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Galaxy Evolution and Star Formation Efficiency in the Last Half of the Universe
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SF2A 2007
J. Bouvier, A. Chalabaev, C. Charbonnel (eds)
GALAXY EVOLUTION AND STAR FORMATION EFFICIENCY IN THE LAST HALF
OF THE UNIVERSE
F. Combes1, S. Garc??a-Burillo2 , J. Braine3, E. Schinnerer4, F. Walter4 , L. Colina5 and M. Gerin6
Abstract. We present the results of a CO(1-0) emission survey with the IRAM 30m of 30 galaxies at
moderate redshift (z ～ 0.2-0.6) to explore galaxy evolution and in particular the star formation efficiency,
in the redshift range filling the gap between local and very high-z objects. Our detection rate is about 50%.
One of the bright objects was mapped at high resolution with the IRAM interferometer, and about 50%
of the total emission found in the 27 arcsec (97 kpc) single dish beam is recovered by the interferometer,
suggesting the presence of extended emission. The FIR-to-CO luminosity ratio is enhanced, following the
increasing trend observed between local and high-z ultra-luminous starbursts.
1 Introduction
It has been established that the star formation in the universe was one or two orders of magnitude larger in
the past, had a maximum around z = 2, and then again faded towards zero beyond z = 6 ? 8 (e.g. Bouwens
Illingworth 2006). When the universe was half of its current age, at z = 0.7 ? 0.8, the star formation rate
was ten times larger than today. Observations also tend to show that the star forming objects were not the
same, in the first half of the Hubble time. Starbursts occur first in the bigger galaxies, that are then quenched,
while in the second half of the universe, star formation occurs in smaller objects, and this phenomenon is called
downsizing.
This strong evolution of star-forming galaxies is not really understood, although it is suspected that galaxy
interactions and mergers are certainly one of the main factors. The frequency of mergers with time has been
estimated, through the perturbed morphologies, the asymmetry index and the numb
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