We present deep mid-IR spectroscopy with Spitzer of 13 SMGs in the GOODS-N field. We find strong PAH emission in all of our targets, which allows us to measure mid-IR spectroscopic redshifts and place constraints on the contribution from star formation and AGN activity to the mid-IR emission. In the high-S/ N composite spectrum, we find that the hot dust continuum from an AGN contributes at most 30% of the mid-IR luminosity. Individually, only 2/13 SMGs have continuum emission dominating the mid-IR luminosity; one of these SMGs, C1, remains undetected in the deep X-ray images but shows a steeply rising continuum in the mid-IR indicative of a Compton-thick AGN. We find that the mid-IR properties of SMGs are distinct from those of 24 m-selected ULIRGs at z $ 2; the former are predominantly dominated by star formation, while the latter are a more heterogeneous sample with many showing significant AGN activity. We fit the IRS spectrum and the mid-IR to radio photometry of SMGs with template SEDs to determine the best estimate of the total IR luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multiwavelength analysis shows that the total IR luminosity, L IR , in SMGs is dominated by star formation. We find that high-redshift SMGs lie on the relation between L IR and L PAH;6:2 (or L PAH;7:7 or L PAH;11:3 ) that has been established for local starburst galaxies. This suggests that PAH luminosity can be used as a proxy for the SFR in SMGs. SMGs are consistent with being a short-lived cool phase in a massive merger where the AGN does not appear to have become strong enough to heat the dust and dominate the mid-or far-IR emission.

ALMA observations of the long wavelength dust continuum are used to estimate the interstellar medium (ISM) masses in a sample of 708 galaxies at z=0.3 to 4.5 in the COSMOS field. The galaxy sample has known farinfrared luminosities and, hence, star formation rates (SFRs) and stellar masses ( * M ) from the optical-infrared spectrum fitting. The galaxies sample SFRs from the main sequence (MS) to 50 times above the MS. The derived ISM masses are used to determine the dependence of gas mass on redshift, * M , and specific SFR (sSFR) relative to the MS. The ISM masses increase approximately with the 0.63 power of the rate of increase in SFRs with redshift and the 0.32 power of the sSFR/sSFR MS . The SF efficiencies also increase as the 0.36 power of the SFR redshift evolution and the 0.7 power of the elevation above the MS; thus the increased activities at early epochs are driven by both increased ISM masses and SF efficiency. Using the derived ISM mass function, we estimate the accretion rates of gas required to maintain continuity of the MS evolution (>  M 100 yr −1 at z > 2.5). Simple power-law dependencies are similarly derived for the gas accretion rates. We argue that the overall evolution of galaxies is driven by the rates of gas accretion. The cosmic evolution of total ISM mass is estimated and linked to the evolution of SF and active galactic nucleus activity at early epochs.

We present spectral energy distributions (SEDs), Spitzer colours, and infrared (IR) luminosities for 850‐μm selected galaxies in the Great Observatories Origins Deep Survey Northern (GOODS‐N) field. Using the deep Spitzer Legacy images and new data and reductions of the Very Large Array‐Hubble Deep Field (VLA‐HDF) radio data, we find statistically secure counterparts for 60 per cent (21/35) of our submillimetre (submm) sample, and identify tentative counterparts for another 12 objects. This is the largest sample of submm galaxies with statistically secure counterparts detected in the radio and with Spitzer. Half of the secure counterparts have spectroscopic redshifts, while the other half have photometric redshifts. We find that in most cases the 850‐μm emission is dominated by a single 24‐μm source, with a median flux density of 241 μJy, leading to a median 24‐to‐850‐μm flux density ratio of 0.040. A composite rest‐frame SED shows that the submm sources peak at longer wavelengths than those of local ultraluminous infrared galaxies (ULIRGs). Using a basic grey‐body model, 850‐μm selected galaxies appear to be cooler than local ULIRGs of the same luminosity. This demonstrates the strong selection effects, both locally and at high redshift, which may lead to an incomplete census of the ULIRG population. The SEDs of submm galaxies are also different from those of their high‐redshift neighbours, the near‐IR selected BzK galaxies, whose mid‐IR‐to‐radio SEDs are more like those of local ULIRGs. Using 24‐μm, 850‐μm and 1.4‐GHz observations, we fit templates that span the mid‐IR through radio to derive the integrated IR luminosity (LIR) of the submm galaxies and find a median value of LIR(8–1000 μm) = 6.0 × 1012 L⊙. By themselves, 24‐μm and radio fluxes are able to predict LIR reasonably well because they are relatively insensitive to temperature. However, the submm flux by itself consistently overpredicts LIR when using spectral templates which obey the local ULIRG temperature–luminosity relation. The shorter Spitzer wavelengths sample the stellar bump at the redshifts of the submm sources, and we find that the Spitzer photometry alone provides a model‐independent estimate of the redshift, σ[Δz/(1 +z)]= 0.07. The median redshift for our secure submm counterparts is 2.0. Using X‐ray and mid‐IR data, only 5 per cent of our secure counterparts (1/21) show strong evidence for an active galactic nucleus dominating the LIR.

ALMA Cycle 2 observations of the long wavelength dust emission in 145 star-forming galaxies are used to probe the evolution of star-forming ISM. We also develop the physical basis and empirical calibration (with 72 low-z and z ∼2 galaxies) for using the dust continuum as a quantitative probe of interstellar medium (ISM) masses. The galaxies with highest star formation rates (SFRs) at < z > = 2.2 and 4.4 have gas masses up to 100 times that of the Milky Way and gas mass fractions reaching 50 to 80%, i.e. gas masses 1 -4 × their stellar masses. We find a single high-z star formation law:-an approximately linear dependence on the ISM mass and an increased star formation efficiency per unit gas mass at higher redshift. Galaxies above the Main Sequence (MS) have larger gas masses but are converting their ISM into stars on a timescale only slightly shorter than those on the MS -thus these 'starbursts' are largely the result of having greatly increased gas masses rather than and increased efficiency for converting gas to stars. At z > 1, the entire population of star-forming galaxies has ∼ 2 -5 times shorter gas depletion times than low-z galaxies. These shorter depletion times indicate a different mode of star formation in the early universe -most likely dynamically driven by compressive, high-dispersion gas motions -a natural consequence of the high gas accretion rates.