Warm Gas and Ionizing Photons in the Local Group (ApJL version)

Abstract

Several lines of argument suggest that a large fraction of the baryons in the universe may be in the form of warm (T\sim 10^5-10^7 K) gas. In particular, loose groups of galaxies may contain substantial reservoirs of such gas. Observations of the cosmic microwave background by COBE place only weak constraints on such an intragroup medium within the Local Group. The idea of a Local Group corona dates back at least forty years (Kahn & Woltjer 1959). Here we show that gas at T\sim 2-3 X 10^6 K (the approximate virial temperature of the Local Group) -- extremely difficult to observe directly -- can in principle radiate a large enough flux of ionizing photons to produce detectable H alpha emission from embedded neutral clouds. However, additional constraints on the corona -- the most stringent being pulsar dispersion measures towards the Magellanic Clouds, and the timing mass -- rule out an intragroup medium whose ionizing flux dominates over the cosmic background or the major Local Group galaxies. A cosmologically significant coronal gas mass could remain invisible to H alpha observations. More massive galaxy groups could contain extensive coronae which are important for the baryon mass and produce a strong, local ionizing flux.

12 pages, 3 figures; to appear in ApJ Letters

References in corpus (4)

• The Cosmic Baryon Budget
• High Velocity Clouds: Building Blocks of the Local Group
• The baryon content of the Universe
• The Escape of Ionizing Photons from the Galaxy

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• X-ray Properties of Groups of Galaxies
• H_alpha Emission from High-Velocity Clouds and their Distances
• Massive Coronae of Galaxies
• The Source of Ionization along the Magellanic Stream
• X-Ray IGM in the Local Group
• The Planetary Nebulae Spectrograph: the green light for Galaxy Kinematics
• Fossil imprint of a powerful flare at the Galactic Centre along the Magellanic Stream
• The large-scale ionization cones in the Galaxy
• In search of cool flow accretion onto galaxies $-$ where does the disk gas end?
• A Tunable Lyot Filter at Prime Focus: a Method for Tracing Supercluster Scales at z ~ 1
• The Contribution of the Intergalactic Medium to Cosmic Microwave Background Anisotropies
• Constraints on a Local Group X-ray Halo
• Physical effects on compact high-velocity clouds in the circumgalactic medium
• Warm gas accretion onto the Galaxy
• Dissipative phenomena in extended-bodies interactions I: Methods Dwarf galaxies of the Local Group and their synthetic CMDs
• The kinetic Sunyaev-Zel'dovich effect from the diffuse gas in the Local Group
• The Kinetic Sunyaev-Zel'dovich effect of the Milky Way Halo
• Galaxies as Fluctuations in the Ionizing Background Radiation at Low Redshift
• Timing Mass of the Local Group