Search

Luminous and ultraluminous infrared galaxies have been an active area of galaxy research since their discovery more than three decades ago. With its vast increase in sensitivity in the infrared, Spitzer played a major role in exploring these galaxies in the local Universe, and at high redshifts. In this Review, we highlight some of the discoveries made with the Infrared Spectrograph on Spitzer, through observations of luminous and ultraluminous infrared galaxies to cosmic noon. These include measuring the role of starbursts and actively accreting supermassive black holes as power sources, finding evidence for energetic feedback on the atomic and molecular interstellar gas and dust and identifying the physical properties of luminous infrared galaxies on and off the galaxy star-forming main sequence. Finally, we briefly discuss how future infrared telescopes will build upon the discoveries of Spitzer to better understand the evolution of this important population since the epoch of reionization. Spitzer revealed the properties of luminous and ultraluminous infrared galaxies, the role of starbursts and actively accreting supermassive black holes in powering these sources and found evidence for energetic feedback on their interstellar gas and dust.

PKS 1413+135 is one of the most peculiar blazars known. Its strange properties led to the hypothesis almost four decades ago that it is gravitationally lensed by a mass concentration associated with an intervening galaxy. It exhibits symmetric achromatic variability, a rare form of variability that has been attributed to gravitational milli-lensing. It has been classified as a BL Lac object, and is one of the rare objects in this class with a visible counterjet. BL Lac objects have jet axes aligned close to the line of sight. It has also been classified as a compact symmetric object, which have jet axes not aligned close to the line of sight. Intensive efforts to understand this blazar have hitherto failed to resolve even the questions of the orientation of the relativistic jet, and the host galaxy. Answering these two questions is important as they challenge our understanding of jets in active galactic nuclei and the classification schemes we use to describe them. We show that the jet axis is aligned close to the line of sight and PKS 1413+135 is almost certainly not located in the apparent host galaxy, but is a background object in the redshift range $0.247 < z < 0.5$. The intervening spiral galaxy at $z = 0.247$ provides a natural host for the putative lens responsible for symmetric achromatic variability and is shown to be a Seyfert 2 galaxy. We also show that, as for the radio emission, a "multizone" model is needed to account for the high-energy emission., Accepted for publication in The Astrophysical Journal. 28 pages, 15 figures

We present rest-frame optical spectroscopic observations of 24 Hot Dust-Obscured Galaxies (Hot DOGs) at redshifts 1.7–4.6 with KECK/NIRES. Our targets are selected, based on their extreme red colors, to be the highest-luminosity sources from the WISE infrared survey. In 20 sources with well-detected emission, we fit the key [O iii], Hβ, Hα, [N ii], and [S ii] diagnostic lines to constrain physical conditions. Of the 17 targets with a clear detection of the [O iii]λ5007 Å emission line, 15 display broad blueshifted and asymmetric line profiles, with widths ranging from 1000 to 8000 km s−1 and blueshifts up to 3000 km s−1. These kinematics provide strong evidence for the presence of massive ionized outflows of up to , with a median of . As many as eight sources show optical emission line ratios consistent with vigorous star formation. Balmer-line star formation rates, uncorrected for reddening, range from 30 to 1300 , with a median of . Estimates of the SFR from Spectral Energy Distribution fitting of mid- and far-infrared photometry suggest significantly higher values. We estimate the central black hole masses to be of order , assuming the present-day relation. The bolometric luminosities and the estimated masses of the central black holes of these galaxies suggest that many of the active galactic nucleus-dominated Hot DOGs are accreting at or above their Eddington limit. The combination of ongoing star formation, massive outflows, and high Eddington ratios suggest Hot DOGs are a transitional phase in galaxy evolution.

The Frontier Fields are a director's discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combine the power of HST and Spitzer with the natural gravitational telescopes of massive high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters - Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370 - were selected based on their lensing strength, sky darkness, Galactic extinction, parallel field suitability, accessibility to ground-based facilities, HST, Spitzer and JWST observability, and pre-existing ancillary data. These clusters have been targeted by the HST ACS/WFC and WFC3/IR with coordinated parallels of adjacent blank fields for over 840 HST orbits. The Spitzer Space Telescope has dedicated > 1000 hours of director's discretionary time to obtain IRAC 3.6 and 4.5 micron imaging to ~26.5, 26.0 ABmag 5-sigma point-source depths in the six cluster and six parallel Frontier Fields. The Frontier Field parallel fields are the second-deepest observations thus far by HST with ~29th ABmag 5-sigma point source depths in seven optical - near-infrared bandpasses. Galaxies behind the Frontier Field cluster lenses experience typical magnification factors of a few, with small regions near the critical curves magnified by factors 10-100. Therefore, the Frontier Field cluster HST images achieve intrinsic depths of ~30-33 magnitudes over very small volumes. Early studies of the Frontier Fields have probed galaxies fainter than any seen before during the epoch of reionization 6 < z < 10, mapped out the cluster dark matter to unprecedented resolution, and followed lensed transient events.

We present SHARC-II 350um imaging of twelve 24um-bright (F_24um > 0.8 mJy) Dust-Obscured Galaxies (DOGs) and CARMA 1mm imaging of a subset of 2 DOGs, all selected from the Bootes field of the NOAO Deep Wide-Field Survey. Detections of 4 DOGs at 350um imply IR luminosities which are consistent within a factor of 2 of expectations based on a warm dust spectral energy distribution (SED) scaled to the observed 24um flux density. The 350um upper limits for the 8 non-detected DOGs are consistent with both Mrk231 and M82 (warm dust SEDs), but exclude cold dust (Arp220) SEDs. The two DOGs targeted at 1mm were not detected in our CARMA observations, placing strong constraints on the dust temperature: T_dust > 35-60 K. Assuming these dust properties apply to the entire sample, we find dust masses of ~3x10^8 M_sun. In comparison to other dusty z ~ 2 galaxy populations such as sub-millimeter galaxies (SMGs) and other Spitzer-selected high-redshift sources, this sample of DOGs has higher IR luminosities (2x10^13 L_sun vs. 6x10^12 L_sun for the other galaxy populations), warmer dust temperatures (>35-60 K vs. ~30 K), and lower inferred dust masses (3x10^8 M_sun vs. 3x10^9 M_sun). Herschel and SCUBA-2 surveys should be able to detect hundreds of these power-law dominated DOGs. We use HST and Spitzer/IRAC data to estimate stellar masses of these sources and find that the stellar to gas mass ratio may be higher in our 24um-bright sample of DOGs than in SMGs and other Spitzer-selected sources. Although larger sample sizes are needed to provide a definitive conclusion, the data are consistent with an evolutionary trend in which the formation of massive galaxies at z~2 involves a sub-millimeter bright, cold-dust and star-formation dominated phase followed by a 24um-bright, warm-dust and AGN-dominated phase., 16 pages, 7 figures, 6 tables; accepted to the ApJ

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6 micron stellar photospheric feature redshifted into Spitzer's 3-8 micron IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 =1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 micron stellar bump can be efficiently used to select highly obscured starforming galaxies at z~2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 micron flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 micron detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on the shapes of their SEDs. Finally, we conclude that z~2 sources with a detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to veil the 7.7 micron PAH band.

Optically bright Galactic stars (V < 13 mag) having fv(24 um) > 1 mJy are identified in Spitzer mid-infrared surveys within 8.2 square degrees for the Bootes field of the NOAO Deep Wide-Field Survey and within 5.5 square degrees for the First Look Survey (FLS). 128 stars are identified in Bootes and 140 in the FLS, and their photometry is given. (K-[24]) colors are determined using K magnitudes from the 2MASS survey for all stars in order to search for excess 24 um luminosity compared to that arising from the stellar photosphere. Of the combined sample of 268 stars, 141 are of spectral types F, G, or K, and 17 of these 141 stars have 24 um excesses with (K-[24]) > 0.2 mag. Using limits on absolute magnitude derived from proper motions, at least 8 of the FGK stars with excesses are main sequence stars, and estimates derived from the distribution of apparent magnitudes indicate that all 17 are main sequence stars. These estimates lead to the conclusion that between 9% and 17% of the main sequence FGK field stars in these samples have 24 um infrared excesses. This result is statistically similar to the fraction of stars with debris disks found among previous Spitzer targeted observations of much brighter, main sequence field stars., Accepted for publication in The Astronomical Journal

The Spitzer Space Telescope was launched in August 2003. Scientists from around the world have applied its orders-of-magnitude gain in imaging and spectroscopic capability to a wide array of topics in extragalactic research. Spitzer studies have found massive galaxies at redshifts greater than 6, resolved the cosmic background at 200 μm > λ > 20 μm into the dusty infrared-luminous galaxies that comprise it, directly detected dust-enshrouded star formation, and measured the star formation history of the universe to z > 3. In this review we examine a small fraction of the extragalactic studies from Spitzer that have been conducted in its first three years of operations.

Spitzer MIPS images in the Bootes field of the NOAO Deep Wide-Field Survey have revealed a class of extremely dust obscured galaxy (DOG) at z~2. The DOGs are defined by very red optical to mid-IR (observed-frame) colors, R - [24 um] > 14 mag, i.e. f_v (24 um) / f_v (R) > 1000. They are Ultra-Luminous Infrared Galaxies with L_8-1000 um > 10^12 -10^14 L_sun, but typically have very faint optical (rest-frame UV) fluxes. We imaged three DOGs with the Keck Laser Guide Star Adaptive Optics (LGSAO) system, obtaining ~0.06'' resolution in the K'-band. One system was dominated by a point source, while the other two were clearly resolved. Of the resolved sources, one can be modeled as a exponential disk system. The other is consistent with a de Vaucouleurs profile typical of elliptical galaxies. The non-parametric measures of their concentration and asymmetry, show the DOGs to be both compact and smooth. The AO images rule out double nuclei with separations of greater than 0.1'' (< 1 kpc at z=2), making it unlikely that ongoing major mergers (mass ratios of 1/3 and greater) are triggering the high IR luminosities. By contrast, high resolution images of z~2 SCUBA sources tend to show multiple components and a higher degree of asymmetry. We compare near-IR morphologies of the DOGs with a set of z=1 luminous infrared galaxies (LIRGs; L_IR ~ 10^11 L_sun) imaged with Keck LGSAO by the Center for Adaptive Optics Treasury Survey. The DOGs in our sample have significantly smaller effective radii, ~1/4 the size of the z=1 LIRGs, and tend towards higher concentrations. The small sizes and high concentrations may help explain the globally obscured rest-frame blue-to-UV emission of the DOGs., 9 pages, 7 figures, 2 tables, accepted for publication in the Astronomical Journal

Observations with Spitzer Space Telescope have recently revealed a significant population of high-redshift z~2 dust-obscured galaxies (DOGs) with large mid-IR to UV luminosity ratios. These galaxies have been missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-z population based solely on the ratio of the observed mid-IR 24um to optical R-band flux density. In the 8.6 sq.deg Bootes NDWFS Field, we uncover ~2,600 DOG candidates (= 0.089/sq.arcmin) with 24um flux densities F24>0.3mJy and (R-[24])>14 (i.e., F[24]/F[R] > 1000). These galaxies have no counterparts in the local universe, and become a larger fraction of the population at fainter F24, representing 13% of the sources at 0.3~mJy. DOGs exhibit evidence of both star-formation and AGN activity, with the brighter 24um sources being more AGN- dominated. We have measured spectroscopic redshifts for 86 DOGs, and find a broad z distribution centered at ~2.0. Their space density is 2.82E-5 per cubic Mpc, similar to that of bright sub-mm-selected galaxies at z~2. These redshifts imply very large luminosities LIR>~1E12-14 Lsun. DOGs contribute ~45-100% of the IR luminosity density contributed by all z~2 ULIRGs, suggesting that our simple selection criterion identifies the bulk of z~2 ULIRGs. DOGs may be the progenitors of ~4L* present-day galaxies seen undergoing a luminous,short- lived phase of bulge and black hole growth. They may represent a brief evolution phase between SMGs and less obscured quasars or galaxies. [Abridged], Accepted for publication in the Astrophysical Journal

We present near-infrared spectroscopic observations for a sample of ten optically faint luminous infrared galaxies (R-[24]> 14) using Keck NIRSPEC and Gemini NIRI. The sample is selected from a 24 micron Spitzer MIPS imaging survey of the NDWFS Bootes field. We measure accurate redshifts in the range 1.3 1900 km/s) Halpha or Hbeta emission lines; the remaining three are type II AGN. Given their large mid-IR luminosities and faint optical magnitudes, we might expect these sources to be heavily extincted quasars, and therefore only visible as type II AGN. The visibility of broad lines in 70% of the sources suggests that it is unlikely that these AGN are being viewed through the mid-plane of a dusty torus. For four of the sources we constrain the Halpha/Hbeta Balmer decrement and estimate the extinction to the emission line region to be large for both type I and type II AGN, with A_Halpha > 2.4-5 mag. Since the narrow-line region is also extincted and the UV continuum emission from the host galaxies is extremely faint, this suggests that much of the obscuration is contributed by dust on large (~kpc) scales within the host galaxies. These sources may be examples of "host-obscured" AGN which could have space densities comparable or greater to that of optically luminous type I AGN with similar bolometric luminosities.

The existence of flux-ratio anomalies between fold and cusp images in galaxy-scale strong-lens systems has led to an interpretation based on the presence of a high mass-fraction of cold-dark-matter (CDM) substructures around galaxies, as predicted by numerical N-body simulations. The flux-ratio anomaly is particularly evident in the radio-loud quadruple gravitational lens system CLASS B2045+265. In this paper, new high-resolution radio, optical, and infrared imaging of B2045+265 is presented which sheds more light on this anomaly and its possible causes. First, deep Very Long Baseline Array observations show very compact images, possibly with a hint of a jet, but with no evidence for differential scattering or scatter broadening. Second, optical and infrared observations with the Hubble Space Telescope and through Adaptive-Optics imaging with the W. M. Keck Telescope, show a previously undiscovered object -- interpreted as a (tidally disrupted) dwarf satellite based on its colours and slight extension -- between the main lens galaxy and the three anomalous flux-ratio images. Third, colour variations in the early-type lens galaxy indicate recent star-formation, possibly the result of secondary infall of gas-rich satellites. A population of such galaxies around the lens system could explain the previously discovered strong [O II] emission. However, spiral structure and/or normal star formation in the lens galaxy cannot be excluded. In light of these new data, we propose a lens model for the system, including the observed dwarf satellite, which reproduces all positional and flux-ratio constraints, without the need for additional CDM substructure. [abridged], Comment: 10 pages, 6 figures, accepted for publication in MNRAS

Accurate photometric redshifts are calculated for nearly 200,000 galaxies to a 4.5 micron flux limit of ~13 uJy in the 8.5 deg^2 Spitzer/IRAC Shallow survey. Using a hybrid photometric redshift algorithm incorporating both neural-net and template-fitting techniques, calibrated with over 15,000 spectroscopic redshifts, a redshift accuracy of \sigma = 0.06(1+z) is achieved for 95% of galaxies at 01) galaxy clusters. We present one such spectroscopically confirmed cluster at =1.24, ISCS J1434.5+3427. Finally, we present a measurement of the 4.5 micron-selected galaxy redshift distribution., Comment: 14 pages, 9 Figures, 5 Tables. ApJ in press. For a version with full-resolution figures, please see http://www.astro.utoronto.ca/~brodwin/papers/0607450.pdf

Spectra have been obtained with the low-resolution modules of the Infrared Spectrograph (IRS) on the Spitzer Space Telescope (Spitzer) for 58 sources having f$_��$(24 micron) > 0.75 mJy. Sources were chosen from a survey of 8.2 deg$^{2}$ within the NOAO Deep Wide-Field Survey region in Bootes (NDWFS) using the Multiband Imaging Photometer (MIPS) on the Spitzer Space Telescope. Most sources are optically very faint (I > 24mag). Redshifts have previously been determined for 34 sources, based primarily on the presence of a deep 9.7 micron silicate absorption feature, with a median z of 2.2. Spectra are presented for the remaining 24 sources for which we were previously unable to determine a confident redshift because the IRS spectra show no strong features. Optical photometry from the NDWFS and infrared photometry with MIPS and the Infrared Array Camera on the Spitzer Space Telescope (IRAC) are given, with K photometry from the Keck I telescope for some objects. The sources without strong spectral features have overall spectral energy distributions (SEDs) and distributions among optical and infrared fluxes which are similar to those for the sources with strong absorption features. Nine of the 24 sources are found to have feasible redshift determinations based on fits of a weak silicate absorption feature. Results confirm that the "1 mJy" population of 24 micron Spitzer sources which are optically faint is dominated by dusty sources with spectroscopic indicators of an obscured AGN rather than a starburst. There remain 14 of the 58 sources observed in Bootes for which no redshift could be estimated, and 5 of these sources are invisible at all optical wavelengths., Accepted by ApJ

We present the discovery of three z>5 quasars in the AGN and Galaxy Evolution Survey (AGES) spectroscopic observations of the NOAO Deep Wide-Field Survey (NDWFS) Bootes Field. These quasars were selected as part of a larger Spitzer mid-infrared quasar sample with no selection based on optical colors. The highest redshift object, NDWFS J142516.3+325409, z=5.85, is the lowest-luminosity z>5.8 quasar currently known. We compare mid-infrared techniques for identifying z>5 quasars to more traditional optical techniques and show that mid-infrared colors allow for selection of high-redshift quasars even at redshifts where quasars lie near the optical stellar locus and at z>7 where optical selection is impossible. Using the superb multi-wavelength coverage available in the NDWFS Bootes field, we construct the spectral energy distributions (SEDs) of high-redshift quasars from observed Bw-band to 24 microns (rest-frame 600 Angstroms - 3.7 microns). We show that the three high-redshift quasars have quite similar SEDs, and the rest-frame composite SED of low-redshift quasars from the literature shows little evolution compared to our high-redshift objects. We compare the number of z>5 quasars we have discovered to the expected number from published quasar luminosity functions. While analyses of the quasar luminosity function are tenuous based on only three objects, we find that a relatively steep luminosity function with Psi L^(-3.2) provides the best agreement with the number of high-redshift quasars discovered in our survey., Comment: 9 page, 7 figures. Accepted for publication in AJ

We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z>0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 micron sources are selected from a Spitzer/MIPS survey of the NOAO Deep Wide-Field Survey Bo\"otes field and have corresponding 8 micron data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 micron to 8 micron flux ratio. The X-ray detected sources fall within the peak corresponding to a flat spectrum in nufnu, implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 micron to 8 micron flux ratio is likely due to LIRGs whose infrared emission is powered by starbursts. The 24 micron emission is increasingly dominated by AGN at higher 24 micron flux densities (f_24): the AGN fraction of the z>0.6 sources increases from ~9% at f_24 ~ 0.35 mJy to 74+/-20% at f_24 ~ 3 mJy in good agreement with model predictions. Deep 24 micron, small area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for ~ 3-7% of the total 24 micron background.

Measurements of Iapetus as seen at 20 and 2.2 μm in the shadow of Saturn's ring are given, providing the thermal response to a rapidly varying heat input. The 20 μm thermal emission follows the 2.2 μm flux input closely. The observations, plus a simple diffusion calculation, imply that the surface of Iapetus is made of material having a very small thermal inertia, probably / ≾ 10Jm^(-2)s^(-½)K^(-1).

We have combined a survey at 24 microns to 0.3 mJy with the Multiband Imaging Photometer (MIPS) on the Spitzer Space Telescope, a 20 cm A-array VLA survey covering 0.5 deg^2, and an existing 172 ks Chandra X-Ray Observatory exposure to investigate the nature of optically faint radio and X-ray sources in the NOAO Deep Wide-Field Survey (NDWFS) in Bootes. We find little overlap between the radio and infrared selected populations: only 9% of the infrared sources are detected in the radio and only 33% of the radio sources are detected in the infrared. Thirty-six (10%) of the 377 compact radio sources lack optical counterparts in the NDWFS Bw, R, & I images. We refer to these objects as optically invisible radio sources (OIRS). Only four of the thirty-one OIRSs observed with MIPS are detected at 24 microns. Comparisons of the radio and infrared properties of the OIRSs with various galaxy spectral energy distributions demonstrate that most of these sources are powered by AGN rather than starbursts. Similarly, eleven X-ray sources observed by both MIPS and the VLA are classified as optically invisible X-ray sources (OIXS). None are detected at 24 microns or 20 cm. All seven OIXSs detected in Chandra's 0.5-2 keV band have infrared to X-ray flux ratios consistent with their luminosity being dominated by an unobscured AGN. From these results we conclude that both the optically invisible radio and X-ray source populations are primarily AGN, relatively unaffected by dust and most likely at z > 1. No OIRSs are detected in X-ray emission and no OIXSs are detected at 20 cm. However, given the wide range in radio and X-ray properties of known AGN and the size of our samples, this lack of overlap does not necessarily imply AGN source populations.

The Infrared Spectrograph (IRS) on board the Spitzer Space Telescope has been used to obtain low- and moderate-resolution spectra of the dust- and gas-rich quasar APM 08279+5255 (z = 3.91). Broad Paα and Paβ recombination lines of hydrogen were detected at wavelengths of 9.235 and 6.315 μm, respectively, as well as a strong, red continuum that is a smooth power law over the observed (rest-frame) wavelength range 5.3-35 μm (1.08-7.1 μm). The observed Paα/Paβ line flux ratio of 1.05 ± 0.2 is far from the case B value of ~2 and simple models of high-density, high optical depth ionized-gas regions (~1.8). This deviation is opposite in sense to the expected effect of reddening. No evidence is found in the spectrum for either the 3.3 or the 6.2 μm emission features usually attributed to aromatic hydrocarbons in gas-rich galaxies in the local universe. This is consistent with the high-luminosity active galactic nucleus (AGN) nature of APM 08279+5255.

We present broad band imaging observations obtained with the ``peak up'' imagers of the Spitzer Space Telescope Infrared Spectrograph (IRS) at wavelengths of 16micron and 22micron for a number of sources detected primarily at submillimeter wavelengths, which are believed to be at high, though undetermined, redshift. We targeted 11 sources originally detected by SCUBA and 5 submillimeter sources detected at 1.2mm by MAMBO. Two optically discovered quasars with z>6 were also observed to determine if there is detectable dust emission at such high redshifts. Seven of the submillimeter sources and both high-redshift quasars were detected, and upper limits of about ~50microJy apply to the remainder. Using their mid-/far-IR colors, we demonstrate that all of the submillimeter sources are at z>1.4. The mid-IR colors for two of our detections and three of our strong upper limits suggest that these galaxies must be at z>2.5. We also introduce a technique for estimating redshifts and source characteristics based only on the ratio of fluxes in the 16micron and 22micron images.

We present the first rest-frame ~4 micron detection of a Lyman break galaxy. The data were obtained using the 16 micron imaging capability of the Spitzer Infrared Spectrograph. The target object, J134026.44+634433.2, is an extremely luminous Lyman break galaxy at z=2.79 first identified in Sloan Digital Sky Survey spectra (Bentz, Osmer, & Weinberg 2004). The source is strongly detected with a flux of 0.94 +/- 0.02 mJy. Combining Spitzer and SDSS photometry with supporting ground-based J- and K-band data, we show that the spectral energy distribution is consistent with an actively star-forming galaxy. We also detect other objects in the Spitzer field of view, including a very red MIR source. We find no evidence of a strong lens amongst the MIR sources.

We carried out direct measurement of the fraction of dusty sources in a sample of extremely red galaxies with (R - K_s) ≥ 5.3 mag and K_s < 20.2 mag, using 24 μm data from the Spitzer Space Telescope. Combining deep 24 μm Ks- and R-band data over an area of ~64 arcmin^2 in ELAIS N1 of the Spitzer First Look Survey (FLS), we find that 50% ± 6% of our extremely red object (ERO) sample have measurable 24 μm flux above the 3 σ flux limit of 40 μJy. This flux limit corresponds to a star formation rate (SFR) of 12 M_⊙ yr^(-1) at z ~ 1, much more sensitive than any previous long-wavelength measurement. The 24 μm-detected EROs have 24 μm/2.2 μm and 24 μm/0.7 μm flux ratios consistent with infrared luminous, dusty sources at z ≥ 1, and are an order of magnitude too red to be explained by an infrared quiescent spiral or a pure old stellar population at any redshift. Some of these 24 μm-detected EROs could be active galactic nuclei; however, the fraction among the whole ERO sample is probably small, 10%-20%, as suggested by deep X-ray observations as well as optical spectroscopy. Keck optical spectroscopy of a sample of similarly selected EROs in the FLS field suggests that most of the EROs in ELAIS N1 are probably at z ~ 1. The mean 24 μm flux (167 μJy) of the 24 μm-detected ERO sample roughly corresponds to the rest-frame 12 μm luminosity, νL_ν(12 μm), of 3 × 10^(10) L_⊙ at z ~ 1. Using the correlation between IRAS νL_ν(12 μm) and infrared luminosity LIR(8-1000 μm), we infer that the 〈L_(IR)〉 of the 24 μm-detected EROs is 3 × 10^(11) and 1 × 10^(12) L_⊙ at z = 1.0 and 1.5, respectively, similar to that of local luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies (ULIRGs). The corresponding SFR would be roughly 50-170 M_⊙ yr^(-1). If the timescale of this starbursting phase is on the order of 10^8 yr as inferred for the local LIRGs and ULIRGs, the lower limit on the masses of these 24 μm-detected EROs is 5 × 10^9 to 2 × 10^(10) M_⊙. It is plausible that some of the starburst EROs are in the midst of a violent transformation to become massive early type galaxies at the epoch of z ~ 1-2.

We present the first results of the angular auto-correlation function of the galaxies detected by the Infrared Array Camera (IRAC) instrument in the First Look Survey (FLS) of the Spitzer Space Telescope. We detect significant signals of galaxy clustering within the survey area. The angular auto-correlation function of the galaxies detected in each of the four IRAC instrument channels is consistent with a power-law form $w(\theta)=A\theta^{1-\gamma}$ out to $\theta = 0.2\arcdeg$, with the slope ranging from $\gamma = 1.5$ to 1.8. We estimate the correlation amplitudes $A$ to be $2.95 \times 10^{-3}$, $2.03 \times 10^{-3}$, $4.53 \times 10^{-3}$, and $2.34 \times 10^{-3}$ at $\theta=1\arcdeg$ for galaxies detected in the IRAC 3.6$\mu$m, 4.5$\mu$m, 5.8$\mu$m, and 8.0$\mu$m instrument channels, respectively. We compare our measurements at 3.6$\mu$m with the previous K-band measurements, and discuss the implications of these results.

High spatial resolution images at 12.5 μm of the nuclei of five nearby Seyfert galaxies—I Zwicky 1, NGC 1320, NGC 2992, M81, and NGC 7479—have been obtained with the 10 m Keck Telescope. The angular size limits indicate that under typical conditions the Keck Telescope shows an unresolved nucleus for these active galactic nuclei. In all cases, the lower limit to the infrared surface brightness is above 3 × 10^(12) L_⊙ kpc^(−2); this argues that nuclear starbursts do not contribute significantly to the infrared luminosities in these nuclei.

IRAS flux densities, redshifts, and infrared luminosities are reported for all sources identified in the IRAS Revised Bright Galaxy Sample (RBGS), a complete flux-limited survey of all extragalactic objects with total 60 micron flux density greater than 5.24 Jy, covering the entire sky surveyed by IRAS at Galactic latitude |b| > 5 degrees. The RBGS includes 629 objects, with a median (mean) sample redshift of 0.0082 (0.0126) and a maximum redshift of 0.0876. The RBGS supersedes the previous two-part IRAS Bright Galaxy Samples, which were compiled before the final ("Pass 3") calibration of the IRAS Level 1 Archive in May 1990. The RBGS also makes use of more accurate and consistent automated methods to measure the flux of objects with extended emission. Basic properties of the RBGS sources are summarized, including estimated total infrared luminosities, as well as updates to cross-identifications with sources from optical galaxy catalogs established using the NASA/IPAC Extragalactic Database (NED). In addition, an atlas of images from the Digitized Sky Survey with overlays of the IRAS position uncertainty ellipse and annotated scale bars is provided for ease in visualizing the optical morphology in context with the angular and metric size of each object. The revised bolometric infrared luminosity function, phi(L_ir), for infrared bright galaxies in the local Universe remains best fit by a double power law, phi(L_ir) ~ L_ir^alpha, with alpha = -0.6 (+/- 0.1), and alpha = -2.2 (+/- 0.1) below and above the "characteristic" infrared luminosity L_ir ~ 10^{10.5} L_solar, respectively. (Abridged)

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxy's central supermassive black hole. Both Brgamma (2.1661 micron) and He I (2.1126 micron) are seen in absorption in S0-2 with equivalent widths (2.8+-0.3 Ang & 1.7+-0.4 Ang) and an inferred stellar rotational velocity (220+-40 km/s) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive (~15 Msun), young (, 14 pages, Latex, Accepted for Publication in ApJ Letters

Photometric observations in the near-infrared atmospheric windows from 1.25 to 3.7 μm and spectrographic observations from 1.2 to 2.4 μm are presented of supernova SN 1993J for 250 days after the outburst. After an initial rise to a secondary maximum the infrared observations show an exponential drop much like the visual observations. At day 130 after the outburst, however, the 3.7 μm data show an excess, which we take as a signature of dust emission. Broadband spectral energy distributions (SEDs) were derived combining published visual photometry and the infrared photometry. The early SEDs during the supernova rise to its secondary maximum could be fitted with blackbodies, but during the exponential decrease the SEDs were too narrow to be fitted by a blackbody energy distribution. The low-resolution spectra were initially featureless, but beyond about day 30 after the outburst, when SN 1993J was in exponential decline, the spectra were dominated by line emission, probably by hydrogen, helium, and iron lines.

We present near-infrared spectra with resolutions of lambda/dlambda~1200 in the emission lines of Pa-beta, [FeII] (1.2567um), Br-gamma, and H2 v=1-0S(1) of the nuclei and circumnuclear regions of the four Seyfert 2 galaxies Mk 1066, NGC 2110, NGC 4388, and Mk 3. All of these galaxies show strong near-infrared line emission that is detected at radii several times the spatial resolution, corresponding to projected physical scales of 0.07 to 0.7 kpc. Velocity gradients are detected in these nuclei, as are spatial variations in line profiles and flux ratios. We compare the spatial and velocity distribution of the line emission to previously observed optical line and radio emission. The evidence indicates that the [FeII] emission is associated with the Seyfert activity in the galaxies. Our data are consistent with X-ray heating being responsible for most of the [FeII] emission, although differences in [FeII] and Pa-beta line profiles associated with radio emission suggests that the [FeII] emission is enhanced by fast shocks associated with radio outflows. The H2 emission is not as strongly associated with outflows or ionization cones as is the emission in other lines, but rather appears to be primarily associated with the disk of the galaxy.

We present near-infrared spectroscopy for a complete sample of 33 ultraluminous infrared galaxies at a resolution of R ≈ 1000. Most of the wavelength range from 1.80–2.20 μm in the rest frame is covered, including the Paα and Brγ hydrogen recombination lines, and the molecular hydrogen vibration-rotation 1–0 S(1) and S(3) lines. Other species, such as He I, [Fe II], and [Si VI] appear in the spectra as well, in addition to a number of weaker molecular hydrogen lines. Nuclear extractions for each of the individual galaxies are presented here, along with spectra of secondary nuclei, where available. The Paα emission is seen to be highly concentrated on the nuclei, typically with very little emission extending beyond a radius of 1 kpc. This survey was carried out primarily to search for signatures of active nuclei via velocity-broadened hydrogen recombination or the presence of the [Si VI] coronal line. These signatures are rare in the present sample, occurring in only two of the 33 galaxies. The extinction to the hydrogen recombination lines is investigated via the Paα/Brγ line ratio. It is found that visual extinctions to the nuclei in excess of 10 mag are relatively common among ULIRGs and that visual extinctions greater than 25 mag are necessary to conceal a QSO emitting half the total bolometric luminosity. The ionized hydrogen regions in many ULIRGs are sufficiently obscured that dust-enshrouded active galactic nuclei would remain hidden at 2 μm at the current level of sensitivity. The vibration-rotation lines of molecular hydrogen appear to be predominantly thermal in origin, with effective temperatures generally around 2200 K. The relative nuclear velocities between double nucleus ULIRGs are investigated, through which it is inferred that the maximum deprojected velocity difference is ~200 km s-1. This figure is lower than the velocities predicted by physical models of strong interactions/mergers of large, gas-rich galaxies.

We present evidence for a significant overdensity of red galaxies, as much as a factor of 14 over comparable field samples, in the field of the z=1.47 radio galaxy B3 0003+387. The colors and luminosities of the brightest red galaxies are consistent with their being at z>0.8. The radio galaxy and one of the red galaxies are separated by 5" and show some evidence of a possible interaction. However, the red galaxies do not show any strong clustering around the radio galaxy nor around any of the brighter red galaxies. The data suggest that we are looking at a wall or sheet of galaxies, possibly associated with the radio galaxy at z=1.47. Spectroscopic redshifts of these red galaxies will be necessary to confirm this large-scale structure.

Near-infrared spectroscopy using the new Palomar Integral Field Spectrograph indicates the presence of an active galactic nucleus (AGN) in the ultraluminous infrared galaxy IRAS 08311-2459. The high-velocity wings of the Paschen-α hydrogen recombination line are seen to be spatially unresolved, with no positional offset between red and blue high-velocity emission. The [Si VI] coronal line, with a 167 eV excitation potential, is also spatially unresolved, with a velocity width comparable to that of the broad component of the Paα emission. The low-velocity component of the Paα emission is seen to be rotating and is extended over ~2 kpc. Molecular hydrogen is also extended, and it is elongated along the maximum velocity gradient, sharing the same rotation profile as the narrow Paα emission. The simple picture in agreement with the observations is that of an AGN surrounded by a rotating disk of star formation. These observations lend strength to the usage of [Si VI] as a diagnostic of AGN activity in ultraluminous infrared galaxies and also highlight the utility of integral field spectroscopy in elucidating subtle morphological differences in line-emitting regions.

We present Keck high-resolution near-IR (2.2 μm; FWHM ~ 0."15) and mid-IR (12.5 μm; FWHM ~ 0."4) images of APM 08279+5255, a z = 3.91 IR-luminous broad absorption line quasi-stellar object with a prodigious apparent bolometric luminosity of 5 × 10^(15) L_☉, the largest known in the universe. The K-band image shows that this system consists of three components, all of which are likely to be the gravitationally lensed images of the same background object, and the 12.5 μm image shows a morphology consistent with such an image configuration. Our lens model suggests that the magnification factor is ~100 from the rest-frame UV to mid-IR, where most of the luminosity is released. The intrinsic bolometric luminosity and IR luminosity of APM 08279+5255 are estimated to be 5 × 10^(13) L_☉ and 1 × 10^(13) L_☉, respectively. This indicates that APM 08279+5255 is intrinsically luminous, but it is not the most luminous object known. As for its dust contents, little can be determined with the currently available data because of the uncertainties associated with the dust emissivity and the possible effect of differential magnification. We also suggest that the lensing galaxy is likely to be a massive galaxy at z ~ 3.

Images of the Hubble Deep Field (HDF) at 3.2 μm, taken with the 10 m Keck Telescope, are presented. The images cover a total area of ~2.5 arcmin^2. To a 5 σ limit of [3.2]_(tot) ≈ 17.5 mag (Vega relative), 11 sources are detected, nine of which are extragalactic. The integrated galaxy number counts are therefore ~1.3 × 10^4 deg^(-2) to this depth. The galaxies detected at 3.2 μm have a median redshift of z = 0.56. All 3.2 μm sources have 1.6 μm, 1.1 μm, and visual counterparts, all of fairly regular morphology; several also have 6.7 μm, 15 μm, 850 μm, 8.5 GHz, or 1.4 GHz counterparts. No sources are found that are either anomalously red or anomalously blue in their H-[3.2] color, and there are significant detections of most of the known near-infrared HDF sources for which detection in these 3.2 μm data seemed likely.

Near infrared spectra with resolution R ~ 1100 in the rest wavelength range 1.8-2.2 microns have been obtained for a complete sample of 33 ultraluminous infrared galaxies. Of the 33 objects observed, 2 show evidence of a central AGN through either a broad Paschen-alpha line or emission in the 1.963 micron fine structure of [Si VI]. In the median spectrum of the remaining 31 objects, the lines present are recombination lines of Hydrogen, neutral Helium, vibration-rotation lines of H_2, and [Fe II]. There is no indication of AGN activity in the median spectrum, either through broad atomic recombination lines or high ionization lines. No trends in luminosity are apparent when subsets of the 31 non-AGN ULIRGs are binned by luminosity and median combined. When secondary nuclei exist in ULIRGs, they typically have spectra very much like those seen in the primary nuclei., 3 figures

Near infrared imaging and spectroscopic observations of the extremely red object (R-K~ 7 mag) CL 0939+4713 B have been obtained with the Near Infrared Camera on the Keck I Telescope of the W. M. Keck Observatory. The imaging shows a slightly elongated structure, while the spectroscopy shows a continuum break that allows us to determine the redshift of z = 1.58 + 0.01/-0.03 for this system. The fits of a range of models to the infrared spectrum suggests that it is predominantly an old (> 10^9 yrs) stellar system that suffers little extinction, while the measurerd R and I magnitudes suggests an age of ~ 3 x 10^8 years. The limit on the equivalent width of any emission line in the infrared spectrum argues that CL 0939+4713 B is not an actively star forming galaxy. This system, though similar in R-K color to HR 10 [also known as J1645+46] (Dey et al. 1999), is much different in morphology and emission line strengths, demonstrating the heterogeneity of extremely red extragalactic objects (EROs) selected on the basis of large values of R-K.

We present the design, construction, and performance of a near-infrared integral field spectrograph now in operation at the Palomar Observatory 200 inch telescope. The spectrograph acts as a front end to one of two existing infrared cameras, covering the 1-2.5 μm and 1-5 μm wavelength ranges. An all-aluminum image slicer splits the field of view into eight slits, feeding two independent spectrographs within the same dewar. Spectra are simultaneously obtained for all points within the field, with seeing-limited spatial resolution. Two resolution modes of R ≈ 1300 and R ≈ 600 are available, as well as a broadband imaging mode for photometry and target acquisition. The spectrograph was first operated on the 200 inch telescope in 1998 September. Performance has been as predicted, with a total system throughput of ~22% from the top of the atmosphere to electron counts. Sensitivity is estimated to allow detection of a K = 18 mag (0.04 mJy) continuum point source at the 5 σ level in 1 hr of integration at the high-resolution setting. Example data from the spectrograph are presented here.

High-resolution 1.10, 2.05, 2.12, and 2.15 μm imaging of the gravitationally lensed system FSC 10214+4724 are presented. These data extend Hubble Space Telescope (HST) observations of the lens system to redder wavelengths, thus providing the highest resolution images to date of the rest-frame optical and narrow-line (i.e., Hα+[N II]) regions of the background quasar. The length of the arc in the wide-band continuum images increases with increasing wavelength, and the Hα+[N II] emission has a length in between that of the 1.10 and 2.05 μm emission. The structure of the arc changes from having an eastern and western peak at 1.10 μm to having a single peak in the center at 2.05 μm. The changing structure and length of the arc can be understood in terms of a model in which the background quasar consists of a region of scattered active galactic nucleus (AGN) light that dominates at 1.10 μm (rest frame 3300 A), surrounded by a more extended narrow-line region. An even more extended red stellar population would thus contribute light at 2.05 μm (rest frame 6200 A). In addition, the Hα+[N II] emission has structural features similar to the 1.10 μm emission normalized by the (predominantly stellar) 2.05 μm emission, possibly confirming that the 1.10 μm emission is a superposition of the sources associated with the line emission (AGNs/massive stars) and the red stellar component that dominates the 2.05 μm emission. The counterimage of the lensed quasar is detected in the 1.10 and 2.05 μm images, and the rest frame 3300 and 6200 A magnifications of the lensed quasar are calculated to be 50 ± 11 and 25 ± 6, respectively, which translates into a rest-frame optical luminosity for the quasar of ~6 × 10^9 L_☉. These magnification values are lower than the previously measured magnification of ~100 at rest frame 2400 A. If the dust in the primary lensing galaxy is not affecting the measurement of the counterimage flux at 2400 and 3300 A, the magnification of the quasar appears to decrease with increasing wavelength. Flux measurements of the primary lensing galaxy fit the spectral energy distribution of an unevolving elliptical galaxy at a redshift of 0.9, consistent with previous determinations of the redshift.

We present imaging polarimetry of the extremely luminous, redshift 2.3 IRAS source FSC10214+4724. The observations were obtained with HST's Faint Object Camera in the F437M filter, which is free of strong emission lines. The 0.7 arcsec long arc is unresolved to 0.04 arcsec FWHM in the transverse direction, and has an integrated polarization of 28 +/- 3 percent, in good agreement with ground-based observations. The polarization position angle varies along the arc by up to 35 deg. The overall position angle is 62 +/- 3 deg east of north. No counterimage is detected to B = 27.5 mag ($3\sigma$), giving an observed arc to counterimage flux ratio greater than 250, considerably greater than the flux ratio of 100 measured previously in the I-band. This implies that the configuration of the object in the source plane at the B-band is different from that at I-band, and/or that the lensing galaxy is dusty.

With an infrared luminosity of L_(IR) = 6 x 10^(12) L_☉, IRAS 09104+4109 is one of the most luminous objects detected in the IRAS survey. First identified with a faint, R ~ 18 mag, galaxy at z = 0.442 by Kleinmann et al.(1988), IRAS 09104+4109 emits more than 99% of its bolometric luminosity longward of ⋋ ~ 1µm. Like most Ultraluminous Infrared Galaxies (ULIRGs), IRAS 09104+4109 has very strong emission lines. The nucleus exhibits a Seyfert type 2 spectrum, with strong, narrow lines over a wide range of ionization (Kleinmann et al.,1988; Soifer et al.,1996). Two-dimensional, ground-based spectrophotometry shows a "plume" of [OIII] emission-line gas extending almost 500 from the nucleus, along with a strong, blueshifted (ΔV ~ 1200 km s^(−1)) nuclear component (Crawford & Vanderriest, 1996). Although the dominant energy source in most ULIRGs is still a matter of some debate, the highly polarized spectrum (~ 18%), broad (10^4 km s^(−1)), (presumably reflected) MgII emission line, and double-lobed, core-jet radio structure, indicate a quasar at the heart of IRAS 09104+4109 (Hines & Wills, 1993). Recent calculations by Hines et al. (1999) suggest that the buried quasar in IRAS 09104+4109 can account for all of the bolometric luminosity of the system.

Imaging at 1.25 and 2.20 microns has been obtained of the field containing the galaxy (RD1) found at redshift 5.34 by Dey et al.(1998). This galaxy has been detected at 1.25 microns, while the lower redshift (z=4.02) galaxy also found in the same field by Dey et al. was detected at both 1.25 and 2.20 microns. Comparison to stellar population synthesis models indicates that if RD1 is a young (< 100 Myr old) galaxy, significant reddening (A(V) > 0.5 mag) is indicated. Combined with observations of other high redshift systems, these data show that dust is likely to be an important component of young galaxies even at redshifts of z > 5. The extinction-corrected monochromatic luminosity of RD1 at 1500 angstroms is then a factor of about three larger than L(1500)* as determined by Dickinson (1998) for z ~ 3 starburst galaxies. The implied star formation rate in RD1, corrected for extinction, is ~ 50-100 solar masses per year., plain LaTex with 1 postscript figure. ApJ Letters, accepted

We present the first results from the ISO-IRAS Faint Galaxy Survey (IIFGS), a program designed to obtain ISO observations of the most distant and luminous galaxies in the IRAS Faint Source Survey by filling short gaps in the ISO observing schedule with pairs of 12 μm ISOCAM and 90 μm ISOPHOT observations. As of 1997 October, over 500 sources have been observed, with an ISOCAM detection rate over 80%, covering over 1.25 deg^2 of sky to an 11.5 μm point-source completeness limit of approximately 1.0 mJy (corresponding to a ~10 σ detection sensitivity). Observations are presented for nine sources detected by ISOPHOT and ISOCAM early in the survey for which we have ground-based G- and I-band images and optical spectroscopy. The ground-based data confirm that the IIFGS strategy efficiently detects moderate-redshift (z = 0.11-0.38 for this small sample) strong emission line galaxies with L_(60 μm) ≳ 10^(11) L_☉; one of our sample has L_(60 μm) > 10^(12) L_☉ (H_0 = 75 km s^(-1) Mpc^(-1), Ω = 1). The infrared-optical spectral energy distributions are comparable to those of nearby luminous infrared galaxies, which span the range from pure starburst (e.g., Arp 220) to infrared QSO (Mrk 231). Two of the systems show signs of strong interaction, and four show active galactic nucleus (AGN)-like excitation; one of the AGNs, F15390+6038, which shows a high excitation Seyfert 2 spectrum, has an unusually warm far- to mid-infrared color and may be an obscured QSO. The IIFGS sample is one of the largest and deepest samples of infrared-luminous galaxies available, promising to be a rich sample for studying infrared-luminous galaxies up to z ~ 1 and for understanding the evolution of infrared galaxies and the star formation rate in the universe.

High spatial and spectral resolution spectra of the near-infrared H2 emission from the planetary nebula BD +30°3639 are presented. A 7'' × 13'' portion of the nebula was mapped at a resolution of 6000 in the v = 1-0 S(1) emission line of molecular hydrogen, revealing the morphology and kinematics of the H2 emission. These spectra show large velocity shifts in the H2 line across the nebula. Comparison with single-dish CO observations show significant velocity differences between the peaks of H2 emission and CO emission. Such differences are unexpected for a photodissociation region (PDR), since PDRs are supposed to form in the surface of the cold gas (as is the case for NGC 7027). A single long-slit spectrum which includes the peak of the H2 emission and covers the entire K band at a resolution of 1000 is analyzed. Fourteen molecular hydrogen emission lines are detected in a 2'' × 05 region covering the bright northeastern H2 lobe. The line ratios indicate that the H2 emission is radiatively excited, but that the gas density is large enough (n 104.5-105 cm-3) for collisions to thermalize the lowest vibrational states. The measured ratio of ortho-hydrogen to para-hydrogen is 2.51 ± 0.22, slightly smaller than the equilibrium value of 3. Spatial variations of the v = 2-1 vibrational temperature and the v = 1 ortho/para ratio are consistent with a decrease in the relative proportion of collisional excitation as distance from the central star increases.

Ground-based counts and colors of faint galaxies in the U and R bands in one field at high Galactic latitude are presented. Integrated over flux, a total of 1.2x10^5 sources per square degree are found to U=25.5 mag and 6.3x10^5 sources per square degree to R=27 mag, with d log N/dm ~ 0.5 in the U band and d log N/dm ~ 0.3 in the R band. Consistent with these number-magnitude curves, sources become bluer with increasing magnitude to median U-R=0.6 mag at 24, Comment: 17 pages, MNRAS in press

The Spitzer Space Telescope has identified a population of ultraluminous infrared galaxies (ULIRGs) at z ~ 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses (M_*) of two populations of Spitzer-selected ULIRGs that have extremely red R – [24] colors (dust-obscured galaxies, or DOGs) and compare our results with submillimeter-selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their mid-infrared (mid-IR) spectral energy distribution (SED) at rest frame 1.6 μm associated with stellar emission ("bump DOGs"), while the other set of 51 DOGs have power-law mid-IR SEDs that are typical of obscured active galactic nuclei ("power-law DOGs"). We measure M_* by applying Charlot & Bruzual stellar population synthesis models to broadband photometry in the rest-frame ultraviolet, optical, and near-infrared of each of these populations. Assuming a simple stellar population and a Chabrier initial mass function, we find that power-law DOGs and bump DOGs are on average a factor of 2 and 1.5 more massive than SMGs, respectively (median and inter-quartile M_* values for SMGs, bump DOGs, and power-law DOGs are log(M_*/M_☉) = 10.42^(+0.42)_(–0.36), 10.62^(+0.36)_(–0.32), and 10.71^(+0.40)_(–0.34), respectively). More realistic star formation histories drawn from two competing theories for the nature of ULIRGs at z ~ 2 (major merger versus smooth accretion) can increase these mass estimates by up to 0.5 dex. A comparison of our stellar masses with the instantaneous star formation rate (SFR) in these z ~ 2 ULIRGs provides a preliminary indication supporting high SFRs for a given M_*, a situation that arises more naturally in major mergers than in smooth accretion-powered systems.

The Two Micron All Sky Survey (2MASS) will provide a uniform survey of the entire sky at three near-infrared wavebands: J(lambda(sub eff) = 1.25 micrometers), H(lambda(sub eff) = 1.65 micrometers), and K(sub s)(lambda(sub eff) = 2.16 micrometers). A major goal of the survey is to probe large scale structures in the Milky Way and in the Local Universe, exploiting the relatively high transparency of the interstellar medium in the near-infrared, and the high near-infrared luminosities of evolved low- and intermediate-mass stars. A sensitive overview of the near-infrared sky is also an essential next step to maximize the gains achievable with infrared array technology. Our assessment of the astrophysical questions that might be addressed with these new arrays is currently limited by the very bright flux limit of the only preceding large scale near-infrared sky survey, the Two Micron Sky Survey carried out at Caltech in the late 1960's. Near-infrared instruments based on the new array technology have already obtained spectra of objects 1 million times fainter than the limit of the TMSS! This paper summarizes the essential parameters of the 2MASS project and the rationale behind those choices, and gives an overview of results obtained with a prototype camera that has been in operation since May 1992. We conclude with a list of expected data products and a statement of the data release policy.

This paper describes the design, expectations, and prototyping of a new allsky survey, called 2MASS (Two Micron All Sky Survey) to be carried out with the new generation of infrared array detectors.

Detailed analysis of the substructure of Lya nebulae can put important constraints on the physical mechanisms at work and the properties of galaxies forming within them. Using high resolution HST imaging of a Lya nebula at z~2.656, we have taken a census of the compact galaxies in the vicinity, used optical/near-infrared colors to select system members, and put constraints on the morphology of the spatially-extended emission. The system is characterized by (a) a population of compact, low luminosity (~0.1 L*) sources --- 17 primarily young, small (Re~1-2 kpc), disky galaxies including an obscured AGN --- that are all substantially offset (>20 kpc) from the line-emitting nebula; (b) the lack of a central galaxy at or near the peak of the Lya emission; and (c) several nearly coincident, spatially extended emission components --- Lya, HeII, and UV continuum --- that are extremely smooth. These morphological findings are difficult to reconcile with theoretical models that invoke outflows, cold flows, or resonant scattering, suggesting that while all of these physical phenomena may be occurring, they are not sufficient to explain the powering and large extent of Lya nebulae. In addition, although the compact galaxies within the system are irrelevant as power sources, the region is significantly overdense relative to the field galaxy population (by at least a factor of 4). These observations provide the first estimate of the luminosity function of galaxies within an individual Lya nebula system, and suggest that large Lya nebulae may be the seeds of galaxy groups or low-mass clusters., Accepted to ApJ after minor revision; 33 pages in emulateapj format, 21 figures, 4 tables