21 cm absorption by compact hydrogen discs around black holes in radio-loud nuclei ofgalaxies

Abstract

The clumpy maser discs observed in some galactic nuclei mark the outskirts of theaccretion disc that fuels the central black hole and provide a potential site of nuclear starformation. Unfortunately, most of the gas in maser discs is currently not beingprobed; large maser gains favor paths that are characterized by a small velocitygradient and require rare edge-on orientations of the disc. Here we propose amethod for mapping the atomic hydrogen distribution in nuclear discs throughits 21 cm absorption against the radio continuum glow around the central blackhole. In NGC 4258, the 21 cm optical depth may approach unity for high angularresolution (VLBI) imaging of coherent clumps which are dominated by thermalbroadening and have the column density inferred from x-ray absorption data,∼1023 cm−2. Spreading the 21 cm absorption over the full rotation velocity width of thematerial in front of the narrow radio jets gives a mean optical depth of∼0.1. Spectroscopic searches for the 21 cm absorption feature in other galaxies can be used toidentify the large population of inclined gaseous discs which are not masing in ourdirection. Follow-up imaging of 21 cm silhouettes of accelerating clumps within these discscan in turn be used to measure cosmological distances.