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The CO-to-H_2 Conversion Factor and Dust-to-gas Ratio on Kiloparsec Scales in Nearby Galaxies

Sandstrom, K. M. and Schruba, A. and Appleton, P. and Armus, L. (2013) The CO-to-H_2 Conversion Factor and Dust-to-gas Ratio on Kiloparsec Scales in Nearby Galaxies. Astrophysical Journal, 777 (1). Art. No. 5. ISSN 0004-637X. doi:10.1088/0004-637X/777/1/5.

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We present ~kiloparsec spatial resolution maps of the CO-to-H_2 conversion factor (α_(CO)) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for α_(CO) and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H I column density to solve for both αCO and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, ^(12)CO J = (2-1) maps from the IRAM 30 m large program HERACLES, and H I 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our α_(CO) results on the more typically used ^(12)CO J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for α_(CO) and the DGR. On average, α_(CO) = 3.1 M_☉ pc^(–2) (K km s^(–1))^(–1) for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of α_(CO) as a function of galactocentric radius. However, most galaxies exhibit a lower α_(CO) value in the central kiloparsec—a factor of ~2 below the galaxy mean, on average. In some cases, the central α_(CO) value can be factors of 5-10 below the standard Milky Way (MW) value of α_(CO,MW) = 4.4 M_☉ pc^(–2) (K km s^(–1))^(–1). While for α_(CO) we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate α_(CO) for studies of nearby galaxies.

Item Type:Article
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URLURL TypeDescription Paper
Sandstrom, K. M.0000-0002-4378-8534
Appleton, P.0000-0002-7607-8766
Armus, L.0000-0003-3498-2973
Additional Information:© 2013 American Astronomical Society. Received 2012 December 4; accepted 2013 July 9; published 2013 October 8. We thank the referee for useful comments that helped to improve the quality of the manuscript. K.S. thanks R. Shetty, S. Glover, R. Klessen, D. Narayanan, and A. Stutz for helpful conversations. The authors thank R. Feldmann and N. Gnedin for helpful comments regarding the comparison of our measurements to their simulations. K.S. is supported by a Marie Curie International Incoming Fellowship. A.D.B. wishes to acknowledge partial support from a CAREER grant NSF-AST0955836, NASA-JPL1373858, and from a Research Corporation for Science Advancement Cottrell Scholar award. This work is based on observations made with Herschel. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM(France);MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT(Spain). SPIRE has been developed by a consortium of institutes led by Cardiff University (UK) and including University of Lethbridge (Canada); NAOC (China); CEA, LAM(France); IFSI, University of Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, University of Sussex (UK); and Caltech, JPL, NHSC, University of Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). HIPE is a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIRE consortia. 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. This research has made use of NASA’s Astrophysics Data System Bibliographic Services.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Marie Curie FellowshipUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Bundesministerium für Verkehr, Innovation und Technologie (BMVIT)UNSPECIFIED
Commissariat à l’Energie Atomique (CEA)UNSPECIFIED
Deutsches Zentrum für Luft- und Raumfahrt (DLR)UNSPECIFIED
Agenzia Spaziale Italiana (ASI)UNSPECIFIED
Canadian Space Agency (CSA)UNSPECIFIED
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
Istituto Nazionale di Astrofisica (INAF)UNSPECIFIED
Ministerio de Ciencia e Innovación (MCINN)UNSPECIFIED
Swedish National Space Board (SNSB)UNSPECIFIED
Science and Technology Facilities Council (STFC)UNSPECIFIED
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:dust, extinction; galaxies: ISM; infrared: ISM; ISM: molecules
Issue or Number:1
Record Number:CaltechAUTHORS:20131121-154024608
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Official Citation:The CO-to-H2 Conversion Factor and Dust-to-gas Ratio on Kiloparsec Scales in Nearby Galaxies K. M. Sandstrom et al. 2013 ApJ 777 5
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42635
Deposited By: Ruth Sustaita
Deposited On:22 Nov 2013 18:01
Last Modified:10 Nov 2021 16:26

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