Observatoire de Paris, LERMA CNRS UMR 8112, 61 Av. de
2 Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
3 Laboratoire d’Astrophysique de Bordeaux, UMR 5804, Université Bordeaux I, BP 89, 33270 Floirac, France
4 Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
5 Departamento de Astrofisica, Centro de Astrobiologia (CSIC/INTA), Torrejón de Ardoz, 28850 Madrid, Spain
Accepted: 26 November 2012
After new observations of 39 galaxies at z ~ 0.6–1.0 obtained at the IRAM 30-m telescope, we present our full CO line survey covering the redshift range 0.2 < z < 1. Our aim is to determine the driving factors accounting for the steep decline in the star formation rate during this epoch. We study both the gas fraction, defined as Mgas/(Mgas + Mstar), and the star formation efficiency (SFE) defined by the ratio between far-infrared luminosity and molecular gas mass (LFIR/M(H2)), i.e. a measure for the inverse of the gas depletion time. The sources are selected to be ultra-luminous infrared galaxies (ULIRGs), with LFIR greater than 1012L⊙ and experiencing starbursts. When we adopt a standard ULIRG CO-to-H2 conversion factor, their molecular gas depletion time is less than 100 Myr. Our full survey has now filled the gap of CO observations in the 0.2 < z < 1 range covering almost half of cosmic history. The detection rate in the 0.6 < z < 1 interval is 38% (15 galaxies out of 39), compared to 60% for the 0.2 < z < 0.6 interval. The average CO luminosity is L′CO = 1.8 × 1010 Kkms-1 pc2, corresponding to an average H2 mass of 1.45 × 1010M⊙. From observation of 7 galaxies in both CO(2–1) and CO(4–3), a high gas excitation has been derived; together with the dust mass estimation, this supports the choice of our low ULIRG conversion factor between CO luminosity and H2 for our sample sources. We find that both the gas fraction and the SFE significantly increase with redshift, by factors of 3 ± 1 from z = 0 to 1, and therefore both quantities play an important role and complement each other in cosmic star formation evolution.
Key words: galaxies: high-redshift / galaxies: ISM / galaxies: starburst / radio lines: galaxies
Based on observations carried out with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).
Appendix is available in electronic form at http://www.aanda.org
Spectra are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A41
© ESO, 2013