Examining a sample of massive galaxies at 1:4 < z < 2:5 with K Vega < 22 from GOODS, we compare photometry from Spitzer at mid-and far-IR to submillimeter, radio, and rest-frame UV wavelengths, to test the agreement between different tracers of star formation rates (SFRs) and to explore the implications for galaxy assembly. For z $ 2 galaxies with moderate luminosities (L 8 m < 10 11 L ), we find that the SFR can be estimated consistently from the multiwavelength data based on local luminosity correlations. However, 20%Y30% of massive galaxies, and nearly all those with L 8 m > 10 11 L , show a mid-IR excess that is likely due to the presence of obscured active nuclei, as shown in a companion paper. There is a tight and roughly linear correlation between stellar mass and SFR for 24 mYdetected galaxies. For a given mass, the SFR at z ¼ 2 was larger by a factor of $4 and $30 relative to that in star-forming galaxies at z ¼ 1 and 0, respectively. Typical ultraluminous infrared galaxies (ULIRGs) at z ¼ 2 are relatively ''transparent'' to ultraviolet light, and their activity is long lived (k400 Myr), unlike that in local ULIRGs and high-redshift submillimeter-selected galaxies. ULIRGs are the common mode of star formation in massive galaxies at z ¼ 2, and the high duty cycle suggests that major mergers are not the dominant trigger for this activity. Current galaxy formation models underpredict the normalization of the mass-SFR correlation by about a factor of 4 and the space density of ULIRGs by an order of magnitude but give better agreement for z > 1:4 quiescent galaxies.

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.

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.

Aims. We use the deepest existing mid-and far-infrared observations (reaching ∼3 mJy at 70 μm) obtained with Spitzer in the Great Observatories Origins Deep Survey (GOODS) and Far Infrared Deep Extragalactic Legacy survey (FIDEL) fields to derive the evolution of the rest-frame 15 μm, 35 μm, and total infrared luminosity functions of galaxies spanning z < 1.3. We thereby quantify the fractional contribution of infrared luminous galaxies to the comoving star formation rate density over this redshift range. In comparison with previous studies, the present one takes advantage of deep 70 μm observations that provide a more robust infrared luminosity indicator than 24 μm affected by the emission of PAHs at high redshift (z ∼ 1), and we use several independent fields to control cosmic variance. Methods. We used a new extraction technique based on the well-determined positions of galaxies at shorter wavelengths to extract the 24 and 70 μm flux densities of galaxies. It is found that sources separated by a minimum of 0.5 × FWHM are deblended by this technique, which facilitates multi-wavelength associations of counterparts. Using a combination of photometric and spectroscopic redshifts that exist for ∼80% of the sources in our sample, we are able to estimate the rest-frame luminosities of galaxies at 15 μm and 35 μm. By complementing direct detections with a careful stacking analysis, we measured the mid-and far-infrared luminosity functions of galaxies over a factor ∼100 in luminosity (10 11 L <∼ L IR <∼ 10 13 L ) at z < 1.3. A stacking analysis was performed to validate the bolometric corrections and to compute comoving star-formation rate densities in three redshift bins 0.4 < z < 0.7, 0.7 < z < 1.0 and, 1.0 < z < 1.3. Results. We find that the average infrared spectral energy distribution of galaxies over the last 2/3 of the cosmic time is consistent with that of local galaxies, although individual sources do present significant scatter. We also measured both the bright and faint ends of the infrared luminosity functions and find no evidence for a change in the slope of the double power law used to characterize the luminosity function. The redshift evolution of infrared luminous galaxies is consistent with pure luminosity evolution proportional to (1 + z) 3.6±0.4 up to z ∼ 1.3. We do not find evidence of differential evolution between LIRGs and ULIRGs up to z ∼ 1.3, in contrast with previous claims. The comoving number density of infrared luminous galaxies has increased by a factor of ∼100 between 0 < z < 1. By z ∼ 1.0, LIRGs produce half of the total comoving infrared luminosity density.

Selection of active galactic nuclei (AGN) in the infrared allows the discovery of AGN whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess what fraction of AGN with mid-infrared luminosities comparable to quasars are missed in optical quasar surveys due to dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg 2 extragalactic FLS. These quasars occupy a distinct region in mid-infrared color space by virtue of their strong, red, continua. This has allowed us to define a mid-infrared color criterion for selecting AGN candidates. About 2000 FLS objects have colors consistent with them being AGN, but most are much fainter in the mid-infrared than the SDSS quasars, which typically have 8µm flux densities, S 8.0 , ∼ 1mJy. We have investigated the properties of the 43 objects with S 8.0 ≥ 1mJy satisfying our AGN color selection. This sample should contain both unobscured quasars, and AGN which are absent from the SDSS survey due to extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects which are likely to be obscured quasars or luminous Seyfert-2 galaxies. This suggests the numbers of obscured and unobscured AGN are similar in samples selected in the mid-infrared at S 8.0 ∼ 1mJy.