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Description
After new observations of 39 galaxies at z=0.6-1.0 obtained at the IRAM 30m 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 (L_FIR_/M(H_2_), i.e. a measure for the inverse of the gas depletion time. The sources are selected to be ultra-luminous infrared galaxies (ULIRGs), with L_FIR_ greater than 10^12^L_{sun}_, and experiencing starbursts. When we adopt a standard ULIRG CO-to-H_2_ conversion factor, their molecular gas depletion time is less than 100Myr. 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_=1.8x10^10^K.km/s.pc^2^, corresponding to an average H_2_ mass of 1.45x10^10^M_{sun}_. 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 H_2_ 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.
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