EXTREMELY INEFFICIENT STAR FORMATION IN THE OUTER DISKS OF NEARBY GALAXIES
- Department of Astronomy, Radio Astronomy Laboratory, University of California, Berkeley, CA 94720 (United States)
- Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg (Germany)
- Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB (United Kingdom)
- Observatories of the Carnegie Institution of Washington, Pasadena, CA 91101 (United States)
We combine data from The H I Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (H I) and far-ultraviolet (FUV) emission outside the optical radius (r{sub 25}) in 17 spiral and 5 dwarf galaxies. In this regime, H I is likely to represent most of the interstellar medium (ISM) and FUV emission to trace recent star formation with little bias due to extinction, so that the two quantities closely trace the underlying relationship between gas and star formation rate (SFR). The azimuthally averaged H I and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the H I profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15'' resolution) between local FUV and H I intensities; near r{sub 25} this correlation is quite strong, in fact stronger than anywhere inside r{sub 25} (where H I is not a good tracer for the bulk of the ISM), and shows a decline toward larger radii. The star formation efficiency (SFE)-defined as the ratio of FUV/H I and thus the inverse of the gas depletion time-decreases with galactocentric radius across the outer disks, though much shallower than across the optical disks. On average, we find the gas depletion times to be well above a Hubble time ({approx}10{sup 11} yr). We observe a clear relationship between FUV/H I and H I column in the outer disks, with the SFE increasing with increasing H I column. Despite observing systematic variations in FUV/H I, we find no clear evidence for step-function-type star formation thresholds, though we emphasize that it may not be realistic to expect them. When compared with results from inside r{sub 25}, we find outer disk star formation to be distinct in several ways: it is extremely inefficient (depletion times of many Hubble times which are also long compared to either the free fall or orbital timescale) with column densities and SFRs lower than found anywhere inside the optical disks. It appears that the H I column is one of the key environmental factors-perhaps the key factor-in setting the SFR in outer galaxy disks.
- OSTI ID:
- 21443207
- Journal Information:
- Astronomical Journal (New York, N.Y. Online), Vol. 140, Issue 5; Other Information: DOI: 10.1088/0004-6256/140/5/1194; ISSN 1538-3881
- Country of Publication:
- United States
- Language:
- English
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