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"Godzilla" is a peculiar object within the gravitationally lensed Sunburst Arc at $z=2.37$. Despite being very bright, it appears in only one of the twelve lensed images of the source galaxy, and shows exotic spectroscopic properties not found in any other clumps. We use JWST's unique combination of spatial resolution and spectroscopic sensitivity to provide a unified, coherent explanation of the physical nature of Godzilla. We measure fluxes and kinematic properties of rest-optical emission lines in Godzilla and surrounding regions. Using standard line ratio-based diagnostic methods in combination with NIRCam imaging and ground based rest-UV spectra, we characterize Godzilla and its surroundings. Among around 60 detected lines, we find a cascade of strong O I lines pumped by intense Ly$\beta$ emission, as well as Ly$\alpha$-pumped rest-optical Fe II lines, reminiscent of the Weigelt blobs in the local LBV star Eta Carinae. Spectra and images of Godzilla and two faint adjacent images, and the detection of a low-surface brightness foreground galaxy in the NIRCam data, support the interpretation that Godzilla is an extremely magnified object due to the alignment with lensing caustics. We find that Godzilla is part of a previously identified clump, comprising $\sim10 - 25$ % of it, with magnifications in the range of $\approx600-25,000$ depending on the models and images in comparison. The unique O I source in Godzilla is well explained by a non-erupting LBV accompanied by a hotter companion and/or gas condensations exposed to more intense radiation compared to the Weigelt blobs. If Godzilla is confirmed to contain an LBV star, it expands the distance to the furthest known LBV from a dozen Mpc to several Gpc., Comment: 22 pages, 19 figures. Submitted to A&A

We report the detection of a population of Wolf-Rayet (WR) stars in the Sunburst Arc, a strongly gravitationally lensed galaxy at redshift $z=2.37$. As the brightest known lensed galaxy, the Sunburst Arc has become an important cosmic laboratory for studying star and cluster formation, Lyman $\alpha$ radiative transfer, and Lyman Continuum (LyC) escape. Here, we present the first results of JWST/NIRSpec IFU observations of the Sunburst Arc, focusing on a stacked spectrum of the 12-fold imaged LyC-emitting (Sunburst LCE) cluster. In agreement with previous studies, we find that the cluster is massive and compact, with $M_{\text{dyn}} = (9\pm1) \times 10^{6} M_{\odot}$, Our age estimate of 4.2--4.5 Myr is much larger than the crossing time of $t_{\text{cross}} = 183 \pm 9 $ kyr, indicating that the cluster is dynamically evolved and consistent with being gravitationally bound. We find a significant nitrogen enhancement of the low ionization state ISM, with $\log(N/O) = -0.74 \pm 0.09$, which is $\approx 0.8$ dex above typical values for H II regions of similar metallicity in the local Universe. We find broad stellar emission complexes around He II$\lambda 4686$ and C IV$\lambda 5808$ with associated nitrogen emission -- this is the first time WR signatures have been directly observed at redshifts above $\sim 0.5$. The strength of the WR signatures cannot be reproduced by stellar population models that only include single-star evolution. While models with binary evolution better match the WR features, they still struggle to reproduce the nitrogen-enhanced WR features. JWST reveals the Sunburst LCE to be a highly ionized, proto-globular cluster with low oxygen abundance and extreme nitrogen enhancement that hosts a population of Wolf-Rayet stars, and possibly Very Massive stars (VMSs), which are rapidly enriching the surrounding medium., Comment: 8 pages, 4 figures, 3 tables. Accepted for publication in A&A

Early galaxy formation, initiated by the dark matter and gas assembly, evolves through frequent mergers and feedback processes into dynamically hot, chaotic structures. In contrast, dynamically cold, smooth rotating disks have been observed in massive evolved galaxies merely 1.4 billion years after the Big Bang, suggesting rapid morphological and dynamical evolution in the early Universe. Probing this evolution mechanism necessitates studies of young galaxies, yet efforts have been hindered by observational limitations in both sensitivity and spatial resolution. Here we report high-resolution observations of a strongly lensed and quintuply imaged, low-luminosity, young galaxy at $z=6.072$ (dubbed the Cosmic Grapes), 930 million years after the Big Bang. Magnified by gravitational lensing, the galaxy is resolved into at least 15 individual star-forming clumps with effective radii of $r_{\rm e}\simeq$ 10--60 parsec (pc), which dominate $\simeq$ 70\% of the galaxy's total flux. The cool gas emission unveils a smooth, underlying rotating disk characterized by a high rotational-to-random motion ratio and a gravitationally unstable state (Toomre $Q \simeq$ 0.2--0.3), with high surface gas densities comparable to local dusty starbursts with $\simeq10^{3-5}$ $M_{\odot}$/pc$^{2}$. These gas properties suggest that the numerous star-forming clumps are formed through disk instabilities with weak feedback effects. The clumpiness of the Cosmic Grapes significantly exceeds that of galaxies at later epochs and the predictions from current simulations for early galaxies. Our findings shed new light on internal galaxy substructures and their relation to the underlying dynamics and feedback mechanisms at play during their early formation phases, potentially explaining the high abundance of bright galaxies observed in the early Universe and the dark matter core-cusp problem., Comment: Submitted. 44 pages, 14 figures, 5 tables. Comments are welcome! See also the companion papers on arXiv. Valentino+2024: arXiv:2402.17845 Gim\'enez-Arteaga+2024: arXiv:2402.17875

Star-forming galaxies (SFGs) with stellar masses below $10^{10}\,M_\odot$ make up the bulk of the galaxy population at $z>2$. The properties of the cold gas in these galaxies can only be probed in very deep ALMA observations or by targeting strongly lensed galaxies. Here we report the results of a pilot survey using the Atacama Compact Array (ACA) of molecular gas in the most strongly magnified galaxies selected as giant arcs in optical data. The selection in rest-frame UV wavelengths ensures that sources are regular star forming galaxies, without a priori indications of intense dusty starburst activity. We conducted Band 4 and Band 7 observations to detect mid-$J$ CO, [C I] and thermal continuum as molecular gas tracers from four strongly lensed systems at $z\approx2-3$: our targets are SGAS J1226651.3+215220 (A and B), SGAS J003341.5+024217 and the Sunburst Arc. The measured molecular mass is then projected onto the source plane with detailed lens models developed from high resolution HST observations. Multiwavelength photometry is then used to obtain the intrinsic stellar mass and star formation rate via SED fitting. In only one of the sources are the three tracers robustly detected, while in the others they are either undetected or detected in continuum only. The implied molecular gass masses range from $4\times 10^{9}\,M_\odot$ in the detected source to an upper limit of $\lesssim 10^9\,M_\odot$ in the most magnified source. The inferred gas fraction and gas depletion timescale are found to lie approximately 0.5 to 1.0 dex below the established scaling relations based on previous studies of unlensed massive galaxies. Our results indicate that the cold gas content of intermediate to low mass galaxies should not be extrapolated from the trends seen in more massive high-$z$ galaxies. (Abridged abstract), Comment: 17 pages, 7 figures. Accepted for publication in Astronomy & Astrophysics

We present gravitational-arc tomography of the cool-warm enriched circumgalactic medium (CGM) of an isolated galaxy (``G1'') at $z \approx 0.77$. Combining VLT/MUSE adaptive-optics and Magellan/MagE echelle spectroscopy we obtain partially-resolved kinematics of MgII in absorption and [OII] in emission. The unique arc configuration allows us to probe 42 spatially independent arc positions transverse to G1, plus 4 positions in front of it. The transverse positions cover G1's minor and major axes at impact parameters of $\approx 10-30$ kpc and $\approx 60$ kpc, respectively. We observe a direct kinematic connection between the cool-warm enriched CGM (traced by MgII) and the interstellar medium (traced by [OII]). This provides strong evidence for the existence of an extended disc that co-rotates with the galaxy out to tens of kiloparsecs. The MgII velocity dispersion ($\sigma \approx 30-100$ km s$^{-1}$, depending on position) is of the same order as the modeled galaxy rotational velocity ($v_{\rm rot} \approx 80$ km s$^{-1}$), providing evidence for the presence of a turbulent and pressure-supported CGM component. We regard the absorption to be modulated by a galactic-scale outflow, as it offers a natural scenario for the observed line-of-sight dispersion and asymmetric profiles observed against both the arcs and the galaxy. An extended enriched co-rotating disc together with the signatures of a galactic outflow, are telltale signs of metal recycling in the $z\sim 1$ CGM., Comment: MNRAS accepted. Moderate changes after addressing referee feedback, including a correction in the systemic redshift; all qualitative results and conclusions remain the same

For the extremely bright lensed galaxy SDSS J1723+3411 at z=1.3293 , we analyze spatially integrated MMT, Keck, and Hubble Space Telescope spectra that fully cover the rest-frame wavelength range of 1400 to 7200 Angstroms. We also analyze near-IR spectra from Gemini that cover H alpha for a portion of the lensed arc. We report fluxes for 42 detected emission lines, and upper limits for an additional 22. This galaxy has extreme emission line ratios and high equivalent widths that are characteristic of extreme emission-line galaxies. We compute strong emission line diagnostics from both the rest-frame optical and rest-frame ultraviolet (UV), to constrain physical conditions and test the spectral diagnostics themselves. We tightly determine the nebular physical conditions using the most reliable diagnostics, and then compare to results from other diagnostics. We find disappointing performance from the UV--only diagnostics: they either are unable to measure the metallicity or dramatically under-estimate it; they over-estimate the pressure; and the UV diagnostic of ionization parameter has a strong metallicity dependence in this regime. Based on these results, we suggest that upcoming James Webb Space Telescope spectroscopic surveys of galaxies in the reionization epoch should invest the additional integration time to capture the optical [O II] and [O III] emission lines, and not rely solely on the rest-frame UV emission lines. We make available the spectra; they represent one of the highest-quality emission line spectral atlases of star-forming galaxy available beyond the local universe, and will aid planning observations with JWST., Comment: Accepted for publication in ApJ

We present spatially-resolved echelle spectroscopy of an intervening MgII-FeII-MgI absorption-line system detected at $z_{\rm abs}=0.73379$ toward the giant gravitational arc PSZ1 G311.65-18.48. The absorbing gas is associated to an inclined disk-like star-forming galaxy, whose major axis is aligned with the two arc-segments reported here. We probe in absorption the galaxy's extended disk continuously, at $\approx 3$ kpc sampling, from its inner region out to $15\times$ the optical radius. We detect strong ($W_0^{2796}>0.3$ \r{A}) coherent absorption along $13$ independent positions at impact parameters $D=0$--$29$ kpc on one side of the galaxy, and no absorption at $D=28$--$57$ kpc on the opposite side (all de-lensed distances at $z_{\rm abs}$). We show that: (1) the gas distribution is anisotropic; (2) $W_0^{2796}$, $W_0^{2600}$, $W_0^{2852}$, and the ratio $W_0^{2600}\!/W_0^{2796}$, all anti-correlate with $D$; (3) the $W_0^{2796}$-$D$ relation is not cuspy and exhibits significantly less scatter than the quasar-absorber statistics; (4) the absorbing gas is co-rotating with the galaxy out to $D \lesssim 20$ kpc, resembling a `flat' rotation curve, but at $D\gtrsim 20$ kpc velocities decline below the expectations from a 3D disk-model extrapolated from the nebular [OII] emission. These signatures constitute unambiguous evidence for rotating extra-planar diffuse gas, possibly also undergoing enriched accretion at its edge. Arguably, we are witnessing some of the long-sought processes of the baryon cycle in a single distant galaxy expected to be representative of such phenomena., Comment: Accepted for publication in MNRAS

In the past decade, our understanding of how stars and galaxies formed during the first 5 billion years after the Big Bang has been revolutionized by observations that leverage gravitational lensing by intervening masses, which act as natural cosmic telescopes to magnify background sources. Previous studies have harnessed this effect to probe the distant universe at ultraviolet, optical, infrared and millimeter wavelengths. However, strong lensing studies of young, star-forming galaxies have never extended into X-ray wavelengths, which uniquely trace high-energy phenomena. Here we report an X-ray detection of star formation in a highly magnified, strongly lensed galaxy. This lensed galaxy, seen during the first third of the history of the Universe, is a low--mass, low--metallicity starburst with elevated X-ray emission, and is a likely analog to the first generation of galaxies. Our measurements yield insight into the role that X-ray emission from stellar populations in the first generation of galaxies may play in re-ionizing the Universe. This observation paves the way for future strong lensing-assisted X-ray studies of distant galaxies reaching orders of magnitude below the detection limits of current deep fields, and previews the depths that will be attainable with future X-ray observatories., Comment: Published in Nature Astronomy

We infer the properties of massive star populations using the far-ultraviolet stellar continua of 61 star-forming galaxies: 42 at low-z observed with HST and 19 at z~2 from the Megasaura sample. We fit each stellar continuum with a linear combination of up to 50 single age and single metallicity Starburst99 models. From these fits, we derive light-weighted ages and metallicities, which agree with stellar wind and photospheric spectral features, and infer the spectral shapes and strengths of the ionizing continua. Inferred light-weighted stellar metallicities span 0.05-1.5 Z$_\odot$ and are similar to the measured nebular metallicities. We quantify the ionizing continua using the ratio of the ionizing flux at 900\AA\ to the non-ionizing flux at 1500\AA\ and demonstrate the evolution of this ratio with stellar age and metallicity using theoretical single burst models. These single burst models only match the inferred ionizing continua of half of the sample, while the other half are described by a mixture of stellar ages. Mixed age populations produce stronger and harder ionizing spectra than continuous star formation histories, but, contrary to previous studies that assume constant star formation, have similar stellar and nebular metallicities. Stellar population age and metallicity affect the far-UV continua in different and distinguishable ways; assuming a constant star formation history diminishes the diagnostic power. Finally, we provide simple prescriptions to determine the ionizing photon production efficiency ($\xi_{ion}$) from the stellar population properties. $\xi_{ion}$ has a range of log($\xi_{ion})=24.4-25.7$ Hz erg$^{-1}$ that depends on stellar age, metallicity, star formation history, and contributions from binary star evolution. These stellar population properties must be observationally determined to determine the number of ionizing photons generated by massive stars., Comment: 31 pages, 23 figures, resubmitted to ApJ after incorporating the referee's comments. Comments encouraged

We present the first spatially resolved analysis of rest-frame optical and UV imaging and spectroscopy for a lensed galaxy at z = 2.39 hosting a Seyfert active galactic nucleus (AGN). Proximity to a natural guide star has enabled high signal-to-noise VLT SINFONI + adaptive optics observations of rest-frame optical diagnostic emission lines, which exhibit an underlying broad component with FWHM ~ 700 km/s in both the Balmer and forbidden lines. Measured line ratios place the outflow robustly in the region of the ionization diagnostic diagrams associated with AGN. This unique opportunity - combining gravitational lensing, AO guiding, redshift, and AGN activity - allows for a magnified view of two main tracers of the physical conditions and structure of the interstellar medium in a star-forming galaxy hosting a weak AGN at cosmic noon. By analyzing the spatial extent and morphology of the Ly-alpha and dust-corrected H-alpha emission, disentangling the effects of star formation and AGN ionization on each tracer, and comparing the AGN induced mass outflow rate to the host star formation rate, we find that the AGN does not significantly impact the star formation within its host galaxy., Comment: 16 pages, 5 figures, accepted for publication in ApJ

The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early universe. However, this escape fraction cannot be measured from high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to indirectly measure the escape fraction of high-redshift galaxies, it is unlikely that we will know what reionized the universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use the H I covering fractions, column densities, and dust attenuations measured in a companion paper to predict the escape fraction of ionizing photons. We find good agreement between the predicted and observed Lyman continuum escape fractions (within $1.4\sigma$) using both the H I and ISM absorption lines. The ionizing photons escape through holes in the H I, but we show that dust attenuation reduces the fraction of photons that escape galaxies. This means that the average high-redshift galaxy likely emits more ionizing photons than low-redshift galaxies. Two other indirect methods accurately predict the escape fractions: the Ly$\alpha$ escape fraction and the optical [O III]/[O II] flux ratio. We use these indirect methods to predict the escape fraction of a sample of 21 galaxies with rest-frame UV spectra but without Lyman continuum observations. Many of these galaxies have low escape fractions ($f_{\rm esc} \le 1$\%), but 11 have escape fractions $>1$\%. The methods presented here will measure the escape fractions of high-redshift galaxies, enabling future telescopes to determine whether star-forming galaxies reionized the early universe., Comment: Accepted for publication in A&A. 12 pages, 5 figures

We stack the rest-frame ultraviolet spectra of N=14 highly magnified gravitationally lensed galaxies at redshifts 1.6

We introduce Project MEGaSaURA: The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas. MEGaSaURA comprises medium-resolution, rest-frame ultraviolet spectroscopy of N=15 bright gravitationally lensed galaxies at redshifts of 1.68$<$z$<$3.6, obtained with the MagE spectrograph on the Magellan telescopes. The spectra cover the observed-frame wavelength range $3200 < \lambda_o < 8280$ \AA ; the average spectral resolving power is R=3300. The median spectrum has a signal-to-noise ratio of $SNR=21$ per resolution element at 5000 \AA . As such, the MEGaSaURA spectra have superior signal-to-noise-ratio and wavelength coverage compared to what COS/HST provides for starburst galaxies in the local universe. This paper describes the sample, the observations, and the data reduction. We compare the measured redshifts for the stars, the ionized gas as traced by nebular lines, and the neutral gas as traced by absorption lines; we find the expected bulk outflow of the neutral gas, and no systemic offset between the redshifts measured from nebular lines and the redshifts measured from the stellar continuum. We provide the MEGaSaURA spectra to the astronomical community through a data release., Comment: Resubmitted to AAS Journals. Data release will accompany journal publication. v2 addresses minor comments from referee

Galactic outflows regulate the amount of gas galaxies convert into stars. However, it is difficult to measure the mass outflows remove because they span a large range of temperatures and phases. Here, we study the rest-frame ultraviolet spectrum of a lensed galaxy at z~2.9 with prominent interstellar absorption lines from O I, tracing neutral gas, up to O VI, tracing transitional phase gas. The O VI profile mimics weak low-ionization profiles at low velocities, and strong saturated profiles at high velocities. These trends indicate that O VI gas is co-spatial with the low-ionization gas. Further, at velocities blueward of -200 km/s the column density of the low-ionization outflow rapidly drops while the O VI column density rises, suggesting that O VI is created as the low-ionization gas is destroyed. Photoionization models do not reproduce the observed O VI, but adequately match the low-ionization gas, indicating that the phases have different formation mechanisms. Photoionized outflows are more massive than O VI outflows for most of the observed velocities, although the O VI mass outflow rate exceeds the photoionized outflow at velocities above the galaxy's escape velocity. Therefore, most gas capable of escaping the galaxy is in a hot outflow phase. We suggest that the O VI absorption is a temporary by-product of conduction transferring mass from the photoionized phase to an unobserved hot wind, and discuss how this mass-loading impacts the observed circum-galactic medium., Comment: 17 pages, 12 figures, accepted for publication in MNRAS

For lensed galaxy SGAS J111020.0+645950.8 at redshift z=2.481, which is magnified by a factor of 28 +- 8, we analyze the morphology of star formation as traced by rest-frame ultraviolet emission, in both the highly-magnified source plane, and in simulations of how this galaxy would appear without lensing magnification. Were this galaxy not lensed but drawn from an HST deep field, we would conclude that almost all its star formation arises from an exponential disk (S\'ersic index of 1.0 +- 0.4) with an effective radius of r_e = 2.7 +- 0.3 kpc measured from two-dimensional fitting to F606W using Galfit, and r_e=1.9 +- 0.1 kpc measured by fitting a radial profile to F606W elliptical isophotes. At the normal spatial resolution of the deep fields, there is no sign of clumpy star formation within SGAS J111020.0+645950.8 . However, the enhanced spatial resolution enabled by gravitational lensing tells a very different story: much of the star formation arises in two dozen clumps with sizes of r=30--50 pc spread across the 7 kpc length of the galaxy. The color and spatial distribution of the diffuse component suggests that still smaller clumps are unresolved. Despite this clumpy, messy morphology, the radial profile is still well-characterized by an exponential profile. In this lensed galaxy, stars are forming in complexes with sizes well below 100 pc; such sizes are wholly unexplored by surveys of galaxy evolution at 1

We present two NuSTAR observations of the local Seyfert 2 active galactic nucleus (AGN) and an ultraluminous X-ray source (ULX) candidate in NGC 5643. Together with archival data from Chandra, XMM-Newton and Swift-BAT, we perform a high-quality broadband spectral analysis of the AGN over two decades in energy ($\sim$0.5-100 keV). Previous X-ray observations suggested that the AGN is obscured by a Compton-thick (CT) column of obscuring gas along our line-of-sight. However, the lack of high-quality $\gtrsim$ 10 keV observations, together with the presence of a nearby X-ray luminous source, NGC 5643 X-1, had left significant uncertainties in the characterization of the nuclear spectrum. NuSTAR now enables the AGN and NGC 5643 X-1 to be separately resolved above 10 keV for the first time and allows a direct measurement of the absorbing column density toward the nucleus. The new data show that the nucleus is indeed obscured by a CT column of $N_{\rm{H}}$ $\gtrsim$ 5 $\times$ 10$^{24}$ cm$^{-2}$. The range of 2-10 keV absorption-corrected luminosity inferred from the best fitting models is $L_{2-10,\rm{int}} =$ (0.8-1.7) $\times$ 10$^{42}$ erg s$^{-1}$, consistent with that predicted from multiwavelength intrinsic luminosity indicators. We also study the NuSTAR data for NGC 5643 X-1, and show that it exhibits evidence for a spectral cut-off at energy, $E$ $\sim$ 10 keV, similar to that seen in other ULXs observed by NuSTAR. Along with the evidence for significant X-ray luminosity variations in the 3-8 keV band from 2003-2014, our results further strengthen the ULX classification of NGC 5643 X-1., Comment: Resubmitted to ApJ on Sept. 10, 2015 after incorporating the referee's comments; 14 pages, 6 figures and 3 tables

We report first results from an ongoing monitoring campaign to measure time delays between the six images of the quasar SDSS\,J2222$+$2745, gravitationally lensed by a galaxy cluster. The time delay between A and B, the two most highly magnified images, is measured to be $\tau_{\rm AB} = 47.7 \pm 6.0$ days (95\% confidence interval), consistent with previous model predictions for this lens system. The strong intrinsic variability of the quasar also allows us to derive a time delay value of $\tau_{\rm CA} = 722 \pm 24$ days between image C and A, in spite of modest overlap between their light curves in the current data set. Image C, which is predicted to lead all the other lensed quasar images, has undergone a sharp, monotonic flux increase of 60-75\% during 2014. A corresponding brightening is firmly predicted to occur in images A and B during 2016. The amplitude of this rise indicates that time delays involving all six known images in this system, including those of the demagnified central images D-F, will be obtainable from further ground-based monitoring of this system during the next few years., Comment: 9 pages, 9 figures, Version accepted for publication in ApJ

We examine the Mg II 2796, 2803 Angstrom, Lyman alpha, and nebular line emission in five bright star-forming galaxies at 1.66

We present high-energy (3--30 keV) {\it NuSTAR} observations of the nearest quasar, the ultraluminous infrared galaxy (ULIRG) Markarian 231 (Mrk 231), supplemented with new and simultaneous low-energy (0.5--8 keV) data from {\it Chandra}. The source was detected, though at much fainter levels than previously reported, likely due to contamination in the large apertures of previous non-focusing hard X-ray telescopes. The full band (0.5--30 keV) X-ray spectrum suggests the active galactic nucleus (AGN) in Mrk 231 is absorbed by a patchy and Compton-thin (N$_{\rm H} \sim1.2^{+0.3}_{-0.3}\times10^{23}$ cm$^{-2}$) column. The intrinsic X-ray luminosity (L$_{\rm 0.5-30 keV}\sim1.0\times10^{43}$ erg s$^{1}$) is extremely weak relative to the bolometric luminosity where the 2--10 keV to bolometric luminosity ratio is $\sim$0.03% compared to the typical values of 2--15%. Additionally, Mrk 231 has a low X-ray-to-optical power law slope ($\alpha_{\rm OX}\sim-1.7$). It is a local example of a low-ionization broad absorption line (LoBAL) quasar that is intrinsically X-ray weak. The weak ionizing continuum may explain the lack of mid-infrared [O IV], [Ne V], and [Ne VI] fine-structure emission lines which are present in sources with otherwise similar AGN properties. We argue that the intrinsic X-ray weakness may be a result of the super-Eddington accretion occurring in the nucleus of this ULIRG, and may also be naturally related to the powerful wind event seen in Mrk 231, a merger remnant escaping from its dusty cocoon., Comment: 15 pages in ApJ style including 6 figures and 1 table. Accepted for publication in ApJ

We present HST/WFC3 narrowband imaging of the H-alpha emission in a sample of eight gravitationally-lensed galaxies at z = 1 - 1.5. The magnification caused by the foreground clusters enables us to obtain a median source plane spatial resolution of 360pc, as well as providing magnifications in flux ranging from ~10x to ~50x. This enables us to identify resolved star-forming HII regions at this epoch and therefore study their H-alpha luminosity distributions for comparisons with equivalent samples at z ~ 2 and in the local Universe. We find evolution in the both luminosity and surface brightness of HII regions with redshift. The distribution of clump properties can be quantified with an HII region luminosity function, which can be fit by a power law with an exponential break at some cut-off, and we find that the cut-off evolves with redshift. We therefore conclude that `clumpy' galaxies are seen at high redshift because of the evolution of the cut-off mass; the galaxies themselves follow similar scaling relations to those at z = 0, but their HII regions are larger and brighter and thus appear as clumps which dominate the morphology of the galaxy. A simple theoretical argument based on gas collapsing on scales of the Jeans mass in a marginally unstable disk shows that the clumpy morphologies of high-z galaxies are driven by the competing effects of higher gas fractions causing perturbations on larger scales, partially compensated by higher epicyclic frequencies which stabilise the disk., Comment: 16 pages, 9 figures, 2 tables, accepted for publication in MNRAS

We present spectroscopic observations in the rest-frame optical and near- to mid-infrared wavelengths of four gravitationally lensed infrared (IR) luminous star-forming galaxies at redshift 1 < z < 3 from the LUCIFER instrument on the Large Binocular Telescope and the Infrared Spectrograph on Spitzer. The sample was selected to represent pure, actively star-forming systems, absent of active galactic nuclei. The large lensing magnifications result in high signal-to-noise spectra that can probe faint IR recombination lines, including Pa-alpha and Br-alpha at high redshifts. The sample was augmented by three lensed galaxies with similar suites of unpublished data and observations from the literature, resulting in the final sample of seven galaxies. We use the IR recombination lines in conjunction with H-alpha observations to probe the extinction, Av, of these systems, as well as testing star formation rate (SFR) indicators against the SFR measured by fitting spectral energy distributions to far-IR photometry. Our galaxies occupy a range of Av from ~0 to 5.9 mag, larger than previously known for a similar range of IR luminosities at these redshifts. Thus, estimates of SFR even at z ~ 2 must take careful count of extinction in the most IR luminous galaxies. We also measure extinction by comparing SFR estimates from optical emission lines with those from far-IR measurements. The comparison of results from these two independent methods indicates a large variety of dust distribution scenarios at 1 < z < 3. Without correcting for dust extinction, the H-alpha SFR indicator underestimates the SFR; the size of the necessary correction depends on the IR luminosity and dust distribution scenario. Individual SFR estimates based on the 6.2 micron PAH emission line luminosity do not show a systematic discrepancy with extinction, although a considerable, ~0.2 dex scatter is observed., Comment: Accepted for publication in The Astrophysical Journal; 14 pages, 8 figures

Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the deployment of the first hard X-ray imaging telescope that, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (~20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a >10x increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population show enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions., Comment: 43 pages, 10 figures, accepted by the ApJ

We report rest-frame optical Keck/NIRSPEC spectroscopy of the bright lensed galaxy RCSGA 032727-132609 at z=1.7037. From precise measurements of the nebular lines, we infer a number of physical properties: redshift, extinction, star formation rate, ionization parameter, electron density, electron temperature, oxygen abundance, and N/O, Ne/O, and Ar/O abundance ratios. The limit on [O III]~4363 A tightly constrains the oxygen abundance via the "direct" or electron temperature method, for the first time in an average-metallicity galaxy at z~2. We compare this result to several standard "bright-line" O abundance diagnostics, thereby testing these empirically-calibrated diagnostics in situ. Finally, we explore the positions of lensed and unlensed galaxies in standard diagnostic diagrams, and explore the diversity of ionization conditions and mass--metallicity ratios at z=2., Comment: ApJ in press. 15 pages, 7 figures

The fate of ionizing radiation is vital for understanding cosmic ionization, energy budgets in the interstellar and intergalactic medium, and star formation rate indicators. The low observed escape fractions of ionizing radiation have not been adequately explained, and there is evidence that some starbursts have high escape fractions. We examine the spectral energy distributions of a sample of local star-forming galaxies, containing thirteen local starburst galaxies and ten of their ordinary star-forming counterparts, to determine if there exist significant differences in the fate of ionizing radiation in these galaxies. We find that the galaxy-to-galaxy variations in the SEDs is much larger than any systematic differences between starbursts and non-starbursts. For example, we find no significant differences in the total absorption of ionizing radiation by dust, traced by the 24um, 70um, and 160um MIPS bands of the Spitzer Space Telescope, although the dust in starburst galaxies appears to be hotter than that of non-starburst galaxies. We also observe no excess ultraviolet flux in the GALEX bands that could indicate a high escape fraction of ionizing photons in starburst galaxies. The small H-alpha fractions of the diffuse, warm ionized medium in starburst galaxies are apparently due to temporarily boosted H-alpha luminosity within the star-forming regions themselves, with an independent, constant WIM luminosity. This independence of the WIM and starburst luminosities contrasts with WIM behavior in non-starburst galaxies and underscores our poor understanding of radiation transfer in both ordinary and starburst galaxies., Comment: 10 pages, 8 figures, accepted to ApJ 10/11/10

We report the discovery of two strongly-lensed z ~ 3 Lyman Break Galaxies (LBGs) discovered as u-band dropouts as part of the SDSS Giant Arcs Survey (SGAS). The first, SGAS J122651.3+215220 at z = 2.9233 is lensed by one of several sub-clusters, SDSS J1226+2152, in a complex massive cluster at z = 0.43. Its (g, r, i) magnitudes are (21.14, 20.60, 20.51) which translate to surface brightnesses, mu_{g,r,i}, of (23.78, 23.11, 22.81). The second, SGAS J152745.1+065219, is an LBG at z = 2.7593 lensed by the foreground SDSS J1527+0652 at z = 0.39, with (g, r, z)=(20.90, 20.52, 20.58) and mu_{g,r,z}=(25.15, 24.52, 24.12). Moderate resolution spectroscopy confirms the redshifts suggested by photometric breaks, and shows both absorption and emission features typical of LBGs. Lens mass models derived from combined imaging and spectroscopy reveal that SGAS J122651.3+215220 is a highly magnified source (M ~40), while SGAS J152745.1+065219 is magnified by no more than M ~ 15. Compared to LBG survey results (Steidel et al. 2003), the luminosities and lensing-corrected magnitudes suggest that SGAS J122651.3+215220 is among the faintest 20% of LBGs in that sample. SGAS J152745.1+065219, on the other hand, appears to be more representative of the average LBG, similar to the "Cosmic Eye"., Comment: 5 pages, 3 figures, 1 Table (emulateapj). Submitted to ApJL

We compare [O IV] 25.89 micron emission line luminosities with very hard (10-200 keV) X-rays from Swift, Integral, and BeppoSAX for a complete sample of 89 Seyferts from the Revised Shapley-Ames sample. Using Seyfert 1s, we calibrate [O IV] as a measure of AGN intrinsic luminosity, for particular use in high-obscuration environments. With this calibration, we measure the average decrement in 14-195 keV X-ray to [O IV] luminosity ratio for Seyfert 2s compared to type 1s. We find a decrement of 3.1 +- 0.8 for Seyfert 2s, and a decrement of 5.0 +- 2.7 for known Compton-thick Seyfert 2s. These decrements imply column densities of approximately log N(H)=24.6 and 24.7 cm^-2, respectively. Thus, we infer that the average Seyfert 2 is more highly obscured and intrinsically more luminous than would be inferred even from the very hard X-rays. We demonstrate two applications of the hard X-ray to [O IV] ratio. We measure a column density for the extremely obscured NGC 1068 of log N(H)=25.3-25.4 cm^-2. Finally, by comparing [O IV] luminosities to total infrared luminosities for twelve bright ultraluminous infrared galaxies, we find that four have substantial AGN contributions., Comment: ApJ in press

We use extensive new observations of the very rich z ~ 0.4 cluster of galaxies A851 to examine the nature and origin of starburst galaxies in intermediate-redshift clusters. New HST observations, Spitzer photometry and ground-based spectroscopy cover most of a region of the cluster about 10 arcmin across, corresponding to a clustercentric radial distance of about 1.6 Mpc. This spatial coverage allows us to confirm the existence of a morphology-density relation within this cluster, and to identify several large, presumably infalling, subsystems. We confirm our previous conclusion that a very large fraction of the starforming galaxies in A851 have recently undergone starbursts. We argue that starbursts are mostly confined to two kinds of sites: infalling groups and the cluster center. At the cluster center it appears that infalling galaxies are undergoing major mergers, resulting in starbursts whose optical emission lines are completely buried beneath dust. The aftermath of this process appears to be proto-S0 galaxies devoid of star formation. In contrast, major mergers do not appear to be the cause of most of the starbursts in infalling groups, and fewer of these events result in the transformation of the galaxy into an S0. Some recent theoretical work provides possible explanations for these two distinct processes, but it is not clear whether they can operate with the very high efficiency needed to account for the very large starburst rate observed., Comment: Accepted for publication in ApJ

Spitzer-MIPS 24 micron and ground-based observations of the rich galaxy cluster Abell 851 at z=0.41 are used to derive star formation rates from the mid-IR 24 micron and from [O II] 3727 emission. Many cluster galaxies have SFR(24 um)/SFR([O II]) >> 1, indicative of star formation highly obscured by dust. We focus on the substantial minority of A851 cluster members where strong Balmer absorption points to a starburst on a 10^{8-9} year timescale. As is typical, A851 galaxies with strong Balmer absorption occur in two types: with optical emission (starforming), and without (post-starburst). Our principal result is the former, so-called e(a) galaxies, are mostly detected (9 out of 12) at 24 um -- for these we find typically SFR(24 um)/SFR([O II]) ~ 4. Strong Balmer absorption and high values of SFR(24 um)/SFR([O II]) both indicate moderately active starbursts and support the picture that e(a) galaxies are the active starbursts that feed the post-starburst population. While 24 um detections are frequent with Balmer-strong objects (even 6 out of 18 of the supposedly "post-starburst'' galaxies are detected) only 2 out of 7 of the continuously starforming `e(c)' galaxies (with weak Balmer absorption) are detected -- for them, SFR(24 um)/SFR([O II]) ~ 1. Their optical spectra resemble present-epoch spirals that dominate today's universe; we strengthen this association by that SFR(24 um)/SFR([O II]) ~ 1 is the norm today. That is, not just the amount of star formation, but its mode, has evolved strongly from z ~ 0.4 to the present. By fitting spectrophotometric models we measure the strength and duration of the bursts to quantify the evolutionary sequence from active- to post-starburst, and to harden the evidence that moderately active starbursts are the defining feature of starforming cluster galaxies at z ~ 0.4., Comment: Accepted to The Astrophysical Journal. Abstract abridged

We study the stellar and star formation (SF) properties of the host galaxies of 58 X-ray selected AGN in the GOODS portion of the Chandra Deep Field South (CDF-S) region at z~0.5-1.4. The AGN are selected such that their rest-frame UV to near-IR SEDs are dominated by stellar emission, i.e., they show a prominent 1.6micron bump, thus minimizing the AGN emission 'contamination'. This AGN population comprises approximately 50% of the X-ray selected AGN at these redshifts. Using models of stellar and dust emission we model their SEDs to derive stellar masses (M*) and total (UV+IR) star formation rates (SFR). AGN reside in the most massive galaxies at the redshifts probed here. Their characteristic stellar masses (M*~7.8x10^10 Msun and M*~1.2x10^11 Msun at median z of 0.67 and 1.07, respectively) appear to be representative of the X-ray selected AGN population at these redshifts, and are intermediate between those of local type 2 AGN and high redshift (z~2) AGN. The inferred black hole masses (MBH~ 2x10^8 Msun) of typical AGN are similar to those of optically identified quasars at similar redshifts. Since the AGN in our sample are much less luminous (L(2-10keV)<10^44 erg/s) than quasars, typical AGN have low Eddington ratios (eta~0.01-0.001). This suggests that, at least at intermediate z, the cosmic AGN 'downsizing' is due to both a decrease in the characteristic stellar mass of typical host galaxies, and less efficient accretion. Finally there is no strong evidence in AGN host galaxies for either highly suppressed SF (expected if AGN played a role in quenching SF) or elevated SF when compared to mass selected (i.e., IRAC-selected) galaxies of similar stellar masses and redshifts. This may be explained by the fact that galaxies with M*~5x 10^10 - 5x10^11 Msun are still being assembled at the redshifts probed here., Comment: Accepted for Publication in ApJ

We present Spitzer/IRS mid-infrared spectra for 15 gravitationally lensed, 24 micron--selected galaxies, and combine the results with 4 additional very faint galaxies with IRS spectra in the literature. The median intrinsic 24 micron flux density of the sample is 130 microJy, enabling a systematic survey of the spectral properties of the very faint 24 micron sources that dominate the number counts of Spitzer cosmological surveys. Six of the 19 galaxy spectra (32%) show the strong mid-IR continuua expected of AGN; X-ray detections confirm the presence of AGN in three of these cases, and reveal AGNs in two other galaxies. These results suggest that nuclear accretion may contribute more flux to faint 24 micron--selected samples than previously assumed. Almost all the spectra show some aromatic (PAH) emission features; the measured aromatic flux ratios do not show evolution from z=0. In particular, the high S/N mid-IR spectrum of SMM J163554.2+661225 agrees remarkably well with low--redshift, lower--luminosity templates. We compare the rest-frame 8 micron and total infrared luminosities of star--forming galaxies, and find that the behavior of this ratio with total IR luminosity has evolved modestly from z=2 to z=0. Since the high aromatic--to--continuum flux ratios in these galaxies rule out a dominant contribution by AGN, this finding implies systematic evolution in the structure and/or metallicity of infrared sources with redshift. It also has implications for the estimates of star forming rates inferred from 24 micron measurements, in the sense that at z ~2, a given observed frame 24 micron luminosity corresponds to a lower bolometric luminosity than would be inferred from low-redshift templates of similar luminosity at the corresponding rest wavelength., Comment: Accepted to the Astrophysical Journal. Full-res version at ftp://ftp.ociw.edu/pub/jrigby/jrigby-irs.pdf

We describe two peculiar galaxies falling into the massive galaxy clusters Abell 1689 (z~0.18) and 2667 (z~0.23) respectively. Hubble Space Telescope images show extraordinary trails composed of bright blue knots (-16.5

We define a sample of 62 galaxies in the Chandra Deep Field-North whose Spitzer IRAC SEDs exhibit the characteristic power-law emission expected of luminous AGN. We study the multiwavelength properties of this sample, and compare the AGN selected in this way to those selected via other Spitzer color-color criteria. Only 55% of the power-law galaxies are detected in the X-ray catalog at exposures of >0.5 Ms, although a search for faint emission results in the detection of 85% of the power-law galaxies at the > 2.5 sigma detection level. Most of the remaining galaxies are likely to host AGN that are heavily obscured in the X-ray. Because the power-law selection requires the AGN to be energetically dominant in the near- and mid-infrared, the power-law galaxies comprise a significant fraction of the Spitzer-detected AGN population at high luminosities and redshifts. The high 24 micron detection fraction also points to a luminous population. The power-law galaxies comprise a subset of color-selected AGN candidates. A comparison with various mid-infrared color selection criteria demonstrates that while the color-selected samples contain a larger fraction of the X-ray luminous AGN, there is evidence that these selection techniques also suffer from a higher degree of contamination by star-forming galaxies in the deepest exposures. Considering only those power-law galaxies detected in the X-ray catalog, we derive an obscured fraction of 68% (2:1). Including all of the power-law galaxies suggests an obscured fraction of < 81% (4:1)., Comment: Accepted for publication in the Astrophysical Journal, 27 pages, 20 figures, 5 tables, version with high-resolution figures and online-only tables available at: http://frodo.as.arizona.edu/~jdonley/powerlaw/

We present the results of a Spitzer search for obscured AGN in the Chandra Deep Field-North, using both radio-excess and mid-infrared power-law selection. AGN selected via the former technique tend to lie at z ~ 1, have SEDs dominated by the 1.6 micron stellar bump, and have Seyfert-like X-ray luminosities (when detected in the X-ray). In contrast, the IRAC (3.6-8.0 micron) power-law selected AGN lie at higher redshifts of z ~ 2, and comprise a significant fraction of the most X-ray luminous AGN in the CDF-N. While there is almost no overlap in the AGN samples selected via these two methods, their X-ray detection fractions are very similar. Only 40% and 55% of the radio-excess and power-law samples are detected in the 2 Ms X-ray catalog, respectively. The majority of the AGN selected via both methods are consistent with being obscured (N_H > 10^(22) cm^-2), but not Compton-thick (N_H > 10^(24) cm^-2), although Compton-thick candidates exist in both samples. We place an upper limit of <82% (or < 4:1) on the obscured fraction of the power-law sample, consistent with predictions from the cosmic X-ray background. The sources selected via the power-law criteria comprise a subset of AGN selected via other IRAC color-color cuts. While smaller in number than the color-selected samples in the deep fields, the power-law sample suffers from less contamination by star-forming galaxies., Comment: 6 pages, 2 figures, To appear in the proceedings of 'At the Edge of the Universe' (9-13 October 2006, Sintra, Portugal)

We have observed the cluster RX J0152.7-1357 (z$\sim$0.83) at 24$\mu$m with the Multiband Imaging Photometer for Spitzer (MIPS). We detected twenty-two sources associated with spectroscopically confirmed cluster members, while ten more have photometric redshifts compatible with membership. Two of the 32 likely cluster members contain obvious active nuclei while the others are associated with dusty star formation. The median IR-determined star formation rate among the remaining galaxies is estimated at 22 M$_\odot$$ $yr$^{-1}$, significantly higher than in previous estimates from optical data. Most of the MIR-emitting galaxies also have optical emission lines, but a few do not and hence have completely hidden bursts of star formation or AGN activity. An excess of MIR-emitting galaxies is seen in the cluster in comparison to the field at the same redshift. The MIR cluster members are more associated with previously detected infalling late type galaxies rather than triggered by the ongoing merging of bigger X-ray clumps. Rough estimates also show that ram pressure may not be capable of stripping the gas away from cluster outskirt galaxies but it may disturb the gas enough to trigger the star formation activity. Harassment can also play a role if for example these galaxies belong to poor galaxy groups. Thus, bursts of star formation occur in the cluster environment and could also help consume the galaxy gas content in addition to ram pressure, harassment or galaxy-galaxy strong interactions., Comment: 12 pages, 6 figures, accepted for publication in ApJ. A full resolution version and fig.2 are available at http://mips.as.arizona.edu/~dmarci/

Deep X-ray surveys by Chandra and XMM-Newton have resolved about 80% of the 2-10 keV cosmic extragalactic X-ray background (CXRB) into point sources, the majority of which are obscured AGN. The obscuration might be connected to processes within the host galaxy, possibly the star-formation rate. Here, we use the results of CXRB synthesis calculations as input to detailed Cloudy simulations in order to predict the evolution of AGN properties at several mid-IR wavelengths. Computations were performed for three different evolutions of the AGN type 2/type 1 ratio between z=0 and 1: where the ratio increased as (1+z)^{0.9}, as (1+z)^{0.3} and one with no redshift evolution. Models were calculated with the inner radius of the absorbing gas and dust at 1 pc or at 10 pc. Comparing the results of the calculations to combined X-ray and Spitzer data of AGN shows that the predicted spectral energy distributions are a good description of average AGNs found in the deep surveys. The existing data indicates that the mid-IR emission from an average AGN is best described by models where the attenuating material is ~10 pc from the central engine. We present the expected Spitzer cumulative number count distributions and the evolution of the total AGN (type 1 + type 2) luminosity function (LF) between z=0 and 1 at rest-frame 8 microns and 30 microns for the three evolutionary scenarios. The mid-IR AGN LF will be an excellent tool to measure the evolution of the covering factor of the gas and dust from z~0 to 1., Comment: 27 pages, 17 figures (12 in color), accepted by the ApJ

We have studied the infrared properties of the brightest cluster galaxies (BCGs) located in the cores of X-ray-luminous clusters at 0.15 < z < 0.35. The majority of the BCGs are not particularly infrared-luminous compared with other massive early-type galaxies, suggesting that the cluster environment has little influence on the infrared luminosities of the BCGs. The exceptions, however, are the BCGs in the three X-ray-brightest clusters in the sample, A1835, Z3146, and A2390. These BCGs have a prominent far-infrared peak in their spectral energy distributions (SEDs), and two of them (those in A1835 and Z3146) can be classified as luminous infrared galaxies (LIRGs: L_{IR} > 10^{11} L_{sun}). Although radio AGNs are found to be prevalent among the BCGs, the infrared luminosities of these three BCGs, judged from the infrared SED signatures, are likely to be powered by star formation. Considering the overall trend that clusters with shorter radiative gas cooling times harbor more infrared-luminous BCGs, the enhanced star formation may be caused by the cooling cluster gas accreting onto the BCGs., Comment: 35 pages, 8 figures, 4 tables; Accepted for publication in ApJ

We are currently undertaking a Spitzer GTO program to image ~30 massive lensing clusters at moderate redshift with both IRAC and MIPS. By taking advantage of the gravitatinoal lensing power of these clusters, we will study the population of faint galaxies that are below the nominal Spitzer detection limits. Here, we present a few examples of our science programs., Comment: 4 pages, 4 figures; To appear in the conference proceedings "The Spitzer Space Telescope: New Views of the Cosmos", ed. Lee Armus, 9-12 November 2004, Pasadena

We investigate why half of X-ray--selected active galactic nuclei (AGN) in deep surveys lack signs of accretion in their optical spectra. The majority of these ``optically--dull'' AGN are no more than 6 times fainter than their host galaxies in rest-frame R-band; as such, AGN lines are unlikely to be overwhelmed by stellar continuum in at least half the sample. We find that optically--dull AGN have the mid--infrared emission and L(x)/L(IR) ratios characteristic of local Seyferts, suggesting that the cause of optical dullness is not missing UV--optical continua. We compare the morphologies of 22 optically--dull and 9 optically--active AGN at 0.5

Mid-infrared observations of Active Galactic Nuclei (AGN) are important for understanding of the physical conditions around the central accretion engines. Chandra and XMM-Newton X-ray observations of a 300 arcmin^2 region in the Extended Groth Strip are used to select a sample of~150 AGN. The Spitzer instruments IRAC and MIPS detect 68-80% of these sources, which show a wide range of mid-infrared properties. About 40% of the sources have red power-law spectral energy distributions (f_nu ~ nu^alpha, alpha<0) in the 3.6-8 um IRAC bands. In these sources the central engine dominates the emission at both X-ray and IR wavelengths. Another 40% of the sources have blue mid-IR spectral energy distributions (alpha>0) with their infrared emission dominated by the host galaxy; the remaining 20% are not well-fit by a power law. Published IRAC color criteria for AGN select most of the red sources, but only some of the blue sources. As with all other known methods, selecting AGN with mid-IR colors will not produce a sample that is simultaneously complete and reliable. The IRAC SED type does not directly correspond to X-ray spectral type (hard/soft). The mid-IR properties of X-ray-detected Lyman-break, radio, submillimeter, and optically-faint sources vary widely and, for the most part, are not distinct from those of the general X-ray/infrared source population. X-ray sources emit 6-11% of the integrated mid--IR light, making them significant contributors to the cosmic infrared background., Comment: Replaced to correct minor error in Figure 3 axis label and typo in author list. ApJ, in press; 40 pages, 12 figures

We investigate the nature of a sample of 92 Spitzer/MIPS 24 micron selected galaxies in the CDFS, showing power law-like emission in the Spitzer/IRAC 3.6-8 micron bands. The main goal is to determine whether the galaxies not detected in X-rays (47% of the sample) are part of the hypothetical population of obscured AGN not detected even in deep X-ray surveys. The majority of the IR power-law galaxies are ULIRGs at z>1, and those with LIRG-like IR luminosities are usually detected in X-rays. The optical to IR spectral energy distributions (SEDs) of the X-ray detected galaxies are almost equally divided between a BLAGN SED class (similar to an optically selected QSO) and a NLAGN SED (similar to the BLAGN SED but with an obscured UV/optical continuum). A small fraction of SEDs resemble warm ULIRG galaxies (e.g., Mrk231). Most galaxies not detected in X-rays have SEDs in the NLAGN+ULIRG class as they tend to be optically fainter, and possibly more obscured. Moreover, the IR power-law galaxies have SEDs significantly different from those of high-z (z_sp>1) IR (24 micron) selected and optically bright (VVDS I_AB<=24) star-forming galaxies whose SEDs show a very prominent stellar bump at 1.6 micron. The galaxies detected in X-rays have 2-8 keV rest-frame luminosities typical of AGN. The galaxies not detected in X-rays have global X-ray to mid-IR SED properties that make them good candidates to contain IR bright X-ray absorbed AGN. If all these sources are actually obscured AGN, we would observe a ratio of obscured to unobscured 24 micron detected AGN of 2:1, whereas models predict a ratio of up to 3:1. Additional studies using Spitzer to detect X-ray-quiet AGN are likely to find more such obscured sources., Comment: 43 pages, 12 figures, accepted for publication in ApJ

We define a sample of 27 radio-excess AGN in the Chandra Deep Field North by selecting galaxies that do not obey the radio/infrared correlation for radio-quiet AGN and star-forming galaxies. Approximately 60% of these radio-excess AGN are X-ray undetected in the 2 Ms Chandra catalog, even at exposures of > 1 Ms; 25% lack even 2-sigma X-ray detections. The absorbing columns to the faint X-ray-detected objects are 10^22 cm^-2 < N_H < 10^24 cm^-2, i.e., they are obscured but unlikely to be Compton thick. Using a local sample of radio-selected AGN, we show that a low ratio of X-ray to radio emission, as seen in the X-ray weakly- and non-detected samples, is correlated with the viewing angle of the central engine, and therefore with obscuration. Our technique can explore the proportion of obscured AGN in the distant Universe; the results reported here for radio-excess objects are consistent with but at the low end of the overall theoretical predictions for Compton-thick objects., Comment: Accepted for publication in the Astrophysical Journal, 15 pages, 10 figures, 4 tables

We report MIPS photometry of 20 radio-loud quasars and galaxies at 24 and 70 um (and of five at 160 um). We combine this sample with additional sources detected in the far infrared by IRAS and ISO for a total of 47 objects, including 23 steep spectrum Type I AGNs: radio-loud quasars and broad line radio galaxies; and 24 Type II AGNs: narrow line and weak line FR II radio galaxies. Of this sample, the far infrared emission of all but 3C 380 appears to be dominated by emission by dust heated by the AGN and by star formation. The AGN appears to contribute more than 50% of the far infrared luminosity in most of sources. It is also expected that the material around the nucleus is optically thin in the far infrared. Thus, the measurements at these wavelengths can be used to test the orientation-dependent unification model. As predicted by the model, the behavior of the sources is consistent with the presence of an obscuring circumnuclear torus; in fact, we find it may still have significant optical depth at 24 um.In addition, as expected for the radio-loud quasars, there is a significant correlation between the low frequency radio (178 MHz) and the 70 um emission, two presumably isotropic indicators of nuclear activity. This result is consistent with the simple unified scheme. However, there is a population of radio galaxies that are underluminous at 70 um compared with the radio-loud quasars and hence are a challenge to the simple unified model., Comment: Accepted for publication in ApJ, 33 pages, 7 figures

Optically--faint X-ray sources (those with f_X/f_R > 10) constitute about 20% of X-ray sources in deep surveys, and are potentially highly obscured and/or at high redshift. Their faint optical fluxes are generally beyond the reach of spectroscopy. For a sample of 20 optically--faint sources in CDFS, we compile 0.4--24 um photometry, relying heavily on Spitzer. We estimate photometric redshifts for 17 of these 20 sources. We find that these AGN are optically--faint both because they lie at significantly higher redshifts (median z ~ 1.6) than most X-ray--selected AGN, and because their spectra are much redder than standard AGN. They have 2--8 keV X-ray luminosities in the Seyfert range, unlike the QSO--luminosities of optically--faint AGN found in shallow, wide--field surveys. Their contribution to the X-ray Seyfert luminosity function is comparable to that of z>1 optically--bright AGN., Comment: Accepted for publication in the Astrophysical Journal

We report the detection of a z~7 galaxy strongly lensed by the massive galaxy cluster Abell 2218 (z=0.175) at 3.6 and 4.5 um using the Spitzer Observatory and at 1.1 um using the Hubble Space Telescope. The new data indicate a refined photometric redshift in the range of 6.6-6.8 depending on the presence of Ly-alpha emission. The spectral energy distribution is consistent with having a significant Balmer break, suggesting that the galaxy is in the poststarburst stage with an age of at least ~50 Myr and quite possibly a few hundred Myr. This suggests the possibility that a mature stellar population is already in place at such a high redshift. Compared with typical Lyman break galaxies at z~3-4, the stellar mass is an order of magnitude smaller (~10^{9} Msun), but the specific star formation rate (star formation rate/M_{star}) is similarly large (> 10^{-9} yr^{-1}), indicating equally vigorous star-forming activity., Comment: 11 pages, 2 figures, 2 tables; Accepted for publication in ApJL

We present Spitzer IRAC (3.6, 4.5, 5.8, 8.0 um) and MIPS (24 um) observations of the SCUBA submillimeter sources and uJy VLA radio sources in a 5'x5' area in the Lockman Hole East region. Out of the ~40 SCUBA/VLA sources in the field, Spitzer counterparts were detected for nearly all except for the few low-weight SCUBA detections. We show that the majority (80-90%) of the detected sources are cold (i.e., starburst-like) infrared-luminous galaxies (L_{IR} > 10^{11} Lsun) at redshift 0.5

We investigate the luminous X-ray sources in the Lockman Hole (LH) and the Extended Groth Strip (EGS) detected at 24microns using MIPS and also with IRAC on board Spitzer. We assemble optical/infrared spectral energy distributions (SEDs) for 45 X-ray/24micron sources in the EGS and LH. Only about 1/4 of the hard X-ray/24micron sources show pure type 1 AGN SEDs. More than half of the X-ray/24micron sources have stellar-emission-dominated or obscured SEDs, similar to those of local type 2 AGN and spiral/starburst galaxies. One-third of the sources detected in hard X-rays do not have a 24micron counterpart. Two such sources in the LH have SEDs resembling those of S0/elliptical galaxies. The broad variety of SEDs in the optical-to-Spitzer bands of X-ray selected AGN means that AGN selected according to the behavior in the optical/infrared will have to be supplemented by other kinds of data (e.g., X-ray) to produce unbiased samples of AGN., Comment: Accepted to the ApJS Spitzer Special Issue