A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES

Journal Article · Mon Aug 15 04:00:00 EDT 2011 · Astronomical Journal (New York, N.Y. Online)

DOI:https://doi.org/10.1088/0004-6256/142/2/37· OSTI ID:21583043

Schruba, Andreas; Walter, Fabian; Dumas, Gaelle; Sandstrom, Karin ^[1]; Leroy, Adam K ^[2]; Bigiel, Frank ^[3]; Brinks, Elias ^[4]; Kramer, Carsten ^[5]; Rosolowsky, Erik ^[6]; Schuster, Karl ^[7]; Usero, Antonio ^[8]; Weiss, Axel; Wiesemeyer, Helmut ^[9]

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1. Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg (Germany)
2. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 (United States)
3. Department of Astronomy, Radio Astronomy Laboratory, University of California, Berkeley, Berkeley, CA 94720 (United States)
4. Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB (United Kingdom)
5. IRAM, Avenida Divina Pastora 7, 18012 Granada (Spain)
6. Department of Physics and Astronomy, University of British Columbia Okanagan, 3333 University Way, Kelowna, BC V1V 1V7 (Canada)
7. IRAM, 300 rue de la Piscine, 38406 St. Martin d'Heres (France)
8. Observatorio Astronomico Nacional, C/Alfonso XII, 3, 28014, Madrid (Spain)
9. MPIfR, Auf dem Huegel 69, 53121 Bonn (Germany)

We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using 21 cm line atomic hydrogen (H I) data, mostly from THINGS, we predict the local mean CO velocity based on the mean H I velocity. By re-normalizing the CO velocity axis so that zero corresponds to the local mean H I velocity we are able to stack spectra coherently over large regions. This enables us to measure CO intensities with high significance as low as I{sub CO} {approx} 0.3 K km s{sup -1} ({Sigma}{sub H{sub 2}}{approx}1 M{sub sun} pc{sup -2}), an improvement of about one order of magnitude over previous studies. We detect CO out to galactocentric radii r{sub gal} {approx} r{sub 25} and find the CO radial profile to follow a remarkably uniform exponential decline with a scale length of {approx}0.2 r{sub 25}. Here we focus on stacking as a function of radius, comparing our sensitive CO profiles to matched profiles of H I, H{alpha}, far-UV (FUV), and Infrared (IR) emission at 24 {mu}m and 70 {mu}m. We observe a tight, roughly linear relationship between CO and IR intensity that does not show any notable break between regions that are dominated by molecular gas ({Sigma}{sub H{sub 2}}>{Sigma}{sub H{sub i}}) and those dominated by atomic gas ({Sigma}{sub H{sub 2}}<{Sigma}{sub H{sub i}}). We use combinations of FUV+24 {mu}m and H{alpha}+24 {mu}m to estimate the recent star formation rate (SFR) surface density, {Sigma}{sub SFR}, and find approximately linear relations between {Sigma}{sub SFR} and {Sigma}{sub H{sub 2}}. We interpret this as evidence of stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H{sub 2} ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relationships between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between atomic and molecular gas depends strongly on the total gas surface density and galactocentric radius. It must also depend on additional parameters. Our results reinforce and extend to lower surface densities, a picture in which star formation in galaxies can be separated into two processes: the assembly of star-forming molecular clouds and the formation of stars from H{sub 2}. The interplay between these processes yields a total gas-SFR relation with a changing slope, which has previously been observed and identified as a star formation threshold.

• OSTI ID: 21583043
• Journal Information: Astronomical Journal (New York, N.Y. Online), Journal Name: Astronomical Journal (New York, N.Y. Online) Journal Issue: 2 Vol. 142; ISSN 1538-3881
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ELEMENTS
EMISSION
EVOLUTION
GALACTIC EVOLUTION
GALAXIES
HYDROGEN
NONMETALS
STAR EVOLUTION
STARS
VELOCITY