Searching for HI around MHONGOOSE Galaxies via Spectral Stacking

TL;DR

This study uses spectral stacking to search for low-density neutral hydrogen around nearby galaxies, finding limited emission outside galactic disks and highlighting detection challenges for future telescopes.

Contribution

It introduces an advanced spectral stacking method combined with a line finder to probe faint HI signals in the CGM and IGM of galaxies.

Findings

Detected minimal HI emission outside galaxy disks

Achieved a column density sensitivity of ~10^{17} cm^{-2}

Indicated future detection of neutral CGM/IGM may be difficult

Abstract

The observed star formation rates of galaxies in the Local Universe suggests that they are replenishing their gas reservoir across cosmic time. Cosmological simulations predict that this accretion of fresh gas can occur in a hot or a cold mode, yet the existence of low column density ($\sim10^{17}$ cm$^{-2}$) neutral atomic hydrogen (HI) tracing the cold mode has not been unambiguously confirmed by observations. We present the application of unconstrained spectral stacking to attempt to detect the emission from this HI in the Circum-Galactic Medium (CGM) and Inter-Galactic Medium (IGM) of 6 nearby star forming galaxies from the MHONGOOSE sample for which full-depth observations are available. Our stacking procedure consists of a standard spectral stacking algorithm coupled with a one-dimensional spectral line finder designed to extract reliable signal close to the noise level. In agreement with previous studies, we found that the amount of signal detected outside the HI disk is much smaller than implied by simulations. Furthermore, the column density limit that we achieve via stacking ($\sim10^{17}$ cm$^{-2}$) suggests that direct detection of the neutral CGM/IGM component might be challenging in the future, even with the next generation of radio telescopes.