Infrared Luminosity Research Articles

From major merger to radio galaxy: low surface-brightness stellar counterpart to the giant H I ring around B2 0648+27

We present the detection of a low surface-brightness stellar counterpart to an enormous (190 kpc) ring of neutral hydrogen (HI) gas that surrounds the nearby radio galaxy B2 0648+27. This system is currently in an evolutionary stage between major merger and (radio-loud) early-type galaxy. In a previous paper we investigated in detail the timescales between merger, starburst and AGN activity in B2 0648+27, based on its unusual multi-wavelength properties (large-scale HI ring, dominating post-starburst stellar population and infra-red luminosity). In this Research Note we present deep optical B- and V-band imaging that provides further evidence for the merger origin of B2 0648+27. The host galaxy shows a distorted optical morphology and a broad tidal arm is clearly present. A low surface-brightness stellar tail or partial ring curls around more than half the host galaxy at a distance of up to 55 kpc from the centre of the galaxy, following the large-scale, ring-like HI structure that we detected previously around this system. The gas and stars in this ring are most likely tidally expelled material that slowly fell back onto the host galaxy after the merger event. There also appear to be sites of star formation in the HI ring that may have formed within the gaseous tidal debris after the merger. We argue that the observed properties of the gas and stars in B2 0648+27, as well as the apparent time-delay between the merger and the starburst event, may be the logical result of a merger between two gas-rich disk galaxies with a prominent bulge, or of a merger between an elliptical and a gas-rich spiral galaxy. There also appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity.

Measuring the Fraction of Obscured Quasars by the Infrared Luminosity of Unobscured Quasars

Recent work has suggested that the fraction of obscured AGNs declines with increasing luminosity, but it has been difficult to quantify this trend. Here we attempt to measure this fraction as a function of luminosity by studying the ratio of mid-infrared to intrinsic nuclear bolometric luminosity in unobscured AGNs. Because the mid-infrared is created by dust reprocessing of shorter wavelength nuclear light, this ratio is a diagnostic of -->fobsc, the fraction of solid angle around the nucleus covered by obscuring matter. In order to eliminate possible redshift dependences while also achieving a large dynamic range in luminosity, we have collected archival 24 μm MIPS photometry from objects with -->z ~ 1 in the Sloan Digital Sky Survey, the Great Observatories Origins Deep Survey, and the Cosmic Evolution Survey. To measure the bolometric luminosity for each object, we used archival optical data supplemented by GALEX data. We find that the mean ratio of 24 μm to bolometric luminosity decreases by a factor of ~3 in the -->Lbol = 1044–3 × 1047 ergs s -->−1 range, but there is also a large scatter at constant -->Lbol. Using radiation transfer solutions for model geometries, we show how the IR/bolometric ratio relates to -->fobsc and compare these values with those obtained from samples of X-ray-selected AGNs. Although we find approximate agreement, our method indicates somewhat higher values of -->fobsc, particularly in the middle range of luminosities, suggesting that there may be a significant number of heavily obscured AGNs missed by X-ray surveys.

Formaldehyde Densitometry of Starburst Galaxies

With a goal toward deriving the physical conditions in external galaxies, we present a survey of the formaldehyde emission in a sample of starburst systems. By extending a technique used to derive the spatial density in star formation regions in our own Galaxy, we show how the relative intensity of the 110-111 and 211-212 K-doublet transitions of H2CO can provide an accurate densitometer for the active star formation environments found in starburst galaxies. Relying on an assumed kinetic temperature and cospatial emission and absorption from both H2CO transitions, our technique is applied to a sample of 19 infrared-bright galaxies which exhibit various forms of starburst activity. In the five galaxies of our sample where both H2CO transitions were detected, we have derived spatial densities. We also use H2CO to estimate the dense gas mass in our starburst galaxy sample, finding similar mass estimates for the dense gas-forming stars in these objects as derived using other dense gas tracers. A related trend can be seen when one compares LIR to our derived n(H2) for the five galaxies within which we have derived spatial densities. Even though our number statistics are small, there appears to be a trend toward higher spatial density for galaxies with higher infrared luminosity. This is likely another representation of the LIR-Mdense correlation.

SpitzerMid‐Infrared Spectroscopy of 70 μm–selected Distant Luminous Infrared Galaxies

We present mid-infrared spectroscopy obtained with the Spitzer Space Telescope of a sample of 11 optically faint, infrared luminous galaxies selected from a Spitzer MIPS 70 μm imaging survey of the NDWFS Bootes field. These are the first Spitzer IRS spectra presented of distant 70 μm-selected sources. All the galaxies lie at redshifts 0.3 < z < 1.3 and have very large infrared luminosities of L_(IR) ~ (0.1–17) × 10^(12)Lסּ. Seven of the galaxies exhibit strong emission features attributed to polycyclic aromatic hydrocarbons (PAHs). The average IRS spectrum of these sources is characteristic of classical starburst galaxies but with much larger infrared luminosities. The PAH luminosities of ν νL_ν(7.7 μ m) ~ (0.4–7) × 10^(11) Lסּ imply star formation rates of ~40-720 Mסּ yr^(−1). Four of the galaxies show deep 9.7 μm silicate absorption features and no significant PAH emission features (6.2 μm equivalent widths <0.03 μm). The large infrared luminosities and low ν νf_ν(70 μ m)/νf_ν(24 μ m) flux density ratios suggests that these sources have AGNs as the dominant origin of their large mid-infrared luminosities, although deeply embedded but luminous starbursts cannot be ruled out. If the absorbed sources are AGN-dominated, a significant fraction of all far-infrared bright, optically faint sources may be dominated by AGNs.

Infrared Spectral Energy Distributions ofz∼ 0.7 Star‐forming Galaxies

We analyze the infrared (IR) spectral energy distributions (SEDs) for 10 μm < λrest < 100 μm for ~600 galaxies at z ~ 0.7 in the extended Chandra Deep Field South by stacking their Spitzer 24, 70, and 160 μm images. We place interesting constraints on the average IR SED shape in two bins: the brightest 25% of z ~ 0.7 galaxies detected at 24 μm, and the remaining 75% of individually detected galaxies. Galaxies without individual detections at 24 μm were not well detected at 70 and 160 μm even through stacking. We find that the average IR SEDs of z ~ 0.7 star-forming galaxies fall within the diversity of z ~ 0 templates. While dust obscuration LIR/LUV seems to be only a function of star formation rate (SFR; ~LIR + LUV), not of redshift, the dust temperature of star-forming galaxies (with SFR ~ 10 M☉ yr-1 ) at a given IR luminosity was lower at z ~ 0.7 than today. We suggest an interpretation of this phenomenology in terms of dust geometry: intensely star-forming galaxies at z ~ 0 are typically interacting, and host dense centrally concentrated bursts of star formation and warm dust temperatures. At z ~ 0.7, the bulk of intensely star-forming galaxies are relatively undisturbed spirals and irregulars, and we postulate that they have large amounts of widespread lower density star formation, yielding lower dust temperatures for a given IR luminosity. We recommend which IR SEDs are most suitable for modeling intermediate-redshift galaxies with different SFRs.

SpitzerMid‐ to Far‐Infrared Flux Densities of Distant Galaxies

We study the infrared (IR) properties of high-redshift galaxies using deep Spitzer 24, 70, and 160 micron data. Our primary interest is to improve the constraints on the total IR luminosities, L(IR), of these galaxies. We combine the Spitzer data in the southern Extended Chandra Deep Field with a K-band-selected galaxy sample and photometric redshifts from the Multiwavelength Survey by Yale-Chile. We used a stacking analysis to measure the average 70 and 160 micron flux densities of 1.5 250 micro-Jy and 1.5 250 micro-Jy have S(70)/S(24) flux ratios comparable to sources with X-ray detections or red rest-frame IR colors, suggesting that warm dust possibly heated by AGN may contribute to the high 24 micron emission. Based on the average 24-160 micron flux densities, nearly all 24 micron-selected galaxies at 1.5 6 x 10^12 solar luminosities are quite rare, with a surface density ~ 30 +/- 10 per sq. deg, corresponding to ~ 2 +/- 1 x 10^-6 Mpc^-3 over 1.5 < z < 2.5.

Star formation in M 33: Spitzer photometry of discrete sources

Aims. Combining the relative vicinity of the Local Group spiral galaxy M 33 with the Spitzer images, we investigate the properties of infrared (IR) emission sites and assess the reliability of the IR emission as a star formation tracer. Methods. The mid- and far-IR emission of M 33 was obtained from IRAC and MIPS images from the Spitzer archive. We compared the photometric results for several samples of three known types of discrete sources (Hll regions, supernovae remnants and planetary nebulae) with theoretical diagnostic diagrams, and derived the spectral energy distribution (from 3.6 to 24 μm) of each type of object. Moreover, we generated a catalogue of 24 μm sources and inferred their nature from the observed and theoretical colours of the known type sources. We estimated the star formation rate in M 33 both globally and locally, from the IR emission and from the Ha emission line. Results. The colours of the typical IR emissions of HII regions, supernovae remnants and planetary nebulae are continuous among the different samples, with overlapping regions in the diagnostic diagrams. The comparison between the model results and the colours of HII regions indicates a dusty envelope at relatively high temperatures ∼600 K, and moderate extinction A v ≤ 10. The 24 μm sources IR colours follow the regions observationally defined by the three classes of known objects but the majority of them represent HII regions. The derived total IR luminosity function is in fact very similar to the HII luminosity function observed in the Milky Way and in other late type spirals. Even though our completeness limit is 5 ×10 37 erg s -1 , in low density regions we are able to detect sources five times fainter than this, corresponding to the faintest possible HII region. The 8 and 24 μm luminosities within the central 5 kpc of M 33 are comparable and of order 4 ×10 28 erg s -1 Hz -1 (vL v (8) = 1.5 x 10 42 and vL v (24) = 4.4 x 10 41 erg s -1 ). We estimate the total IR emission in the same region of M 33 to be 10 9 L ⊙ . The discrete sources account for about one third of the 24 μm emission while the rest is diffuse. From the IR emission, we derive a star formation rate for the inner disk equal to 0.2 M⊙ yr -1 , consistent with the star formation rate obtained from the Her emission.

A census of the Carina Nebula - II. Energy budget and global properties of the nebulosity

The first paper in this series took a direct census of energy input from the known OB stars in the Carina Nebula, and in this paper we study the global properties of the surrounding nebulosity. This detailed comparison may prove useful for interpreting observations of extragalactic giant H II regions and ultraluminous infrared (IR) galaxies. We find that the total IR luminosity of Carina is about 1.2 x 10 7 L ⊙ , accounting for only about 50-60 per cent of the known stellar luminosity from Paper I. Similarly, the ionizing photon luminosity derived from the integrated radio continuum is about 7 x 10 50 s -1 , accounting for ∼75 per cent of the expected Lyman continuum from known OB stars. The total kinetic energy of the nebula is about 8 x 10 51 erg, or ∼30 per cent of the mechanical energy from stellar winds over the lifetime of the nebula, so there is no need to invoke a supernova (SN) explosion based on energetics. Warm dust grains residing in the HII region interior dominate emission at 10-30 μm, but cooler grains at 30-40 K dominate the IR luminosity and indicate a likely gas mass of ∼10 6 M ⊙ . We find an excellent correlation between the radio continuum and 20-25 μm emission, consistent with the idea that the ∼80-K grain population is heated by trapped Lyα photons. Similarly, we find a near perfect correlation between the far-IR optical depth map of cool grains and 8.6-μm hydrocarbon emission, indicating that most of the nebular mass resides as atomic gas in photodissociation regions and not in dense molecular clouds. Synchronized star formation around the periphery of Carina provides a strong case that star formation here was indeed triggered by stellar winds and ultraviolet radiation. This second generation appears to involve a cascade toward preferentially intermediate- and low-mass stars, but this may soon change when η Carinae and its siblings explode. If the current reservoir of atomic and molecular gas can be tapped at that time, massive star formation may be rejuvenated around the periphery of Carina much as if it were a young version of Gould's Belt. Furthermore, when these multiple SNe occur, the triggered second generation will be pelted repeatedly with SN ejecta bearing short-lived radioactive nuclides. Carina may therefore represent the most observable analogue to the cradle of our own Solar system.

Infrared and radio features of galaxies with UV excess

This is a statistical study of galaxies with a UV excess. A sample of 702 Kazarian galaxies (KG) is used. The KGs are identified with objects from the MAPS, IRAS FSC, IRAS PSC, and NVSS catalogs. The O and E magnitudes are known for more than 92% of the KGs. It is shown that the KG sample is complete up to 16m.0 in the blue and to 16m.5 in the red. More than 36% of the KGs are identified with infrared (IR) sources at wavelengths of 12, 25, 60, and 100 m. Calculations of the far IR (FIR) luminosities show that 4% of the KGs are strong FIR emitters (LFIR ≥ 1011 L⊙). More than 32% of the KGs have been identified as radio sources at a frequency of 1.4 GHz. A determination of the radio luminosities shows that the sample of KGs with known radial velocities include one powerful, LR ≥ 1025 W/Hz, radio object (Kaz 273) which is a BL Lac object. A close correlation (r=0.93) is bound between the FIR and radio luminosities for galaxies with a UV excess. An examination of the relationship between the FIR and radio luminosities for galaxies in different spectral classes shows that the correlation coefficient is higher (r=0.99) and the slope of the fit curve is larger (a=1.18) for Seyfert galaxies. Calculations of the logarithm of the ratio of the FIR and radio fluxes indicate that the sample includes 4 KGs with a radio excess, while there are none with an IR excess.

Multiwavelength study of X-ray selected star-forming galaxies within the Chandra Deep Field-South

We have combined multiwavelength observations of a selected sample of star-forming galaxies with galaxy evolution models in order to compare the results obtained for different star formation rate (SFR) tracers and to study the effect that the evolution of the star-forming regions has on them. We also aimed at obtaining a better understanding of the corrections due to extinction and nuclear activity on the derivation of the SFR. We selected the sample from Chandra data for the well studied region Chandra Deep Field-South (CDFS) and chose the objects that also have ultraviolet (UV) and infrared (IR) data from Galaxy Evolution Explorer (GALEX) and Great Observatories Origins Deep Survey (GOODS) Spitzer, respectively. Our main finding is that there is good agreement between the extinction corrected SFR(UV) and the SFR(X), and we confirm the use of X-ray luminosities as a trustful tracer of recent star formation activity. Nevertheless, at SFR(UV) larger than about 5 M⊙ yr−1 there are several galaxies with an excess of SFR(X) suggesting the presence of an obscured active galactic nucleus (AGN) not detected in the optical spectra. We conclude that the IR luminosity is driven by recent star formation even in those galaxies where the SFR(X) is an order of magnitude higher than the SFR(UV) and therefore may harbour an AGN. One object shows SFR(X) much lower than expected based on the SFR(UV); this SFR(X) ‘deficit’ may be due to an early transient phase before most of the massive X-ray binaries were formed. An X-ray deficit could be used to select extremely young bursts in an early phase just after the explosion of the first supernovae associated with massive stars and before the onset of massive X-ray binaries.

The Infrared Luminosity Function of Galaxies at Redshiftsz= 1 andz∼ 2 in the GOODS Fields

We present the rest-frame 8 micron luminosity function (LF) at redshifts z=1 and ~2, computed from Spitzer 24 micron-selected galaxies in the GOODS fields over an area of 291 sq. arcmin. Using classification criteria based on X-ray data and IRAC colours, we identify the AGN in our sample. The rest-frame 8 micron LF for star-forming galaxies at redshifts z=1 and ~2 have the same shape as at z~0, but with a strong positive luminosity evolution. The number density of star-forming galaxies with log_{10}(nu L_nu(8 micron))>11 increases by a factor >250 from redshift z~0 to 1, and is basically the same at z=1 and ~2. The resulting rest-frame 8 micron luminosity densities associated with star formation at z=1 and ~2 are more than four and two times larger than at z~0, respectively. We also compute the total rest-frame 8 micron LF for star-forming galaxies and AGN at z~2 and show that AGN dominate its bright end, which is well-described by a power-law. Using a new calibration based on Spitzer star-forming galaxies at 0<z<0.6 and validated at higher redshifts through stacking analysis, we compute the bolometric infrared (IR) LF for star-forming galaxies at z=1 and ~2. We find that the respective bolometric IR luminosity densities are (1.2+/-0.2) x 10^9 and (6.6^{+1.2}_{-1.0}) x 10^8 L_sun Mpc^{-3}, in agreement with previous studies within the error bars. At z~2, around 90% of the IR luminosity density associated with star formation is produced by luminous and ultraluminous IR galaxies (LIRG and ULIRG), with the two populations contributing in roughly similar amounts. Finally, we discuss the consistency of our findings with other existing observational results on galaxy evolution.

The ultraviolet properties of luminous infrared galaxies atz ~ 0.7

Aims.The total infrared (TIR: 8-1000 μm) and far-ultraviolet (FUV: ~1500 Å) luminosity functions of galaxies and the related luminosity densities and are known to evolve at differemt rates from to z ~1: the galaxy populations appear to be brighter in the past at both wavelengths, but the evolution in the TIR is larger than in the FUV. This leads to an increase of the ratio of TIR to FUV luminosity densities which can be interpreted as a global increase of the dust attenuation from to z ~ 1. Our aim is to understand the origin of this increase: is it entirely due to a variation of the dust attenuation with the luminosity of the galaxies as seen as or are properties of galaxies evolving with the redshift?

Observations of Herbig Ae Disks with Nulling Interferometry

We present the results of 10 micron nulling interferometric observations of 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 m telescopes. A portion of the observations was completed with the adaptive secondary at the MMT. We have conclusively spatially resolved 3 of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter two recently resolved using adaptive optics in combination with nulling interferometry. For the resolved objects we find that the 10 $\mu$m emitting regions have a spatial extent of 15-30 AU in diameter. We also have some evidence for resolved emission surrounding an additional 2 stars (V892 Tau and R CrA). For those objects in our study with mid-IR SEDs classifications from Meeus et al. (2001), we find that the Group I objects (those with constant to increasing mid-IR flux) are more likely to be resolved, within our limited sample. This trend is evident in correlations in the inferred disk sizes vs. the sub-millimeter SED slope and disk size vs. fractional infrared luminosity of the systems. We explore the spatial distribution and orientation of the warm dust in the resolved systems and constrain physical models which are consistent with their observational signatures.

Infrared 2–4 Micrometer Spectroscopy and Millimeter Interferometric HCN and HCO+ Observations of the Individual Merging Components of Arp 299

Abstract We present ground-based infrared $K$–(2–2.5$\, {\mu \mathrm {m}}$) and $L$-band (2.8–4.1$\, {\mu \mathrm {m}}$) spectroscopy, as well as interferometric observations at $\lambda \sim 3 \,\mathrm{mm}$, for the individual merging components (A, B, and C) of the luminous infrared galaxy Arp 299. The presence and location of a buried active galactic nucleus (AGN) are investigated. Our sub-arcsec-resolution infrared spectra clearly reveal that the putative buried AGN resides in the nucleus B1 (a subcomponent of B), based on a very low equivalent width of 3.3$\, {\mu \mathrm {m}}$ polycyclic aromatic hydrocarbon emission, a weak 2.3$\, {\mu \mathrm {m}}$ CO absorption feature, and a large time variation of the $K$–and $L$-band continuum fluxes. Our interferometric observations simultaneously obtain HCN ($J = 1 \hbox{--} 0$) and $\mathrm{HCO}^{+}$ ($J = 1 \hbox{--} 0$) emission lines with $\sim 4 {{}^{\prime \prime }}$ resolution, and we find the HCN to $\mathrm{HCO}^{+}$ brightness-temperature ratios to be as low as those found in starburst nuclei in all of the major merging components of Arp 299. The low ratio even in the AGN-hosting nucleus B may be due to the presence of a large amount of high-density molecular gas whose chemistry is dominated by coexisting starbursts and/or shocks, rather than by the central strong X-ray-emitting AGN.

The Local Universe as Seen in the Far‐Infrared and Far‐Ultraviolet: A Global Point of View of the Local Recent Star Formation

We select far-infrared (FIR: 60 μm) and far-ultraviolet (FUV: 530 Å) samples of nearby galaxies in order to discuss the biases encountered by monochromatic surveys (FIR or FUV). Very different volumes are sampled by each selection, and much care is taken to apply volume corrections to all the analyses. The distributions of the bolometric luminosity of young stars are compared for both samples: they are found to be consistent with each other for galaxies of intermediate luminosities, but some differences are found for high (>5 × 1010 L☉) luminosities. The shallowness of the IRAS survey prevents us from securing a comparison at low luminosities (<2 × 109 L☉). The ratio of the total infrared (TIR) luminosity to the FUV luminosity is found to increase with the bolometric luminosity in a similar way for both samples up to 5 × 1010 L☉. Brighter galaxies are found to have a different behavior according to their selection: the LTIR/LFUV ratio of the FUV-selected galaxies brighter than 5 × 1010 L☉ reaches a plateau, whereas LTIR/LFUV continues to increase with the luminosity of bright galaxies selected in FIR. The volume-averaged specific star formation rate (SFR per unit galaxy stellar mass, SSFR) is found to decrease toward massive galaxies within each selection. The mean values of the SSFR are found to be larger than those measured for optical and NIR-selected samples over the whole mass range for the FIR selection, and for masses larger than 1010 M☉ for the FUV selection. Luminous and massive galaxies selected in FIR appear as active as galaxies with similar characteristics detected at z ∼ 0.7.

Mid‐Infrared High Spatial Resolution Observations of NGC 1569: Detection of Embedded Embryos of Star Formation

We present high spatial resolution mid-infrared (MIR) imaging and spectroscopic observations of the dwarf galaxy NGC 1569 with COMICS on the Subaru telescope. The [S IV] 10.5 μm image clearly shows four pointlike sources and one extended source together with the diffuse emission. The brightest MIR source (MIR1) coincides with the intense H II region, which does not have a corresponding optical source and should thus be a source embedded in dust clouds. The infrared luminosity suggests that there are about 40 O7 stars associated with MIR1. The second brightest source located ~15 south of MIR1 (MIR2) is detected for the first time. This source is not seen in the radio continuum. These characteristics can be accounted for if ionizing regions in the source are compact and optically thick in the radio. The large infrared luminosity (~2 × 108 L☉) suggests that there are about 950 O7 stars in MIR2 and significant contributions from sources other than O-type stars, such as less massive stars and/or pre-main-sequence stars. If this interpretation is correct, MIR2 could be in a very early phase of massive star formation. The unidentified infrared (UIR) band at 11.2 μm is also detected in the spectrum of MIR2. The strength of the UIR band decreases with the [S IV] emission, suggesting that the UIR band weakens and the band carriers are destroyed in strong radiation fields. Super star cluster (SSC) A is also detected in the N-band images. The mid-infrared characteristics support the difference in the ages between two components of SSC A.

Probing Cool and Warm Infrared Galaxies Using Photometric and Structural Measures

We have analyzed a sample of nearby cool and warm infrared (IR) galaxies using photometric and structural parameters. The set of measures include far-infrared color [C = log(S60 μm/S100 μm)], total IR luminosity (LTIR), radio surface brightness, and radio, near-infrared, and optical sizes. In a given luminosity range cool and warm galaxies are considered as those sources that are found approximately 1 σ below and above the mean color in the far-infrared C-LTIR diagram. We find that galaxy radio surface brightness is well correlated with color whereas size is less well correlated with color. Our analysis indicates that IR galaxies that are dominated by cool dust are large, massive spirals that are not strongly interacting or merging and presumably the ones with the least active star formation. Dust in these cool objects is less centrally concentrated than in the more typical luminous and ultraluminous IR galaxies that are dominated by warm dust. Our study also shows that low-luminosity early-type unbarred and transitional spirals are responsible for the large scatter in the C-LTIR diagram. Among highly luminous galaxies, late-type unbarred spirals are predominately warm, and early-type unbarred and barred spirals are systematically cooler. We highlight the significance of the C-LTIR diagram in terms of local and high-redshift submillimeter galaxies.

Searching for a gigamaser in APM-08279+5255, and other short stories

Bolometer arrays on large antennas at high, dry sites have unveiled a dusty population of massive, luminous galaxies - submillimetre galaxies, or SMGs - which make a significant contribution to the star formation rate density at z > 1. The most crucial piece of information required to derive the history of obscured star formation is the redshift distribution of this galaxy population, N(z), which breaks degeneracies in the models and allows the mass and dynamics of the galaxies to be explored via high-resolution three-dimensional imaging in CO and by determining their level of clustering. Many SMGs are extremely faint, optically; some have no plausible counterparts, even in the infrared (IR), making the determination of an unbiased N(z) very difficult. The arrival of the Herschel Space Observatory and next-generation ground-based submm cameras will likely exacerbate this so-called redshift deadlock. Here, we report the first test of a new method for determining redshifts, based on the observed dependence of maser and IR luminosities. We have searched the dusty, lensed, hyperluminous quasar, APM 08279+5255, for the 1612-, 1665- and 1667-MHz hydroxyl lines as well as the 22-GHz water line. At z = 3.9, these are shifted to 329, 340 and 4538 MHz. Our relatively shallow test data reveal no convincing maser activity, but we set a meaningful constraint on the OH maser luminosity and we approach the expected thermal noise levels, meaning progress is possible. As an aside, we present deep new submm and radio imaging of this field. Using a simple shift-and-add technique, we uncover a new submm galaxy, conceivably at the redshift of APM 08279+5255.

A Panoramic Mid‐Infrared Survey of Two Distant Clusters

We present panoramic Spitzer MIPS 24 μm observations, covering ~9 × 9 Mpc2 (25' × 25') fields around two massive clusters, Cl 0024+16 and MS 0451-03, at z = 0.39 and z = 0.55, respectively, reaching a 5 σ flux limit of ~200 μJy. Our observations cover a very wide range of environments within these clusters, from high-density regions around the cores out to the turnaround radius. Cross-correlating the mid-infrared catalogs with deep optical and near-infrared imaging of these fields, we investigate the optical/near-infrared colors of the mid-infrared sources. We find excesses of mid-infrared sources with the optical/near-infrared colors expected of cluster members in the two clusters and test this selection using spectroscopically confirmed 24 μm members. The much more significant excess is associated with Cl 0024+16, whereas MS 0451-03 has comparatively few mid-infrared sources. The mid-infrared galaxy population in Cl 0024+16 appears to be associated with dusty star-forming galaxies (typically redder than the general cluster population by up to AV ~ 1-2 mag) rather than emission from dusty tori around active galactic nuclei in early-type hosts. We compare the star formation rates derived from the total infrared (8-1000 μm) luminosities for the mid-infrared sources in Cl 0024+16 with those estimated from a published Hα survey, finding rates 5 times those found from Hα, indicating significant obscured activity in the cluster population. Compared to previous mid-infrared surveys of clusters from z ~ 0-0.5, we find evidence for strong evolution of the level of dust-obscured star formation in dense environments to z = 0.5, analogous to the rise in the fraction of optically selected star-forming galaxies seen in clusters and the field out to similar redshifts. However, there are clearly significant cluster-to-cluster variations in the populations of mid-infrared sources, probably reflecting differences in the intracluster media and recent dynamical evolution of these systems.

Extragalactic H2O masers and X-ray absorbing column densities

Having conducted a search for the λ ~ 1.3 cm (22 GHz) water vapor line towards galaxies with nuclear activity, large nuclear column densities or high infrared luminosities, we present H2 O spectra for NGC 2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and 400 . The H2 O maser in UGC 5101 is by far the most luminous yet found in an ultraluminous infrared galaxy. NGC 3393 reveals the classic spectrum of a “disk maser”, represented by three distinct groups of Doppler components. As in all other known cases except NGC 4258, the rotation velocity of the putative masing disk is well below 1000 km s-1 . Based on the literature and archive data, X-ray absorbing column densities are compiled for the 64 galaxies with reported maser sources beyond the Magellanic Clouds. For NGC 2782 and NGC 5728, we present Chandra archive data that indicate the presence of an active galactic nucleus in both galaxies. Modeling the hard nuclear X-ray emission, NGC 2782 is best fit by a high energy reflection spectrum with N H 1024 cm-2 . For NGC 5728, partial absorption with a power law spectrum indicates N H ~ 8 1023 cm-2 . The correlation between absorbing column and H2 O emission is analyzed. There is a striking difference between kilo- and megamasers with megamasers being associated with higher column densities. All kilomasers ( cm-2 . Among the H2 O megamasers, 50% arise from Compton-thick and 85% from heavily obscured (>1023 cm-2 ) active galactic nuclei. These values are not larger but consistent with those from samples of Seyfert 2 galaxies not selected on the basis of maser emission. The similarity in column densities can be explained by small deviations in position between maser spots and nuclear X-ray source and a high degree of clumpiness in the circumnuclear interstellar medium.