COMPARISON OF Halpha AND UV STAR FORMATION RATES IN THE LOCAL VOLUME: SYSTEMATIC DISCREPANCIES FOR DWARF GALAXIES
- Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)
- Departmento de Astrofisica, Universidad Complutense de Madrid, Madrid 28040 (Spain)
- Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)
- Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA (United Kingdom)
- NOAO, 950 North Cherry Ave., Tucson, AZ 85719 (United States)
- Department of Astronomy, University of Massachusetts, Amherst, MA 01003 (United States)
- Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States)
- Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071 (United States)
- Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States)
- Division of Astronomy and Astrophysics, University of California, Los Angeles, CA 90095-1562 (United States)
- Department of Astronomy, University of Minnesota, Minneapolis, MN 55455 (United States)
Using a complete sample of approx300 star-forming galaxies within 11 Mpc of the Milky Way, we evaluate the consistency between star formation rates (SFRs) inferred from the far ultraviolet (FUV) non-ionizing continuum and Halpha nebular emission, assuming standard conversion recipes in which the SFR scales linearly with luminosity at a given wavelength. Our analysis probes SFRs over 5 orders of magnitude, down to ultra-low activities on the order of approx10{sup -4} M{sub sun} yr{sup -1}. The data are drawn from the 11 Mpc Halpha and Ultraviolet Galaxy Survey (11HUGS), which has obtained Halpha fluxes from ground-based narrowband imaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies (SFR approx 1 M{sub sun} yr{sup -1}), our results are consistent with previous work which has shown that FUV SFRs tend to be lower than Halpha SFRs before accounting for internal dust attenuation, but that there is relative consistency between the two tracers after proper corrections are applied. However, a puzzle is encountered at the faint end of the luminosity function. As lower luminosity dwarf galaxies, roughly less active than the Small Magellanic Cloud, are examined, Halpha tends to increasingly underpredict the total SFR relative to the FUV. The trend is evident prior to corrections for dust attenuation, which affects the FUV more than the nebular Halpha emission, so this general conclusion is robust to the effects of dust. Although past studies have suggested similar trends, this is the first time this effect is probed with a statistical sample for galaxies with SFR approx< 0.1 M{sub sun} yr{sup -1}. By SFR approx 0.003 M{sub sun} yr{sup -1}, the average Halpha-to-FUV flux ratio is lower than expected by a factor of two, and at the lowest SFRs probed, the ratio exhibits an order of magnitude discrepancy for the handful of galaxies that remain in the sample. A range of standard explanations does not appear to be able to fully account for the magnitude of the systematic. Some recent work has argued for a stellar initial mass function which is deficient in high-mass stars in dwarf and low surface brightness galaxies, and we also consider this scenario. Under the assumption that the FUV traces the SFR in dwarf galaxies more robustly, the prescription relating Halpha luminosity to SFR is re-calibrated for use in the low SFR regime when FUV data are not available.
- OSTI ID:
- 21378164
- Journal Information:
- Astrophysical Journal, Vol. 706, Issue 1; Other Information: DOI: 10.1088/0004-637X/706/1/599; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
DUST-CORRECTED STAR FORMATION RATES OF GALAXIES. I. COMBINATIONS OF Halpha AND INFRARED TRACERS
THE SINS SURVEY: SINFONI INTEGRAL FIELD SPECTROSCOPY OF z approx 2 STAR-FORMING GALAXIES