Project AMIGA: A Minimal Covering Factor for Optically Thick Circumgalactic Gas around the Andromeda Galaxy

Journal Article · Sun Sep 10 04:00:00 EDT 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA87B4· OSTI ID:22875812

Howk, J. Christopher; Wotta, Christopher B.; Berg, Michelle A.; Lehner, Nicolas ^[1]; Lockman, Felix J. ^[2]; Hafen, Zachary; Faucher-Giguère, Claude-André ^[3]; Pisano, D. J.; Wolfe, Spencer A. ^[4]; Wakker, Bart P. ^[5]; Prochaska, J. Xavier; Guhathakurta, Puragra ^[6]; Ribaudo, Joseph ^[7]; Barger, Kathleen A. ^[8]; Corlies, Lauren; Kalirai, Jason; Peeples, Molly S. ^[9]; Fox, Andrew J. ^[10]; Jenkins, Edward B. ^[11]; O’Meara, John M. ^[12] ; others, and

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1. Department of Physics, University of Notre Dame, Notre Dame, IN (United States)
2. Green Bank Observatory, Green Bank, WV (United States)
3. Department of Physics and Astronomy and Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, Evanston, IL (United States)
4. Department of Physics and Astronomy, West Virginia University, Morgantown, WV (United States)
5. Department of Astronomy, University of Wisconsin-Madison, Madison, WI (United States)
6. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA (United States)
7. Department of Physics, Utica College, Utica, NY (United States)
8. Department of Physics and Astronomy, Texas Christian University, Fort Worth, TX (United States)
9. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD (United States)
10. Space Telescope Science Institute, Baltimore, MD (United States)
11. Princeton University Observatory, Princeton, NJ (United States)
12. Department of Chemistry and Physics, Saint Michael’s College, Colchester, VT (United States)

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≲ρ≲340 kpc (0.1≲ρ/R{sub vir}≲1.1). We do not detect any 21 cm emission toward these AGNs to limits of N(H I)≈4×10{sup 17} cm{sup −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{sub c}<0.051 (at 90% confidence) for ρ⩽R{sub 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 ρ≲0.5R{sub vir} at the same N(H I), although the metal ion-to-H I ratios appear to be consistent with those seen in M31.

• OSTI ID: 22875812
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 846; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
COMPUTERIZED SIMULATION
DENSITY
EMISSION
GALAXIES
IMPACT PARAMETER
METALS
QUASARS
RED SHIFT
SPACE
TELESCOPES
ULTRAVIOLET RADIATION