Warm gas accretion onto the Galaxy

Abstract

We present evidence that the accretion of warm gas onto the Galaxy today is at least as important as cold gas accretion. For more than a decade, the source of the bright H-alpha emission (up to 750 mR) along the Magellanic Stream has remained a mystery. We present a hydrodynamical model that explains the known properties of the H-alpha emission and provides new insights on the lifetime of the Stream clouds. The upstream clouds are gradually disrupted due to their interaction with the hot halo gas. The clouds that follow plough into gas ablated from the upstream clouds, leading to shock ionisation at the leading edges of the downstream clouds. Since the following clouds also experience ablation, and weaker H-alpha (100-200 mR) is quite extensive, a disruptive cascade must be operating along much of the Stream. In order to light up much of the Stream as observed, it must have a small angle of attack (~20 deg) to the halo, and this may already find support in new HI observations. Another prediction is that the Balmer ratio will be substantially enhanced due to the slow shock. We find that the clouds are evolving on timescales of 100-200 Myr, such that the Stream must be replenished by the Magellanic Clouds at a fairly constant rate (>0.1 Msun/yr). The ablated material falls onto the Galaxy as a warm drizzle; diffuse ionized gas at 10^4 K is an important constituent of galactic accretion. We consider the stability of HI clouds falling towards the Galactic disk and show that most of these must break down into smaller fragments that become partially ionized. The Galactic halo is expected to have huge numbers of smaller neutral and ionized fragments. When the ionized component is accounted for, the rate of gas accretion is ~0.4 Msun/yr, roughly twice the rate deduced from HI observations alone.

14 pages, 6 figures. "The Galaxy Disk in Cosmological Context," Copenhagen, IAU Symp 254, 241; published version with figures available at http://www.physics.usyd.edu.au/~jbh/share/Bland-Hawthorn-IAU254.pdf

References in corpus (21)

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• Radio observational constraints on Galactic 3D-emission models
• On the mass-to-light ratio of the local Galactic disc and the optical luminosity of the Galaxy
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• The Source of Ionization along the Magellanic Stream
• Smith's Cloud: A High-velocity Cloud Colliding with the Milky Way
• Missing Baryons and the Warm-Hot Intergalactic Medium
• The evolution of the galaxy Red Sequence in simulated clusters and groups
• Balmer-Dominated Shocks Revisited
• Ongoing Galactic Accretion: Simulations and Observations of Condensed Gas in Hot Halos
• Highly ionised plasma in the Large Magellanic Cloud: Evidence for outflows and a possible galactic wind
• Rising Jet-Inflated Bubbles in Clusters of Galaxies
• The scattered debris of the Magellanic Stream
• Morphology of flows and buoyant bubbles in the Virgo cluster

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• The SAMI Galaxy Survey: extraplanar gas, galactic winds, and their association with star formation history
• The Sagittarius dwarf galaxy: Where did all the gas go?
• Cold gas accretion by high-velocity clouds and their connection to QSO absorption-line systems
• The Magellanic System: the puzzle of the leading gas stream
• Physical effects on compact high-velocity clouds in the circumgalactic medium
• The high-velocity cloud complex Galactic Center Negative as seen by EBHIS and GASS. I. Cloud catalog and global properties
• The Smith Cloud: surviving a high speed transit of the Galactic disc
• Modelling mass distribution of the Milky Way galaxy using Gaia billion-star map
• The circum-galactic gas around cosmologically simulated disks