# Completing the Hydrogen Census in the Circumgalactic Medium at z~0

## TL;DR

This paper discusses how combining 21-cm HI radio observations with UV absorption data can complete the census of circumgalactic gas at z~0, revealing its distribution, kinematics, and role in galaxy fueling.

## Contribution

It introduces the use of advanced radio telescopes to map HI emission in the CGM, complementing absorption studies and providing a more complete understanding of gas accretion.

## Key findings

- Radio telescopes can map low-density HI in the CGM.
- Combining radio and UV data constrains gas inflow onto galaxies.
- The approach advances understanding of galaxy fueling processes.

## Abstract

Over the past decade, Lyman-alpha and metal line absorption observations have established the ubiquity of a gas-rich circumgalactic medium (CGM) around star-forming galaxies at z~0.2 potentially tracing half of the missing baryonic mass within galaxy halos. Unfortunately, these observations only provide a statistical measure of the gas in the CGM and do not constrain the spatial distribution and kinematics of the gas. Furthermore, we have limited sensitivity to Lyman-alpha at z~0 with existing instruments. As such, we remain ignorant of how this gas may flow from the CGM onto the disks of galaxies where it can fuel ongoing star-formation in the present day. Fortunately, 21-cm HI observations with radio telescopes can map HI emission providing both spatial and kinematic information for the CGM in galaxies at z=0. Observations with phased array feeds, radio cameras, on single-dish telescopes yield unmatched surface brightness sensitivity and survey speed. These observations can complete the census of HI in the CGM below N(HI)<10^17 cm^-2 and constrain how gas accretion is proceeding in the local universe, particularly when used in concert with UV absorption line data.