HI 21cm emission from the sub-damped Lyman-alpha absorber at z=0.0063 towards PG1216+069

TL;DR

This study investigates the HI 21cm emission from a low-redshift sub-DLA, revealing a surprisingly small HI mass and complex environment, challenging typical galaxy association assumptions.

Contribution

First detailed HI 21cm emission analysis of a sub-DLA at very low redshift, providing new insights into its mass, environment, and origin.

Findings

HI mass is about 3.2x10^7 solar masses, much smaller than typical spirals.

Detected HI emission suggests a complex environment with multiple sources.

The gas has very low metallicity (~1/40 solar), indicating a non-galactic origin.

Abstract

We present HI 21cm emission observations of the z ~ 0.00632 sub-damped Lyman-alpha absorber (sub-DLA) towards PG1216+069 made using the Arecibo Telescope and the Very Large Array (VLA). The Arecibo 21cm spectrum corresponds to an HI mass of ~ 3.2x10^7 solar masses, 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 HI column densities is expected to be dominated by spirals. The 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 HI sources appears to be part of complex "W", believed to lie in the background of the Virgo cluster. While several HI 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