21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies

TL;DR

This paper proposes using 21cm hydrogen absorption to map atomic gas in nuclear disks around black holes, offering a new way to study gas distribution and measure cosmological distances in active galactic nuclei.

Contribution

It introduces a novel method for detecting and imaging atomic hydrogen in galactic nuclei via 21cm absorption, complementing maser observations and enabling distance measurements.

Findings

Potential optical depth of ~1 in NGC 4258 for high-resolution imaging.

Mean optical depth of ~0.1 when spreading absorption over rotation velocity.

Spectroscopic searches can identify inclined gaseous disks not seen in masers.

Abstract

The clumpy maser disks observed in some galactic nuclei mark the outskirts of the accretion disk that fuels the central black hole and provide a potential site of nuclear star formation. Unfortunately, most of the gas in maser disks is currently not being probed; large maser gains favor paths that are characterized by a small velocity gradient and require rare edge-on orientations of the disk. Here we propose a method for mapping the atomic hydrogen distribution in nuclear disks through its 21cm absorption against the radio continuum glow around the central black hole. In NGC 4258, the 21cm optical depth may approach unity for high angular-resolution (VLBI) imaging of coherent clumps which are dominated by thermal broadening and have the column density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm absorption over the full rotation velocity width of the material in front of the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic searches for the 21cm absorption feature in other galaxies can be used to identify the large population of inclined gaseous disks which are not masing in our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps within these disks can in turn be used to measure cosmological distances.