Cold gas accretion in galaxies |
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| Authors: | Renzo Sancisi Filippo Fraternali Tom Oosterloo Thijs van der Hulst |
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| Institution: | (1) Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy;(2) Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands;(3) Astronomy Department, Bologna University, Via Ranzani 1, 40127 Bologna, Italy;(4) ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands |
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| Abstract: | Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing
or recent accretion: (1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. This suggests ongoing minor mergers and recent arrival of external gas. It may be regarded,
therefore, as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material
infall as the stellar streams observed in the halos of our galaxy and M 31. (2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is undoubtedly produced by galactic fountains,
it is likely that a part of it is of extragalactic origin. Also the Milky Way has extra-planar gas complexes: the Intermediate-
and High-Velocity Clouds (IVCs and HVCs). (3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed
as the origin. (4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here
recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging
of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The new gas could be added to the halo
or be deposited in the outer parts of galaxies and form reservoirs for replenishing the inner parts and feeding star formation.
The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean “visible”
accretion rate of cold gas in galaxies of at least  . In order to reach the accretion rates needed to sustain the observed star formation ( ), additional infall of large amounts of gas from the IGM seems to be required. |
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| Keywords: | Galaxies Neutral hydrogen Accretion Extra-planar gas Interactions Mergers |
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