nearby X-ray source. If these nearby X-ray sources are at the redshifts of the sub-DLAs, their estimated 0.2-10 keV luminosities range between 0.8 x 10^{44}h^{-2} and 4.2 x 10^{44}h^{-2} erg s^{-1}, thus ruling out a normal late-type galaxy origin, and suggesting that the emission originates in a galactic nucleus near the center of a protogalaxy. The projected distances of these possible nearby X-ray sources from the background quasars lie in the range of 3-7 h^{-1} kpc, consistent with our hypothesis that they represent AGNs centered on the sub-DLAs. Deeper follow-up X-ray and optical observations are required to confirm the marginal detections of X-rays from these sub-DLA galaxies.