A Herschel Study of 24 micron-Selected AGNs and Their Host Galaxies

We present a sample of 290 24-micron-selected active galactic nuclei (AGNs) mostly at z ~ 0.3 -- 2.5, within 5.2 square degrees distributed as 25' X 25' fields around each of 30 galaxy clusters in the Local Cluster Substructure Survey (LoCuSS). The sample is nearly complete to 1 mJy at 24 microns, and has a rich multi-wavelength set of ancillary data; 162 are detected by Herschel. We use spectral templates for AGNs, stellar populations, and infrared emission by star forming galaxies to decompose the spectral energy distributions (SEDs) of these AGNs and their host galaxies, and estimate their star formation rates (SFRs), AGN luminosities, and host galaxy stellar masses. The set of templates is relatively simple: a standard Type-1 quasar template; another for the photospheric output of the stellar population; and a far infrared star-forming template. For the Type-2 AGN SEDs, we substitute templates including internal obscuration, and some Type-1 objects require a warm component (T > 50 K). The individually Herschel- detected Type-1 AGNs and a subset of 17 Type-2 ones typically have luminosities > 10^{45} ergs/s, and supermassive black holes of ~ 3 X 10^8 Msun emitting at ~ 10% of the Eddington rate. We find them in about twice the numbers of AGN identified in SDSS data in the same fields, i.e., they represent typical high luminosity AGN, not an infrared-selected minority. These AGNs and their host galaxies are studied further in an accompanying paper

Evidence for a Z < 8 Origin of the Source Subtracted Near Infrared Background

This letter extends our previous fluctuation analysis of the near infrared background at 1.6 microns to the 1.1 micron (F110W) image of the Hubble Ultra Deep field. When all detectable sources are removed the ratio of fluctuation power in the two images is consistent with the ratio expected for faint, z<8, sources, and is inconsistent with the expected ratio for galaxies with z>8. We also use numerically redshifted model galaxy spectral energy distributions for 50 and 10 million year old galaxies to predict the expected fluctuation power at 3.6 microns and 4.5 microns to compare with recent Spitzer observations. The predicted fluctuation power for galaxies at z = 0-12 matches the observed Spitzer fluctuation power while the predicted power for z>13 galaxies is much higher than the observed values. As was found in the 1.6 micron (F160W) analysis the fluctuation power in the source subtracted F110W image is two orders of magnitude below the power in the image with all sources present. This leads to the conclusion that the 0.8--1.8 micron near infrared background is due to resolved galaxies in the redshift range z<8, with the majority of power in the redshift range of 0.5--1.5.Comment: Accepted for publication in the Astrophysical Journa

The Nuclear Starburst in NGC 253

We have obtained long-slit spectra of NGC 253 in the J, H, K, and N bands, broadband images in the J, H, and Ks bands, narrowband images centered at the wavelengths of BrGamma and H2(1,0)S(1), and imaging spectroscopy centered on [NeII](12.8um). We use these data and data from the literature in a comprehensive re-assessment of the starburst in this galaxy. We derive the supernova rate from the strength of the infrared [FeII] lines. We find that most of the H2 infrared luminosity is excited by fluorescence in low density gas. We derive a strong upper limit of ~37,000K for the stars exciting the emission lines. We use velocity-resolved infrared spectra to determine the mass in the starburst region. Most of this mass appears to be locked up in the old, pre-existing stellar population. Using these constraints and others to build an evolutionary synthesis model, we find that the IMF originally derived to fit the starburst in M 82 (similar to a Salpeter IMF) also accounts for the properties of NGC 253. The models indicate that rapid massive star formation has been ongoing for 20-30 million years in NGC 253---that is, it is in a late phase of its starburst. We model the optical emission line spectrum expected from a late phase starburst and demonstrate that it reproduces the observed HII/weak-[OI] LINER characteristics.Comment: 48 pages, 14 figures, uses AASTeX macros, to appear in Ap

Absolute Flux Calibration of the IRAC Instrument on the Spitzer Space Telescope using Hubble Space Telescope Flux Standards

The absolute flux calibration of the James Webb Space Telescope will be based on a set of stars observed by the Hubble and Spitzer Space Telescopes. In order to cross-calibrate the two facilities, several A, G, and white dwarf (WD) stars are observed with both Spitzer and Hubble and are the prototypes for a set of JWST calibration standards. The flux calibration constants for the four Spitzer IRAC bands 1-4 are derived from these stars and are 2.3, 1.9, 2.0, and 0.5% lower than the official cold-mission IRAC calibration of Reach et al. (2005), i.e. in agreement within their estimated errors of ~2%. The causes of these differences lie primarily in the IRAC data reduction and secondarily in the SEDs of our standard stars. The independent IRAC 8 micron band-4 fluxes of Rieke et al. (2008) are about 1.5 +/- 2% higher than those of Reach et al. and are also in agreement with our 8 micron result.Comment: 16 pages, 6 figure

On Measuring the Infrared Luminosity of Distant Galaxies with the Space Infrared Telescope Facility

The Space Infrared Telescope Facility (SIRTF) will revolutionize the study of dust-obscured star formation in distant galaxies. Although deep images from the Multiband Imaging Photometer for SIRTF (MIPS) will provide coverage at 24, 70, and 160 micron, the bulk of MIPS-detected objects may only have accurate photometry in the shorter wavelength bands due to the confusion noise. Therefore, we have explored the potential for constraining the total infrared (IR) fluxes of distant galaxies with solely the 24 micron flux density, and for the combination of 24 micron and 70 micron data. We also discuss the inherent systematic uncertainties in making these transitions. Under the assumption that distant star-forming galaxies have IR spectral energy distributions (SEDs) that are represented somewhere in the local Universe, the 24 micron data (plus optical and X-ray data to allow redshift estimation and AGN rejection) constrains the total IR luminosity to within a factor of 2.5 for galaxies with 0.4 < z < 1.6. Incorporating the 70 micron data substantially improves this constraint by a factor < 6. Lastly, we argue that if the shape of the IR SED is known (or well constrained; e.g., because of high IR luminosity, or low ultraviolet/IR flux ratio), then the IR luminosity can be estimated with more certainty.Comment: 4 pages, 3 figures (2 in color). Accepted for Publication in the Astrophysical Journal Letters, 2002 Nov

Near-Infrared and Star-forming properties of Local Luminous Infrared Galaxies

We use HST NICMOS continuum and Pa-alpha observations to study the near-infrared and star-formation properties of a representative sample of 30 local (d ~ 35-75Mpc) luminous infrared galaxies (LIRGs, infrared 8-1000um luminosities of L_IR=11-11.9[Lsun]). The data provide spatial resolutions of 25-50pc and cover the central ~3.3-7.1kpc regions of these galaxies. About half of the LIRGs show compact (~1-2kpc) Pa-alpha emission with a high surface brightness in the form of nuclear emission, rings, and mini-spirals. The rest of the sample show Pa-alpha emission along the disk and the spiral arms extending over scales of 3-7kpc and larger. About half of the sample contains HII regions with H-alpha luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24um and hydrogen recombination (extinction-corrected Pa-alpha) luminosity for these LIRGs, and the HII regions in the central part of M51. This relation holds over more than four decades in luminosity suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of Kennicutt (1998), we derive an empirical calibration of the SFR in terms of the monochromatic 24um luminosity that can be used for luminous, dusty galaxies.Comment: Accepted for publication in ApJ. Contact first author for high qualitity version of figure

MIPS: The Multiband Imaging Photometer for SIRTF

The Multiband Imaging Photometer for SIRTF (MIPS) is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700μm spectral region. It will use high performance photoconductive detectors from 3 to 200μm with integrating JFET amplifiers. From 200 to 700μm, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution