Papers › Project AMIGA: A Minimal Covering Factor for Optically Thick Circumgalactic Gas Around...
Project AMIGA: A Minimal Covering Factor for Optically Thick Circumgalactic Gas Around the Andromeda Galaxy
J. Christopher Howk, Christopher B. Wotta, Michelle A. Berg, Nicolas Lehner, Felix J. Lockman, Zachary Hafen, D. J. Pisano, Claude-Andre Faucher-Giguere, Bart P. Wakker, J. Xavier Prochaska, Spencer A. Wolfe, Joseph Ribaudo, Kathleen A. Barger, Lauren Corlies, Andrew J. Fox, Puragra Guhathakurta, Edward B. Jenkins, Jason Kalirai, John M. O'Meara, Molly S. Peeples, Kyle R. Stewart, Jay Strader
The archive published only this paper's code-link row. Authors, date and abstract are from arXiv's metadata (CC0), read from the Kaggle arXiv metadata snapshot of 2026-09-12 where its title matched the archive's; the title is the archive's.
We present a deep search for HI 21-cm emission from the gaseous halo of Messier 31 as part of Project AMIGA, a large program Hubble Space Telescope program to study the circumgalactic medium of the Andromeda galaxy. Our observations with the Robert C. Byrd Green Bank Telesope target sight lines to 48 background AGNs, more than half of which have been observed in the ultraviolet with the Cosmic Origins Spectrograph, with impact parameters 25 ≲ρ≲330 kpc (0.1 ≲ρ/ Rᵥᵢᵣ ≲1.1). We do not detect any 21-cm emission toward these AGNs to limits of N(HI) ≈4 ×10¹⁷ cm⁻² (5σ, per 2 kpc diameter beam). This column density corresponds to an optical depth of ∼2.5 at the Lyman limit, thus our observations overlap with absorption line studies of Lyman limit systems at higher redshift. Our non-detections place a limit on the covering factor of such optically-thick gas around M31 to f_c < 0.051 (at 90\% confidence) for ρ≤Rᵥᵢᵣ. While individual clouds have previously been found in the region between M31 and M33, the covering factor of strongly optically-thick gas is quite small. Our upper limits on the covering factor are consistent with expectations from recent cosmological "zoom" simulations. Recent COS-Halos ultraviolet measurements of \HI\ absorption about an ensemble of galaxies at z ≈0.2 show significantly higher covering factors within ρ≲0.5 Rᵥᵢᵣ at the same N(H I), although the metal ion-to-H I ratios appear to be consistent with those seen in M31.
Code
Repository list and official/mentioned flags are the archive's, frozen 2025-07-28. Reachability, where shown, is from one Syntology probe window (2026-09-16 to 2026-09-18); repositories not probed show nothing. GitHub stars are not tracked.
Code Syntology ran Syntology
Not run by Syntology. Nothing on this page verifies that the listed code works.
Results from the paper archive 2025-07-28
No leaderboard rows for this paper in the archive.
Report a problem or propose a change · a person checks every report against the paper or source before anything changes; decisions are listed on /corrections