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We present ultra-deep mid-infrared spectra of 48 infrared-luminous galaxies in the GOODS-south field obtained with the Infrared Spectrograph on the Spitzer Space Telescope. These galaxies are selected among faint infrared sources (0.14-0.5 mJy at 24 mm) in two redshift bins (0.76-1.05 and 1.75-2.4) to sample the major contributors to the cosmic infrared background at the most active epochs. We estimate redshifts for 92% of the sample using polycyclic aromatic (PAH) and Si absorption features obtaining, in particular, eight new redshifts difficult to measure from ground-based observations. Only a few of these galaxies (5% at z [?] 1 and 12% at z [?] 2) have their total infrared luminosity dominated by emission from active galactic nuclei (AGNs). The averaged mid-IR spectrum of the z [?] 1 luminous infrared galaxies (LIRGs) is a very good match to the averaged spectrum of local starbursts. The averaged spectrum of the z [?] 2 ultra-luminous infrared galaxies (ULIRGs), because of a deeper Si absorption, is better fitted by the averaged spectrum of H II-like local ULIRGs. Combining this sample with other published data, we find that 6.2 mm PAH equivalent widths (EW) reach a plateau of [?] 1 mm for L 24 mm [?] 1011 L . At higher luminosities, EW6.2 mm anti-correlates with L 24 mm. Intriguingly, high-z ULIRGs and sub-millimeter galaxies (SMGs) lie above the local EW6.2 mm-L 24 mm relationship suggesting that, at a given luminosity, high-z ULIRGs have AGN contributions to their dust emission lower than those of local counterparts. A quantitative analysis of their morphology shows that most of the luminous IR galaxies have morphologies similar to those of IR-quiet galaxies at the same redshift. All z [?] 2 ULIRGs of our sample are IR-excess BzK galaxies and most of them have L FIR/L 1600 ratios higher than those of starburst galaxies at a given UV slope. The "IR excess" is mostly due to strong 7.7 mm PAH emission and underestimation of UV dust extinction. On the basis of the AGN-powered L 6 mm continuum measured directly from the mid-IR spectra, we estimate an average intrinsic X-ray AGN luminosity of L 2-10 keV = (0.1+- 0.6)x 1043 erg s-1, a value substantially lower than the prediction by Daddi et al.

We model the optical to far-infrared spectral energy distributions (SEDs) of a sample of six type-1 and six type-2 quasars selected in the mid-infrared. The objects in our sample are matched in mid-IR luminosity and selected based on their Spitzer IRAC colors. We obtained new targeted Spitzer Infrared Spectrograph and Multiband Imaging Photometer for Spitzer observations and used archival photometry to examine the optical to far-IR SEDs. We investigate whether the observed differences between samples are consistent with orientation-based unification schemes. The type-1 objects show significant emission at 3 mm. They do not show strong polycyclic aromatic hydrocarbon (PAH) emission and have less far-IR emission on average when compared to the type-2 objects. The SEDs of the type-2 objects show a wide assortment of silicate features, ranging from weak emission to deep silicate absorption. Some also show strong PAH features. In comparison, silicate is only seen in emission in the type-1 objects. This is consistent with some of the type-2's being reddened by a foreground screen of cooler dust, perhaps in the host galaxy itself. We investigate the active galactic nucleus contribution to the far-IR emission and find it to be significant. We also estimate the star formation rate (SFR) for each of the objects by integrating the modeled far-IR flux and compare this with the SFR found from PAH emission. We find that the type-2 quasars have a higher average SFR than the type-1 quasars based on both methods, though this could be due to differences in bolometric luminosities of the objects. While we find pronounced differences between the two types of objects, none of them are inconsistent with orientation-based unification schemes.

We present the first detections of the 3 mm water ice and 3.4 mm amorphous hydrocarbon (HAC) absorption features in z [?] 2 ULIRGs. These are based on deep rest-frame 2-8 mm Spitzer Infrared Spectrograph spectra of 11 sources selected for their appreciable silicate absorption. The HAC-to-silicate ratio for our z [?] 2 sources is typically higher by a factor of 2-5 than that observed in the Milky Way. This HAC "excess" suggests compact nuclei with steep temperature gradients as opposed to predominantly host obscuration. Beside the above molecular absorption features, we detect the 3.3 mm polycyclic aromatic hydrocarbon (PAH) emission feature in one of our sources with three more individual spectra showing evidence for it. Stacking analysis suggests that water ice, hydrocarbons, and PAH are likely present in the bulk of this sample even when not individually detected. The most unexpected result of our study is the lack of clear detections of the 4.67 mm CO gas absorption feature. Only three of the sources show tentative signs of this feature at significantly lower levels than has been observed in local ULIRGs. Overall we find that the closest local analogs to our sources, in terms of 3-4 mm color, HAC-to-silicate and ice-to-silicate ratios, as well as low PAH equivalent widths, are sources dominated by deeply obscured nuclei. Such sources form only a small fraction of ULIRGs locally and are commonly believed to be dominated by buried active galactic nuclei (AGNs). Our sample suggests that, in an absolute number, such buried AGNs are at least an order of magnitude more common at z [?] 2 than today. The presence of PAH suggests that significant levels of star formation are present even if the obscured AGNs typically dominate the power budget.

We present a catalog of mid-infrared (MIR) spectra of 150 infrared (IR) luminous galaxies in the Spitzer extragalactic first look survey obtained with the IR spectrograph on board the Spitzer Space Telescope. The sample is selected to be brighter than [?]0.9 mJy at 24 mm and it has a redshift distribution in the range [0.3, 3.5], with a peak at z = 1. It primarily comprises ultraluminous IR galaxies (ULIRGs) at z [?] 1 and luminous IR galaxies at z < 1, as estimated from their monochromatic rest-frame 14 mm luminosities. The number of sources with spectra that are dominated by an active galactic nucleus (AGN) continuum is 49, while 39 sources have strong, star formation related features. For this classification, we used the equivalent width (EW) of the 11.3 mm polycyclic aromatic hydrocarbon (PAH) feature. Several intermediate and high z starbursts have higher PAH EW than local ULIRGs, which could be indicative of an elevated star formation activity. Moreover, an increase in the AGN activity is observed with increasing z and luminosity, based on the decreasing EW of PAHs and the increasing [Ne III]/ [Ne II] ratio. Spectral stacking leads to the detection of weak features such as the 3.3 mm PAH, the H2 0-0 S(1) and S(3) lines, and the [Ne V] line. We observe differences in the flux ratios of PAHs in the stacked spectra of IR-luminous galaxies with redshift or luminosity, which cannot be attributed to extinction effects since both the depth and the profile of the silicate absorption feature at 9.7 mm remain the same at z < 1 and z [?] 1. When placing the observed galaxies on IR color-color diagrams, we find that the wedge defining AGNs comprises most sources with continuum-dominated spectra, but also contains many starbursts and sources with strong silicate absorption at 9.7 mm. The comparison of the 11.3 mm PAH EW and the H-band effective radius, measured from Hubble Space Telescope data, indicates that sources with EWs [?] 2 mm, are typically more extended than [?]3 kpc. However, there is no strong correlation between the MIR spectral type and the near-IR extent of the sources.

We present the FIR, millimeter, and radio photometry and optical and NIR spectroscopy of a sample of 48 z [?] 1-3 Spitzer-selected ULIRGs with IRS MIR spectra. Our goals are to compute their bolometric emission and to determine both the presence and relative strength of their AGN and starburst components. We find that strong-PAH sources tend to have higher MIPS 160 mm and MAMBO 1.2 mm fluxes than weak-PAH sources. The depth of the 9.7 mm silicate feature does not affect MAMBO detectability. We fit the far-IR SEDs of our sample and find an average [?]LIR[?] [?] 7 x 1012 L for our 1.5"/> z > 1.5 sources. Our spectral decomposition suggests that strong-PAH sources typically have ~20%-30% AGN fractions of LIR. The weak-PAH sources by contrast tend to have [?]70% AGN fractions, with a few sources having comparable contributions of AGN and starbursts. The optical line diagnostics support the presence of AGNs in the bulk of the weak-PAH sources. With one exception, our sources are narrow-line sources, show no obvious correspondence between the available optical extinction and the silicate feature depth, and, in two cases, show some evidence for outflows. Radio AGNs are present in both strong- and weak-PAH sources. This is supported by our sample's FIR-to-radio ratios (q) being consistently below the average value of 2.34 for local star-forming galaxies. We use survival analysis to include the lower limits given by the radio-undetected sources, arriving at [?]q[?] = 2.07 +- 0.01 for our 1.5"/> z > 1.5 sample. In total, radio and, where available, optical line diagnostics support the presence of AGNs in 57% of the 1.5"/> z > 1.5 sources, independent of IR-based diagnostics. For higher z sources, the AGN luminosities alone are estimated to be >1012 L, which, supported by the available [O III] luminosities, implies that the bulk of our sources host obscured quasars.

We present Hubble Space Telescope NICMOS H-band imaging of 33 ultraluminous infrared galaxies (ULIRGs) at z [?] 2 that were selected from the 24 mm catalog of the Spitzer Extragalactic First Look Survey. The images reveal that at least 17 of the 33 objects are associated with interactions. Up to one-fifth of the sources in our sample could be minor mergers, whereas only two systems are merging binaries with luminosity ratio [?] 3:1 $3&thinspace;{:}&thinspace; 1$ , which is characteristic of local ULIRGs. The rest-frame optical luminosities of the sources are of the order 1010- 1011 L and their effective radii range from 1.4 to 4.9 kpc. The most compact sources are either those with a strong active nucleus continuum or those with a heavy obscuration in the mid-infrared regime, as determined from Spitzer Infrared Spectrograph data. The luminosity of the 7.7 mm feature produced by polycyclic aromatic hydrocarbon molecules varies significantly among compact systems, whereas it is typically large for extended systems. A bulge-to-disk decomposition performed for the six brightest ( mH < 20) sources in our sample indicates that they are best fit by disklike profiles with small or negligible bulges, unlike the bulge-dominated remnants of local ULIRGs. Our results provide evidence that the interactions associated with ultraluminous infrared activity at z [?] 2 can differ from those at z [?] 0.

We present mid-infrared observations of 18 sources from a sample of 21 z [?] 2 radio-intermediate obscured (type 2) quasars. The mid-infrared spectra of the sources are continuum dominated, and 12 sources show deep silicate absorption with t9.7 [?] 1-2. Combining mid-infrared and optical spectra, we achieve 86% spectroscopic completeness which allows us to confirm that most ( 63+ 14[?]22% ) z [?] 2 radio-intermediate quasars are obscured. The new spectra also prove that many high-redshift type 2 quasars do not show any rest-frame ultraviolet emission lines. From the 18 individual mid-infrared spectra, we classify most of the sources into three subsamples: those with hints of the 7.7 and 6.2 mm polyaromatic hydrocarbons (3/18 sources show PAHs; subsample A), those with an excess of emission around 8 mm but no hint of the 6.2 mm PAH (7/18 cases; subsample B), and pure-continuum sources with no visible excess (4/18 sources; subsample C). The remaining 4/18 sources have spectra that are featureless or too noisy for any features to be visible. In subsample A, averaging the spectra leads to a statistical detection of both 6.2 and 7.7 mm PAHs over the continuum, with the strength of the 7.7 mm PAH comparable to that of submillimeter-selected galaxies (SMGs) at similar redshifts. These sources are in a phase of coeval growth of a supermassive black hole and a host galaxy.

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z ~ 1-3 and likely LIR ~ 1012-1013 L. Our study aims at quantifying the star formation and active galactic nucleus (AGN) processes in these sources that 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 img1.gif 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 mm hot dust continua. However, their steep 30-14 mm slopes suggest that star formation might dominate the total infrared luminosity. Six of our z ~ 2 sources have EW6.2 [?] 0.3 mm and L14 mm [?] 1012 L (implying LIR [?] 1013 L). At these luminosities, such high EW6.2 ULIRGs do not exist in the local universe. We find a median optical depth at 9.7 mm of [?]t9.7 mm[?] = 1.4. This is consistent with local IRAS-selected ULIRGs but differs from early results on SCUBA-selected z ~ 2 ULIRGs. Similar to local ULIRGs, about 25% of our sample shows extreme obscuration (t9.7 mm [?] 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 the mid-infrared spectra obtained with the Spitzer IRS for 52 sources, selected as infrared luminous, z [?] 1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f24 mm [?] 0.9 mJy, nfn(24 mm)/nfn(8 mm) [?] 3.16, and nfn(24 mm)/nfn(0.7 mm) [?] 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 [?] z [?] 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 mm, 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 mm, continuum luminosities (nLn), ranging from 1010.3 to 1012.6 L. Our spectra have mid-IR slope a5-15 mm [?] 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 t9.8 mm > 1. Their L1600 A and LIR 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.

We use a simple parameterization of the mid-IR spectra of a wide range of galaxy types in order to predict their distribution in the Infrared Array Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 mm and Multiband Photometer for Spitzer 24 mm color-color diagrams. We distinguish three basic spectral types by the energetically dominant component in the 3-12 mm regime: stellar-dominated, polycyclic aromatic hydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markov chain Monte Carlo approach to arrive at a more systematic and robust representation of the mid-IR spectra of galaxies than do more traditional approaches. We find that IRAC color-color plots are well suited to distinguishing the above spectral types, while the addition of 24 mm data allows us to suggest practical three-color cuts that preferentially select higher redshift sources of a specific type. We compare our simulations with the color-color plot obtained by the Spitzer First Look Survey and find reasonable agreement. Lastly, we discuss other applications as well as future directions for this work.

We present flux densities and polarization percentages of 159 radio galaxies based on nearly simultaneous Very Large Array observations at four frequencies, 4.86, 8.46, 22.46, and 43.34 GHz. This sample is selected from the high-frequency Australia Telescope 20 GHz (AT20G) survey and consists of all sources with flux density S 20 GHz > 40 mJy in an equatorial field of the Atacama Cosmology Telescope (ACT) survey. For a subset of 25 of these sources, we used the Green Bank Telescope (GBT) to obtain 90 GHz data. The goals of this program are: (1) a characterization of the spectra, polarization, and variability of high-frequency-selected radio sources, (2) extrapolating from the few GHz regime to the [?]150 GHz regime of the ACT survey, allowing for more accurate removal of the radio source signal in our particular field, and (3) providing a data set that will allow more accurate modeling of the high-frequency radio source contamination in current and future Sunyaev-Zeldovich and cosmic microwave background experiments. We find that, as expected, this sample consists of flatter spectrum and more compact or point-like sources than low-frequency-selected samples. In the K band, variability is typically [?]20%, although there are exceptions. The higher frequency data are well suited to the detection of extreme gigahertz peak spectrum sources. The inclusion of the 43 GHz data causes the relative fraction of inverted spectrum sources to go down and of peaked spectrum sources to go up when compared with the AT20G survey results. The trend largely continues with the inclusion of the 90 GHz data, although [?]10% of the sources with GBT data show a spectral upturn from 43 GHz to 90 GHz. The measured polarization fractions are typically <5%, although in some cases they are measured to be up to [?]20%. For sources with detected polarized flux in all four bands, about 40% of the sample, the polarization fractions typically increase with frequency. This trend is stronger for steeper spectrum sources as well as for the lower flux density sources.

We present new results on the physical nature of infrared-luminous sources at 0.5 < z < 2.8 as revealed by HST/NICMOS imaging and Infrared Spectrograph mid-infrared spectroscopy. Our sample consists of 134 galaxies selected at 24 um with a flux of S(24 um)>0.9 mJy. We find many ([?]60%) of our sources to possess an important bulge and/or central point source component, most of which reveal additional underlying structures after subtraction of a best-fit Sersic (or Sersic+PSF) profile. Based on visual inspection of the NIC2 images and their residuals, we estimate that [?]80% of all our sources are mergers. We calculate lower and upper limits on the merger fraction to be 62% and 91%, respectively. At z < 1.5, we observe objects in early (pre-coalescence) merging stages to be mostly disk and star formation dominated, while we find mergers to be mainly bulge dominated and active galactic nucleus (AGN)-starburst composites during coalescence and then AGN dominated in late stages. This is analogous to what is observed in local ULIRGs. At z [?] 1.5, we find a dramatic rise in the number of objects in pre-coalescence phases of merging, despite an increase in the preponderance of AGN signatures in their mid-IR spectra and luminosities above 1012.5 L . We further find the majority of mergers at those redshifts to retain a disk-dominated profile during coalescence. We conclude that, albeit still driven by mergers, these high-z ULIRGs are substantially different in nature from their local counterparts and speculate that this is likely due to their higher gas content. Finally, we observe obscured (t9.7 um>3.36) quasars to live in faint and compact hosts and show that these are likely high-redshift analogs of local dense-core mergers. We find late-stage mergers to possess predominantly unobscured AGN spectra, but do not observe other morphological classes to carry any specific combination of t9.7 um and polycyclic aromatic hydrocarbon (PAH) equivalent width. This suggests a high degree of variation in the PAH emission and silicate absorption properties of these mergers, and possibly throughout the merging process itself.