We use the first systematic data sets of CO molecular line emission in z ∼ 1-3 normal star-forming galaxies (SFGs) for a comparison of the dependence of galaxy-averaged star formation rates on molecular gas masses at low and high redshifts, and in different galactic environments. Although the current high-z samples are still small and biased towards the luminous and massive tail of the actively star-forming 'main-sequence', a fairly clear picture is emerging. Independent of whether galaxy-integrated quantities or surface densities are considered, low-and high-z SFG populations appear to follow similar molecular gas-star formation relations with slopes 1.1 to 1.2, over three orders of magnitude in gas mass or surface density. The gas-depletion time-scale in these SFGs grows from 0.5 Gyr at z ∼ 2 to 1.5 Gyr at z ∼ 0. The average corresponds to a fairly low star formation efficiency of 2 per cent per dynamical time. Because star formation depletion times are significantly smaller than the Hubble time at all redshifts sampled, star formation rates and gas fractions are set by the balance between gas accretion from the halo and stellar feedback.In contrast, very luminous and ultraluminous, gas-rich major mergers at both low and high z produce on average four to 10 times more far-infrared luminosity per unit gas mass. We show that only some fraction of this difference can be explained by uncertainties in gas mass or luminosity estimators; much of it must be intrinsic. A possible explanation is a top-heavy stellar mass function in the merging systems but the most likely interpretation is that the star formation relation is driven by global dynamical effects. For a given mass, the more compact merger systems produce stars more rapidly because their gas clouds are more compressed with shorter dynamical times, so that they churn more quickly through the available gas reservoir than the typical normal disc galaxies. When the dependence on galactic dynamical Based on observations with the Plateau de Bure millimetre interferometre, operated by the Institute for Radio Astronomy in the Millimetre Range (IRAM), which is funded by a partnership of INSU/CNRS (France), MPG (Germany) and IGN (Spain).

2Stars form from cold molecular interstellar gas. Since this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly 1,2 . Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more gas rich. Molecular gas observations in the distant Universe have so far been largely restricted to very luminous, rare objects, including mergers and quasars 3,4,5 . Here we report the results of a systematic survey of molecular gas in samples of typical massive star forming galaxies at ~1.2 and 2.3, when the Universe was 40% and 24% of its current age. Our measurements provide empirical evidence that distant star forming galaxies indeed were gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z= 2.3 and z=1.2 is ~44% and 34%, three to ten times higher than in today's massive spiral galaxies 6 . The slow decrease between z~2 and 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.Direct observations of molecular gas in galaxies as a function of cosmic epoch are required to understand how galaxies have turned their gas into stars. To explore the evolution of cold gas fractions, we selected two samples of star forming galaxies (SFGs) spanning similar ranges in stellar mass and star formation rates: one at redshift z~1.2 (t o~5 .5 Gyr after the Big Bang) and the other at z~2.3 (t 0~3 Gyr). With recent improvements in instrumental sensitivity we can now sample the massive tail of the 3 typical, or 'normal' star forming galaxies in this epoch (Supplementary Information, section 1). These 'main-sequence' galaxies are believed to have high (40-70%) duty cycles of star formation, and most are probably not 'starbursts' in a brief period of activity, such as major dissipative mergers 1,2 . Figure 1 displays the source-integrated spectra in the CO J=3-2 transition for 19 of the galaxies observed, 10 at z~2 and 9 at z~1. For 14 SFGs we have solid (>4σ) detections in both redshift ranges, and for the first time for z>2 SFGs. In 5 galaxies the emission is marginally or not detected, or may be continuum rather than line emission. Table 1 summarizes the observed and derived galaxy properties.For EGS1305123 (z=1.12) and EGS1207881 (z=1.17) we also obtained high quality spatially resolved maps with FWHM resolutions of 0.65" and 1". Figure 2c shows the integrated CO 3-2 emission in EGS1305123, superposed on an optical image from the Hubble Space Telescope (HST). This map resolves for the first time the cold molecular gas distribution in a clearly non-merging high-z SFG. This system looks like a scaled-up version (in terms of star formation rate and gas mass) of normal z~0 gas rich disk galaxies. In the optical images EGS1305123 is a nearly face-on, large spiral disk. The clumpy CO emissio...

We have studied the properties of giant star forming clumps in five z~2 starforming disks with deep SINFONI AO spectroscopy at the ESO VLT 1 . The clumps reside in disk regions where the Toomre Q-parameter is below unity, consistent with their being bound and having formed from gravitational instability. Broad Hα/ [NII] line wings demonstrate that the clumps are launching sites of powerful outflows. The inferred outflow rates are comparable to or exceed the star formation rates, in one case by a factor of eight. Typical clumps may lose a fraction of their original gas by feedback in a few hundred million years, allowing them to migrate into the center. inferred gas phase oxygen abundance are broadly consistent with inside-out growing disks, and/or with inward migration of the clumps..

Using the cosmological baryonic accretion rate and normal star formation (SF) efficiencies, we present a very simple model for star-forming galaxies that accounts for the mass and redshift dependences of the star formation rate (SFR)-mass and Tully-Fisher (TF) relations from z ∼ 2 to the present. The time evolution follows from the fact that each modeled galaxy approaches a steady state where the SFR follows the (net) cold gas accretion rate. The key feature of the model is a halo mass floor M min 10 11 M below which accretion is quenched in order to simultaneously account for the observed slopes of the SFR-mass and TF relations. The same successes cannot be achieved via an SF threshold (or delay) nor by varying the SF efficiency or the feedback efficiency. Combined with the mass ceiling for cold accretion due to virial shock heating, the mass floor M min explains galaxy "downsizing," where more massive galaxies formed earlier and over a shorter period of time. It turns out that the model also accounts for the observed galactic baryon and gas fractions as a function of mass and time, and the cosmic SFR density, which are all resulting from the mass floor M min . The model helps us to understand that it is the cosmological decline of accretion rate that drives the decrease of cosmic SFR density between z ∼ 2 and z = 0 and the rise of the cosmic SFR density from z ∼ 6 to z ∼ 2 that allows us to put a constraint on our main parameter M min 10 11 M . Among the physical mechanisms that could be responsible for the mass floor, our view is that photoionization feedback (from first in situ hot stars) lowering the cooling efficiency is likely to play a large role.