2017ApJ...846..141H

We present a deep search for H I 21 cm emission from the gaseous halo of Messier 31 as part of Project AMIGA, a large Hubble Space Telescope program to study the circumgalactic medium of the Andromeda galaxy. Our observations with the Robert C. Byrd Green Bank Telescope 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 ρ340 kpc (0.1 ρ/R_vir 1.1). We do not detect any 21 cm emission toward these AGNs to limits of N(H I)~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 ρR_vir. Although 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 H I absorption about an ensemble of galaxies at z~0.2 show significantly higher covering factors within ρ0.5R_vir at the same N(H I), although the metal ion-to-H I ratios appear to be consistent with those seen in M31.