No abstract

We analyze the dependence of galaxy structure (size and Sersic index) and mode of star formation (Σ SF R and SF R IR /SF R UV ) on the position of galaxies in the SFR versus Mass diagram. Our sample comprises roughly 640000 galaxies at z ∼ 0.1, 130000 galaxies at z ∼ 1, and 36000 galaxies at z ∼ 2. Structural measurements for all but the z ∼ 0.1 galaxies are based on HST imaging, and SFRs are derived using a Herschel-calibrated ladder of SFR indicators. We find that a correlation between the structure and stellar population of galaxies (i.e., a 'Hubble sequence') is already in place since at least z ∼ 2.5. At all epochs, typical star-forming galaxies on the main sequence are well approximated by exponential disks, while the profiles of quiescent galaxies are better described by de Vaucouleurs profiles. In the upper envelope of the main sequence, the relation between the SFR and Sersic index reverses, suggesting a rapid build-up of the central mass concentration in these starbursting outliers. We observe quiescent, moderately and highly star-forming systems to co-exist over an order of magnitude or more in stellar mass. At each mass and redshift, galaxies on the main sequence have the largest size. The rate of size growth correlates with specific SFR, and so does Σ SF R at each redshift. A simple model using an empirically determined SF law and metallicity scaling, in combination with an assumed geometry for dust and stars is able to relate the observed Σ SF R and SF R IR /SF R UV , provided a more patchy dust geometry is assumed for high-redshift galaxies.

We combine molecular gas masses inferred from CO emission in 500 star forming galaxies (SFGs) between z=0 and 3, from the IRAM-COLDGASS, PHIBSS1/2 and other surveys, with gas masses derived from Herschel far-IR dust measurements in 512 galaxy stacks over the same stellar mass/redshift range. We constrain the scaling relations of molecular gas depletion time scale (t depl ) and gas to stellar mass ratio (M molgas /M * ) of SFGs near the star formation 'main-sequence' with redshift, specific star formation rate (sSFR) and stellar mass (M * ). The CO-and dust-based scaling relations agree remarkably well. This suggests that the CO  H 2 mass conversion factor varies little within ±0.6dex of the main sequence (sSFR(ms,z,M * )), and less than 0.3dex throughout this redshift range. This study builds on and strengthens the results of earlier work. We find that t depl scales as (1+z) -0.3  (sSFR/sSFR(ms,z,M * )) -0.5 , with little dependence on M * . The resulting steep redshift dependence of M molgas /M *  (1+z) 3 mirrors that of the sSFR and probably reflects the gas supply rate. The decreasing gas fractions at high M * are driven by the flattening of the SFR-M * relation. Throughout the redshift range probed a larger sSFR at constant M * is due to a combination of an increasing gas fraction and a decreasing depletion time scale. As a result galaxy integrated samples of the M molgas -SFR rate relation exhibit a super-linear slope, which increases with the range of sSFR. With these new relations it is now possible to determine M molgas with an accuracy of ±0.1dex in relative terms, and ±0.2dex including systematic uncertainties.

Aims. The aim of this work is to investigate the physical, structural and evolutionary properties of old, passive galaxies at z > 1.4 and to place new constraints on massive galaxy formation and evolution. Methods. We combine ultradeep optical spectroscopy from the GMASS project (Galaxy Mass Assembly ultradeep Spectroscopic Survey) with GOODS multi-band (optical to mid-infrared) photometry and HST imaging to study a sample of spectroscopically identified passive galaxies at 1.39 < z < 1.99 selected from Spitzer Space Selescope imaging at 4.5 µm. Results. A stacked spectrum with an equivalent integration time of ∼500 h was obtained and compared with libraries of synthetic stellar population spectra. The stacked spectrum is publicly released. The spectral and photometric SED properties indicate very weak or absent star formation, moderately old stellar ages of ≈1 Gyr (for solar metallicity) and stellar masses in the range of 10 10−11 M , thus implying that the major star formation and assembly processes for these galaxies occurred at z > 2. No X-ray emission was found neither from individual galaxies nor from a stacking analysis of the sample. Only one galaxy shows a marginal detection at 24 µm. These galaxies have morphologies that are predominantly compact and spheroidal. However, their sizes (R e 1 kpc) are much smaller than those of spheroids in the present-day Universe. Their stellar mass surface densities are consequently higher by ≈1 dex if compared to spheroids at z ≈ 0 with the same mass. Their rest-frame B-band surface brightness scales with the effective radius, but the offset with respect to the surface brightness of the local Kormendy relation is too large to be explained by simple passive evolution. At z ≈ 1, a larger fraction of passive galaxies follows the z ≈ 0 size-mass relation. Superdense relics with R e ≈ 1 kpc are extremely rare at z ≈ 0 with respect to z > 1, and absent if R e < 1 kpc. Because of the similar sizes and mass densities, we suggest that the superdense passive galaxies at 1 < z < 2 are the remnants of the powerful starbursts occurring in submillimeter-selected galaxies at z > 2. The results are compared with theoretical models and the main implications discussed in the framework of massive galaxy formation and evolution.