The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGNs), and dark matter (DM) with large-scale structure (LSS) over the redshift range z > 0:5Y 6. The survey includes multiwavelength imaging and spectroscopy from X-rayYtoYradio wavelengths covering a 2 deg 2 area, including HST imaging. Given the very high sensitivity and resolution of these data sets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS data sets, and a summary the science goals.

Using free-free emission measured in the Ka-band (26 − 40 GHz) for 10 star-forming regions in the nearby galaxy NGC 6946, including its starbursting nucleus, we compare a number of star formation rate (SFR) diagnostics that are typically considered to be unaffected by interstellar extinction. These diagnostics include non-thermal radio (i.e., 1.4 GHz), total infrared (IR; 8 − 1000 µm), and warm dust (i.e., 24 µm) emission, along with hybrid indicators that attempt to account for obscured and unobscured emission from star-forming regions including Hα + 24 µm and UV + IR measurements.The assumption is made that the 33 GHz free-free emission provides the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24 µm, Hα + 24 µm and UV + IR SFR calibrations are in good agreement with the 33 GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimate the nuclear SFR by a factor of ∼2 relative to the 33 GHz free-free SFR. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the 33 GHz free-free SFRs for the nucleus, and underestimate the SFRs from the extranuclear star-forming regions by an average factor of ∼2 and ∼4−5 before and after subtracting local background emission, respectively. This result likely arises from the CR electrons decaying within the starburst region with negligible escape, whereas the transient nature of star formation in the young extranuclear star-forming complexes allows for CR electrons to diffuse significantly further than dust heating photons, resulting in an underestimate of the true SFR. Finally, we find that the SFRs estimated using the total 33 GHz flux density appear to agree well with those from using the free-free emission due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33 GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

We explore the redshift evolution of the specific star formation rate (SSFR) for galaxies of different stellar mass by drawing on a deep 3.6 µm-selected sample of > 10 5 galaxies in the 2 deg 2 COSMOS field. The average star formation rate (SFR) for sub-sets of these galaxies is estimated with stacked 1.4 GHz radio continuum emission. We separately consider the total sample and a subset of galaxies that shows evidence for substantive recent star formation in the rest-frame optical spectral energy distributions. At redshifts 0.2 < z < 3 both populations show a strong and mass-independent decrease in their SSFR towards the present epoch. It is best described by a power-law (1 + z) n , where n ∼ 4.3 for all galaxies and n ∼ 3.5 for star forming (SF) sources. The decrease appears to have started at z > 2, at least for high-mass (M * 4 × 10 10 M ⊙ ) systems where our conclusions are most robust. Our data show that there is a tight correlation with power-law dependence, SSFR ∝ M * β , between SSFR and stellar mass at all epochs. The relation tends to flatten below M * ≈ 10 10 M ⊙ if quiescent galaxies are included; if they are excluded from the analysis a shallow index β SFG ≈ −0.4 fits the correlation. On average, higher mass objects always have lower SSFRs, also among SF galaxies. At z > 1.5 there is tentative evidence for an upper threshold in SSFR that an average galaxy cannot exceed, possibly due to gravitationally limited molecular gas accretion. It is suggested by a flattening of the SSFR-M * relation (also for SF sources), but affects massive (> 10 10 M ⊙ ) galaxies only at the highest redshifts. Since z = 1.5 there thus is no direct evidence that galaxies of higher mass experience a more rapid waning of their SSFR than lower mass SF systems. In this sense, the data rule out any strong 'downsizing' in the SSFR. We combine our results with recent measurements of the galaxy (stellar) mass function in order to determine the characteristic mass of a SF galaxy: we find that since z ∼ 3 the majority of all new stars were always formed in galaxies of M * = 10 10.6±0.4 M ⊙ . In this sense, too, there is no 'downsizing'. Finally, our analysis constitutes the most extensive SFR density determination with a single technique out to z = 3. Recent Herschel results are consistent with our results, but rely on far smaller samples.Note. -Median stacking-based average 1.4 GHz radio flux densities and derived average quantities for all our bins in mass and redshift for star forming systems within our mass-selected sample. For details see caption of Tab. 2. † Mass bin contains data below the limit of mass representativeness and yields an upper limit to the average SFR (see Sec. 2.6 for further details.) ⋆ First mass bin above the limit of representativeness (see Sec. 2.6) which contains a low fraction (< 15 %) of optically faint objects with m AB (i + ) ≥ 25.5 for which the photo-z accuracy is degraded (see Sec. 2.2 for further details). The average SFR measured in this bin might be slightly overestimated towards higher values (se...

We compare molecular gas traced by 12 CO(2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between Σ mol and Σ SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, τ mol dep = Σ mol /Σ SFR . At the 1 kpc common resolution of HERACLES, CO emission correlates closely with many tracers of the recent SFR. Weighting each line of sight equally, using a fixed alpha CO equivalent to the Milky Way value, our data yield a molecular gas depletion time, τ mol dep = Σ mol /Σ SFR ≈ 2.2 Gyr with 0.3 dex 1σ scatter, in very good agreement with recent literature data. We apply a forward-modeling approach to constrain the power-law index, N , that relates the SFR surface density and the molecular gas surface density, Σ SFR ∝ Σ N mol . We find N = 1 ± 0.15 for our full data set with some scatter from galaxy to galaxy. This also agrees with recent work, but we caution that a power law treatment oversimplifies the topic given that we observe correlations between τ mol dep and other local and global quantities. The strongest of these are a decreased τ mol dep in low-mass, low-metallicity galaxies and a correlation of the kpc-scale τ mol dep with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H 2 conversion factor (α CO ) that depends on dust shielding, and thus D/G, in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes this the most conservative explanation of the strongest observed τ mol dep trends. After applying a D/G-dependent α CO , some weak correlations between τ mol dep and local conditions persist. In particular, we observe lower τ mol dep and enhanced CO excitation associated with nuclear gas concentrations in a subset of our targets. These appear to reflect real enhancements in the rate of star formation per unit gas and although the distribution of τ dep does not appear bimodal in galaxy centers, τ dep does appear multivalued at fixed Σ mol , supporting the the idea of "disk" and "starburst" modes driven by other environmental parameters.