Dust temperature and CO  →  H₂ conversion factor variations in the SFR-M_∗ plane^⋆

¹ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
e-mail: magnelli@mpe.mpg.de; amelie@mpe.mpg.de
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

Received: 23 July 2012
Accepted: 1 October 2012

Abstract

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, M_dust, 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 (δ_GDR(μ₀)), the CO luminosities and M_dust values are used to derive for each galaxy the CO-to-H₂ conversion factor, α_CO. Like in the local Universe, the value of α_CO is a factor of  ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We additionally uncover a relation between α_CO and dust temperature (T_dust; α_CO decreasing with increasing T_dust) as obtained from modified blackbody fits to the far-infrared data. While the absolute normalization of the α_CO(T_dust) relation is uncertain, the global trend is robust against possible systematic biases in the determination of M_dust, δ_GDR(μ₀) or metallicity. Although we cannot formally distinguish between a step and a smooth evolution of α_CO with the dust temperature, we can unambiguously conclude that in galaxies of near-solar metallicity, a critical value of T_dust = 30 K can be used to determine whether the appropriate α_CO is closer to the “starburst” value (1.0 M_⊙ (K km s^-1 pc²)^-1, when T_dust > 30 K) or closer to the Galactic value (4.35 M_⊙ (K km s^-1 pc²)^-1, when T_dust < 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 T_dust to select the appropriate α_CO is also more indicative of ISM conditions than a fixed L_IR 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)/SSFR_MS(M_∗,z)]) can be used to identify galaxies requiring the use of an α_CO conversion factor lower than the Galactic value (i.e., when Δlog (SSFR)_MS ≳ 0.3 dex).