paper

Parsec-scale HI absorption structure in a low-redshift galaxy seen against a Compact Symmetric Object

arXiv:1607.05995 · doi:10.1093/mnras/stw1786

Abstract

We present global VLBI observations of the 21-cm transition of atomic hydrogen seen in absorption against the radio source J0855+5751. The foreground absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at = 0.026. As the background source is heavily resolved by VLBI, the data allow us to map the properties of the foreground HI gas with a spatial resolution of 2pc. The absorbing gas corresponds to a single coherent structure with an extent 35pc, but we also detect significant and coherent variations, including a change in the HI optical depth by a factor of five across a distance of 6pc. The large size of the structure provides support for the Heiles & Troland model of the ISM, as well as its applicability to external galaxies. The large variations in HI optical depth also suggest that caution should be applied when interpreting measurements from radio-detected DLAs. In addition, the distorted appearance of the background radio source is indicative of a strong jet-cloud interaction in its host galaxy. We have measured its redshift ( = 0.54186) using optical spectroscopy on the William Herschel Telescope and this confirms that J0855+5751 is a FRII radio source with a physical extent of 1kpc and supports the previous identification of this source as a Compact Symmetric Object. These sources often show absorption associated with the host galaxy and we suggest that both HI and OH should be searched for in J0855+5751.

arXiv comment14 pages and 10 figures. Accepted for publication in MNRAS

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