Connecting X-ray absorption and 21 cm neutral hydrogen absorption in obscured radio AGN

Abstract

Many radio galaxies show the presence of dense and dusty gas near the active\nnucleus. This can be traced by both 21cm HI absorption and soft X-ray\nabsorption, offering new insight into the physical nature of the circumnuclear\nmedium of these distant galaxies. To better understand this relationship, we\ninvestigate soft X-ray absorption as an indicator for the detection of\nassociated HI absorption, as part of preparation for the First Large Absorption\nSurvey in HI (FLASH) to be undertaken with the Australian Square Kilometre\nArray Pathfinder (ASKAP). We present the results of our pilot study using the\nBoolardy Engineering Test Array, a precursor to ASKAP, to search for new\nabsorption detections in radio sources brighter than 1 Jy that also feature\nsoft X-ray absorption. Based on this pilot survey, we detected HI absorption\ntowards the radio source PKS 1657-298 at a redshift of z = 0.42. This source\nalso features the highest X-ray absorption ratio of our pilot sample by a\nfactor of 3, which is consistent with our general findings that X-ray\nabsorption predicates the presence of dense neutral gas. By comparing the X-ray\nproperties of AGN with and without detection of HI absorption at radio\nwavelengths, we find that X-ray hardness ratio and HI absorption optical depth\nare correlated at a statistical significance of 4.71{\\sigma}. We conclude by\nconsidering the impact of these findings on future radio and X-ray absorption\nstudies.\n