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Star Formation Thresholds in Galactic Disks

Martin, Crystal L. and Kennicutt, Robert C., Jr. (2001) Star Formation Thresholds in Galactic Disks. Astrophysical Journal, 555 (1). pp. 301-321. ISSN 0004-637X. doi:10.1086/321452.

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We report the first results of a detailed study of the star formation law in a sample of 32 nearby spiral galaxies with well-measured rotation curves, H I and H_2 (as traced by CO) surface density profiles, and new Hα CCD photometry. In this paper we present an atlas of Hα images and radial surface brightness profiles and describe a surface density threshold in the star formation law. Prominent breaks in the Hα surface brightness profiles are identified in nearly all of the actively star-forming disks, confirming previous claims of star formation thresholds based on lower quality data. We measure the ratio of the gas density to the critical density for local gravitational stability at the threshold radii. The outer threshold radii observed in Sab-Sdm galaxies are in general agreement with those expected from the Toomre Q stability criterion, confirming earlier work, but with a significant variation that appears to be weakly correlated with galaxy type. Such a trend could plausibly reflect variations in the relative contribution of the stellar disk to the instability of the gas disk across this range of galaxy types. Among disks with subcritical gas surface densities, and outside the threshold radius in star-forming disks, the number of isolated H II regions increases as the gas surface density approaches the critical density. At the thresholds, the gas surface densities span a wide range, and the atomic/molecular gas fraction is highest in the disks having the lowest total gas surface density. The simple Toomre condition fails to account for the active star formation in the inner disks of low-mass spirals such as NGC 2403 and M33. An alternative stability criterion based on the shear in the disk provides a better description of these disks but is a less accurate indicator of the outer edges of star-forming disks than the Toomre criterion. These results strongly support the view that the formation of gravitationally bound interstellar clouds regulates the onset of widespread star formation, at least in the outer regions of galactic disks.

Item Type:Article
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URLURL TypeDescription Paper
Kennicutt, Robert C., Jr.0000-0001-5448-1821
Additional Information:© 2001. The American Astronomical Society. Received 2000 December 21; accepted 2001 March 5. We thank Rene Walterbos for making the KPNO 36 inch data available and Jean Turner and Pat Crosthwaite for providing a CO map of NGC 5236 in advance of publication. We thank Bruce Elmegreen, Sharda Jogee, and Nick Scoville for their comments on a draft of the paper and enlightening discussions. The comments of an anonymous referee were also appreciated. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. C. L. M. acknowledges support from a Sherman Fairchild fellowship. R. C. K. gratefully acknowledges the support of NSF grant AST-9900789.
Funding AgencyGrant Number
Sherman Fairchild FoundationUNSPECIFIED
Subject Keywords:galaxies: evolution; galaxies: ISM; galaxies: stellar content; stars: formation
Issue or Number:1
Record Number:CaltechAUTHORS:20170408-172009063
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Official Citation:Crystal L. Martin and Robert C. Kennicutt, Jr. 2001 ApJ 555 301
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76327
Deposited By: 1Science Import
Deposited On:13 Mar 2018 19:47
Last Modified:15 Nov 2021 16:59

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