Volume 548, December 2012
|Number of page(s)||15|
|Published online||13 November 2012|
Dust temperature and CO → H2 conversion factor variations in the SFR-M∗ plane⋆
Max-Planck-Institut für Extraterrestrische Physik (MPE),
Postfach 1312, 85741
e-mail: email@example.com; firstname.lastname@example.org
2 Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
3 Department of Physics and Institute of Theoretical and Computational Physics, University of Crete, 71002 Heraklion, Greece
4 Chercheur Associé, Observatoire de Paris, 75014 Paris, France
5 IESL/Foundation for Research and Technology-Hellas, PO Box 1527, 71110 Heraklion, Greece
6 Universität Wien, Institut für Astronomie, Türkenschanzstraße 17, 1180 Wien, Österreich
7 UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
8 Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Ave., Cambridge CB3 0HE, UK
9 Dipartimento di Astronomia, Universita di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
10 INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
11 School of Physics and Astronomy, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
Accepted: 1 October 2012
Deep Herschel PACS/SPIRE imaging and 12CO(2−1) line luminosities from the IRAM Plateau de Bure Interferometer are combined for a sample of 17 galaxies at z > 1 from the GOODS-N field. The sample includes galaxies both on and above the main sequence (MS) traced by star-forming galaxies in the SFR-M∗ plane. The far-infrared data are used to derive dust masses, Mdust, following the Draine & Li (2007, ApJ, 657, 810) models. Combined with an empirical prescription for the dependence of the gas-to-dust ratio on metallicity (δGDR(μ0)), the CO luminosities and Mdust values are used to derive for each galaxy the CO-to-H2 conversion factor, αCO. Like in the local Universe, the value of αCO is a factor of ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We additionally uncover a relation between αCO and dust temperature (Tdust; αCO decreasing with increasing Tdust) as obtained from modified blackbody fits to the far-infrared data. While the absolute normalization of the αCO(Tdust) relation is uncertain, the global trend is robust against possible systematic biases in the determination of Mdust, δGDR(μ0) or metallicity. Although we cannot formally distinguish between a step and a smooth evolution of αCO with the dust temperature, we can unambiguously conclude that in galaxies of near-solar metallicity, a critical value of Tdust = 30 K can be used to determine whether the appropriate αCO is closer to the “starburst” value (1.0 M⊙ (K km s-1 pc2)-1, when Tdust > 30 K) or closer to the Galactic value (4.35 M⊙ (K km s-1 pc2)-1, when Tdust < 30 K). This indicator has the great advantage of being less subjective than visual morphological classifications of mergers/SFGs, which can be difficult at high z because of the clumpy nature of SFGs. Using Tdust to select the appropriate αCO is also more indicative of ISM conditions than a fixed LIR criterion. In the absence of far-infrared data, the offset of a galaxy from the star formation main sequence (i.e., Δlog (SSFR)MS = log [SSFR(galaxy)/SSFRMS(M∗,z)]) can be used to identify galaxies requiring the use of an αCO conversion factor lower than the Galactic value (i.e., when Δlog (SSFR)MS ≳ 0.3 dex).
Key words: galaxies: evolution / infrared: galaxies / galaxies: starburst
Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Based also on observations carried out by the Herschel space observatory. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
© ESO, 2012
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