Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 471, Issue 3, Pages 2952-2973Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stx1679
Keywords
methods: observational; galaxies: active; galaxies: ISM-galaxies: nuclei radio; lines: galaxies; X-rays: galaxies
Categories
Funding
- Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO) [CE110001020]
- Australian Government
- National Collaborative Research Infrastructure Strategy
- Government of Western Australia
- Science and Industry Endowment Fund
- National Aeronautics and Space Administration
- ESA
- NASA
- National Science Foundation
- Space Telescope Science Institute under U.S. Government [NAGW-2166]
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Many radio galaxies show the presence of dense and dusty gas near the active nucleus. This can be traced by both 21 cm HI absorption and soft X-ray absorption, offering new insight into the physical nature of the circumnuclear medium of these distant galaxies. To better understand this relationship, we investigate soft X-ray absorption as an indicator for the detection of associated HI absorption, as part of preparation for the First Large Absorption Survey in HI to be undertaken with the Australian Square Kilometre Array Pathfinder (ASKAP). We present the results of our pilot study using the Boolardy Engineering Test Array, a precursor to ASKAP, to search for new absorption detections in radio sources brighter than 1 Jy that also feature soft X-ray absorption. Based on this pilot survey, we detected HI absorption towards the radio source PKS 1657-298 at a redshift of z = 0.42. This source also features the highest X-ray absorption ratio of our pilot sample by a factor of 3, which is consistent with our general findings that X-ray absorption predicates the presence of dense neutral gas. By comparing the X-ray properties of active galactic nuclei with and without detection of HI absorption at radio wavelengths, we find that X-ray hardness ratio and HI absorption optical depth are correlated at a statistical significance of 4.71 sigma. We conclude by considering the impact of these findings on future radio and X-ray absorption studies.
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