Cold gas accretion in galaxies

Renzo Sancisi*, Filippo Fraternali, Tom Oosterloo, Thijs van der Hulst

*Corresponding author for this work

Research output: Contribution to journalReview articlepeer-review

355 Citations (Scopus)

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 0.2 M(circle dot) year(-1). In order to reach the accretion rates needed to sustain the observed star formation (approximate to 1M(circle dot) year(-1)), additional infall of large amounts of gas from the IGM seems to be required.

Original languageEnglish
Pages (from-to)189-223
Number of pages35
JournalAstronomy and Astrophysics Review
Volume15
Issue number3
DOIs
Publication statusPublished - Jun-2008

Keywords

  • Galaxies
  • neutral hydrogen
  • accretion
  • extra-planar gas
  • interactions
  • mergers
  • HIGH-VELOCITY CLOUDS
  • LOPSIDED SPIRAL GALAXIES
  • NEUTRAL HYDROGEN OBSERVATIONS
  • LOCAL GROUP ANALOGS
  • FAST ALPHA SURVEY
  • EXTRA-PLANAR GAS
  • H-I OBSERVATIONS
  • HIGH-LATITUDE HI
  • DISK GALAXIES
  • DWARF GALAXY

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