H i 21cm emission from the subdamped Lyman-α absorber atz = 0.0063 towards PG 1216+069

Abstract

We present H I 21 cm emission observations of the z ∼ 0.006 32 subdamped Lyman-α absorber (sub-DLA) towards PG 1216+069 made using the Arecibo Telescope and the Very Large Array (VLA). The Arecibo H I 21cm spectrum corresponds to an H I mass of ∼3.2 × 10⁷ M_⊙, two orders of magnitude smaller than that of a typical spiral galaxy. This is surprising since in the local Universe the cross-section for absorption at high H I column densities is expected to be dominated by spirals. The H I 21cm emission detected in the VLA spectral cube has a low signal-to-noise ratio, and represents only half the total flux seen at Arecibo. Emission from three other sources is detected in the VLA observations, with only one of these sources having an optical counterpart. This group of H I sources appears to be part of complex ‘W’, believed to lie in the background of the Virgo cluster. While several H I cloud complexes have been found in and around the Virgo cluster, it is unclear whether the ram pressure and galaxy harassment processes that are believed to be responsible for the creation of such clouds in a cluster environment are relevant at the location of this cloud complex. The extremely low metallicity of the gas, ∼1/40 solar, also makes it unlikely that the sub-DLA consists of material that has been stripped from a galaxy. Thus, while our results have significantly improved our understanding of the host of this sub-DLA, the origin of the gas cloud remains a mystery.