|Annu. Rev. Astron. Astrophys. 2012. 50:
Copyright © 2012 by Annual Reviews. All rights reserved
We have begun to create maps of the multiphase gas in galaxy halos and determine its multiple origins and evolution. The relationship between halo gas and galaxy feedback and accretion mechanisms can be further clarified through additional studies in several areas. For instance, satellite gas is stripped into galaxy halos, but the percentage of halo gas that can be attributed to minor (or potentially past major) mergers remains to be determined. The IGM is rich with baryons, particularly near star forming galaxies, but our understanding of how the IGM accretes onto galaxies is heavily based on simulation results. Feedback mechanisms impact galaxy halos, though how effective they are at transporting mass out of the disk and the primary mechanism responsible remains uncertain. Finally, although spiral galaxies have abundant fueling resources, the continuity of these resources and the physical mechanisms that lead to the conversion of the incoming fuel into molecular gas have yet to be fully determined.
On the observational front, the next generation of telescopes and instruments will greatly aid our understanding of the flow of baryons in the Universe. Our knowledge of the link between the cold and warm gas detected in emission and absorption will become stronger with future HI maps with the Square Kilometer Array and the SKA pathfinders. Three-dimensional maps of the warm gas could be made with large area, high kinematic resolution integral field units (IFUs), and this would show the flow of warm gas relative to the disk. The hot halo gas remains an elusive component beyond the disk-halo region. Highly sensitive x-ray telescopes are needed for a direct detection, but in the meantime we can infer the presence of hot gas with broad warm-hot absorbers in the ultraviolet (e.g., Savage et al. 2011). The motion of the diffuse, hot halo component can potentially be calculated from the properties of the cold and warm clouds embedded within it. The characterization of the multi-phase halo gas in relation to a wide range of spiral galaxy properties (e.g., total and baryonic mass, SFR, environment) will provide significant insight into its origin.
Stars in galaxy halos will be mapped with future large optical surveys, such as PanSTARRS and LSST. This will provide more information on the satellite accretion history and stars with a standard intrinsic color can be used for dust detection in the Galactic halo through reddening effects. Observing the halo stars for absorption lines from halo gas in the optical and ultraviolet will provide tighter distance constraints, and the lines can be used to examine the metallicity and ionization conditions of the halo gas. With additional estimates of the metallicity, halo gas origin scenarios can be further constrained (in particular the level of feedback), and the dust can be characterized.
On the theoretical front, two of the most important steps are to continue to improve the resolution of the simulations and to study the effect of the many relevant physical processes, both separately and in combination, in a cosmological setting. Current cosmological models are unable to reach the length scale of many of the observed small HI clouds in the halo and at the disk-halo interface, nor can they accurately resolve the interaction of the multiple phases of gas with one another. These problems can be partially alleviated in the near future with models that are limited to a small region in space and time; however, then the relationship of the halo gas to galactic and cosmological processes on large scales is lost. Until supercomputers become fast enough to cover the large dynamic range required, clever combinations of cosmological and local models can be used to interpret the broad range of observed halo properties and distinguish between various feedback and fueling mechanisms. Reproducing stellar metallicity distributions with different accretion scenarios will also be an important feature of future simulations. The current chemical evolution models of our Galaxy are largely semi-analytic (although see Few et al. 2012, Kobayashi & Nakasato 2011). Future state-of-the-art simulations can aim to track the metal flows in a galaxy, and through comparisons of observed and simulated stellar metallicities we can learn how feedback and accretion operate throughout time.
We thank our colleagues at Columbia for many useful discussions, in particular comments from Greg Bryan and Jacqueline van Gorkom. M. Putman thanks several hosts while working on the review, the International Centre for Radio Astronomy Research, the University of Sydney, and Arecibo Observatory. We thank George Heald, Tom Oosterloo, and Tobias Westmeier for providing HI data, Bart Wakker for providing HVC distance information and funding from NSF grants AST-1008134, AST-0904059 and the Luce Foundation. JEGP was supported by HST-HF-51295.01A, provided by NASA through a Hubble Fellowship grant from STScI, which is operated by AURA under NASA contract NAS5-26555.