Using very deep Spitzer MIPS 24 µm observations, we present an analysis of the bolometric luminosities and UV extinction properties of more than 200 spectroscopically identified, optically selected (U n GR) z ∼ 2 galaxies in the GOODS-N field. The large spectroscopic sample of rest-UV selected galaxies is supplemented with photometrically identified near-IR-selected ("BzK" and "DRG") galaxies and sub-mm sources at similar redshifts in the same field, providing a representative collection of relatively massive (M * > 10 10 M ⊙ ) galaxies at high redshifts. We focus on the redshift range 1.5 < z < 2.6, for which the 24 µm observations provide a direct measurement of the strength of the mid-IR PAH features in the galaxy spectra; the rest-frame 5 − 8.5 µm luminosities (L 5−8.5µm ) are particularly tightly constrained for the objects in our sample with precise spectroscopic redshifts. We demonstrate, using stacked X-ray observations and a subset of galaxies with Hα measurements, that L 5−8.5µm provides a reliable estimate of L IR for most star forming galaxies at z ∼ 2. We show that the range of L IR in the optical/near IR-selected samples considered extends from ≃ 10 10 L ⊙ to > 10 12 L ⊙ , with a mean L IR ≃ 2 × 10 11 L ⊙ . The LIRG population at z ∼ 2 is essentially the same population of galaxies that are selected by their optical/near-IR colors. Objects with LIRG to ULIRG luminosities are present over the full range of stellar masses in the samples, from 2 × 10 9 M ⊙ to 5 × 10 11 M ⊙ . We use the MIPS 24 µm observations for an independent examination of dust extinction in high redshift galaxies, and demonstrate that, as in the local universe, the obscuration ( LIR L1600 ) is strongly dependent on bolometric luminosity, and ranges in value from < 1 to ∼ 1000 within the sample considered. However, the obscuration is ∼ 10 times smaller at a given L bol (or, equivalently, a similar level of obscuration occurs at luminosities ∼ 10 times larger) at z ∼ 2 than at z ∼ 0. We show that the values of L IR and obscuration inferred from the UV spectral slope β generally agree well with the values inferred from L 5−8.5µm for L bol < 10 12 L ⊙ . As found previously by several investigators, for "ultraluminous" objects with L bol > 10 12 L ⊙ it is common for UV-based estimates to underpredict L IR by a factor of ∼ 10 − 100. Using the specific star formation rate of galaxies (SFR per unit stellar mass) as a proxy for cold gas fraction, we find a wide range in the evolutionary state of galaxies at z ∼ 2, from galaxies that have just begun to form stars to those which have already accumulated most of their stellar mass and are about to become, or already are, passively-evolving.

Abstract. Deep extragalactic surveys with ISOCAM revealed the presence of a large density of faint mid-infrared (MIR) sources. We have computed the 15 µm integrated galaxy light produced by these galaxies above a sensitivity limit of 50 µJy. It sets a lower limit to the 15 µm extragalactic background light of (2.4 ± 0.5) nW m −2 sr −1 . The redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). It peaks around z ∼ 0.8 in agreement with studies in other fields. The rest-frame 15 µm and bolometric infrared (8-1000 µm) luminosities of ISOCAM galaxies are computed using the correlations that we establish between the 6.75, 12, 15 µm and infrared (IR) luminosities of local galaxies. The resulting IR luminosities were double-checked using radio (1.4 GHz) flux densities from the ultra-deep VLA and WSRT surveys of the HDFN on a sample of 24 galaxies as well as on a sample of 109 local galaxies in common between ISOCAM and the NRAO VLA Sky Survey (NVSS). This comparison shows for the first time that MIR and radio luminosities correlate up to z ∼ 1. This result validates the bolometric IR luminosities derived from MIR luminosities unless both the radio-far infrared (FIR) and the MIR-FIR correlations become invalid around z ∼ 1. The fraction of IR light produced by active nuclei was computed from the cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively. We find that at most 20% of the 15 µm integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. About 75% of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies (LIR ≥ 10 11 L ). They exhibit star formation rates of the order of ∼100 M yr −1 . The comoving density of infrared light due to these luminous IR galaxies was more than 40 times larger at z ∼ 1 than today. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140 µm was computed from the MIR-FIR correlations for star forming galaxies and from the spectral energy distribution of the Seyfert 2, NGC 1068, for AGNs. We find that the galaxies unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e. (16 ± 5) nW m −2 sr −1 as compared to the (25 ± 7) nW m −2 sr −1 measured with the COBE satellite, with less than 10% due to AGNs. Since the CIRB contains most of the light radiated over the history of star formation in the universe, this means that a large fraction of present-day stars must have formed during a dusty starburst event similar to those revealed by ISOCAM.

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.

We analyze the internal velocity dispersion p of a sample of 172 nearby galaxy clusters (z ¹ 0.15), each of which has at least 30 available galaxy redshifts and spans a large richness range. Cluster membership selection is based on nonparametric methods. In the estimate of galaxy velocity dispersion, we consider the e †ects of possible velocity anisotropies in galaxy orbits, the infall of late-type galaxies, and velocity gradients. The dynamical uncertainties due to the presence of substructures are also taken into account. Previous p-distributions, based on smaller cluster samples, are complete for the Abell richness class R º 1. In order to improve p completeness, we enlarge our sample by also including poorer clusters. By resampling 153 AbellÈAbell-Corwin-Olowin clusters, according to the richness class frequencies of the Edinburgh-Durham Cluster Catalog, we obtain a cluster sample which can be taken as representative of the nearby universe. Our cumulative p-distribution agrees with previous distributions within their p completeness limit km s~1). We estimate that our distribution is complete for at least p º 650 (p Z 800 km s~1. In this completeness range, a Ðt of the form dN P pa dp gives in fair agreement a \ [(7.4~0 .8 0.7), with results coming from the X-ray temperature distributions of nearby clusters. We brieÑy discuss our results with respect to p-distributions for galaxy groups and to theories of large-scale structure formation.

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z ∼ 1 -3 and likely L IR ∼ 10 12 -10 13 L ⊙ . Our study aims at quantifying the star-formation and AGN processes in these sources which recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum-(i.e. AGN) dominated sources, but ∼ 60% of these show PAH emission, suggesting the presence of star-formation activity. These sources have redder mid-IR colors than typical optically-selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5 µm hot dust continua. However, their steep 30 µm -to -14 µm slopes suggest that star-formation might dominate the total infrared luminosity. Six of our z ∼ 2 sources have EW 6.2 > ∼ 0.3 µm and L 14µm > ∼ 10 12 L ⊙ (implying L IR > ∼ 10 13 L ⊙ ). At these luminosities, such high EW 6.2 ULIRGs do not exist in the local Universe. We find a median optical depth at 9.7 µm of τ 9.7µm = 1.4. This is consistent with local IRAS-selected ULIRGs, but differs from early results on SCUBAselected z ∼ 2 ULIRGs. Similar to local ULIRGs about 25% of our sample show extreme obscuration (τ 9.7µm > ∼ 3) suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs, but are an order of magnitude more luminous. It is not clear whether our z ∼ 2 ULIRGs are simply scaled-up versions of local ULIRGs, or subject to fundamentally different physical processes.

We present optical spectroscopy of a sample of 77 luminous AGN and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and SWIRE XMM-LSS fields, with a typical 24µm flux density of 5mJy. The median redshift is 0.6, with a range of ∼ 0.05 − 4. Only 33% (25/77) of these objects are normal type-1 quasars, with no obscuration. 44% (34/77) are type-2 objects, with highionization, narrow emission lines, 14% (11/77) are dust-reddened type-1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. 9% (7/77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra which could be of either starburst or LINER types. These latter objects are analogous to the X-ray detected population of AGN with weak or non-existent optical AGN emission (the "XBONGs"). 21 of our objects from the SWIRE field fall within moderately-deep XMM exposures. All the unobscured quasars, and about half the obscured quasars are detected in these exposures. This sample, when taken together with other samples of Spitzer selected AGN and quasars, and results from X-ray studies, confirms that obscured AGN dominate the AGN and quasar number counts of all rapidly-accreting supermassive black hole systems, at least for z < ∼ 4. This implies a high radiative efficiency for the black hole accretion process.

Abstract.We discuss the constraints set on galaxy evolution by a variety of data from deep extragalactic surveys performed in the mid-IR and far-IR with the Infrared Space Observatory and with millimetric telescopes at longer wavelengths. These observations indicate extremely high rates of evolution for IR galaxies, exceeding those measured for galaxies at other wavelengths and comparable or larger than the rates observed for quasars. We also match the modelled integrated emission by IR galaxies at any redshifts with the observed spectral intensity of the extragalactic IR background (CIRB), as a further constraint. The multi-wavelength statistics on IR galaxies can be reconciled with each other by assuming for the bulk of the population spectral energy distributions (SED) as typical for starbursts, which we take as an indication that stellar (rather than AGN, see also Fadda et al. 2001) activity powers IR emission by faint galaxies. According to our model and following the analysis of Elbaz et al. (2001), the deep ISO surveys at 15 µm may have already resolved more than 50% of the bolometric CIRB intensity: the faint ISO 15 µm source samples, relatively easy to identify in deep optical images , can then allow to investigate the origin of the CIRB background. From our fits to the observed optical-IR SEDs, these objects appear to mostly involve massive galaxies hosting luminous starbursts (SF R ∼ 100 M /yr). The evolutionary scheme we infer from these data considers a bimodal star formation (SF), including a phase of long-lived quiescent SF, and enhanced SF taking place during transient events recurrently triggered by interactions and merging. We interpret the strong observed evolution as an increase with z of the rate of interactions between galaxies (density evolution) and an increase of their IR luminosity due to the more abundant fuel available in the past (luminosity evolution): both factors enhance the probability to detect a galaxy during the "active" phase at higher z. Very schematically, we associate the origin of the bulk of the optical/NIR background to the quiescent evolution, while the CIRB is interpreted as mostly due the dusty starburst phase. The latter possibly leads to the formation of galaxy spheroids, when the dynamical events triggering the starburst re-distribute already present stellar populations. The large energy contents in the CIRB and optical backgrounds are not easily explained, considering the moderate efficiency of energy generation by stars: a top-heavy stellar IMF associated with the starburst phase (and compared with a more standard IMF during the quiescent SF) would alleviate the problem. The evolution of the IR emissivity of galaxies from the present time to z ∼ 1 is so strong that the combined set of constraints by the observed z-distributions and the CIRB spectrum impose it to turn-over at z > 1: scenarios in which a dominant fraction of stellar formation occurs at very high-z are not supported by our analysis.

We present the mid-infrared spectra obtained with the Spitzer IRS for 52 sources, selected as infrared luminous, z k1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f 24 m k 0:9 mJy, f (24 m)/ f (8 m) k 3:16, and f (24 m)/f (0:7 m) k 10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with 35/47 (74%) at 1:5 P z P 3:2. Keck spectroscopy of a subsample (17/47) agrees with the mid-IR redshift measurements. The observed spectra fall into three categories: (1) 33% (17/52) have strong PAH emission and are probably powered by star formation with total IR luminosity roughly a factor of 5 higher than the local starburst ULIRGs; (2) 33% (17/52) have only deep silicate absorption at 9.8 m, indicatiing deeply embedded dusty systems -these data alone cannot determine the energetic nature of the heating sources in these systems; and (3) the remaining 34% are mid-IR continuum-dominated systems with weak PAH emission and/or silicate absorption. This subsample is probably AGNs. We derived monochromatic, rest-frame 5.8 m, continuum luminosities (L ), ranging from 10 10.3 to 10 12.6 L . Our spectra have mid-IR slope 5Y15 m k 2:1, much redder than the median value of 1.3 for the optically selected PG quasars. From the silicate absorption feature, we estimate that roughly two-thirds of the sample have optical depth 9:8 m > 1. Their L 1600 8 and L IR suggest that our sample is among the most luminous and most dust-enshrouded systems of its epoch. Our study has revealed a significant population of dust-enshrouded galaxies at z $ 2, whose enormous energy output, comparable to that of quasars, is generated by AGNs, as well as starbursts. This IR-luminous population has very little overlap with submillimeter and UV-selected populations.