A. Alonso-Herrero

THE MULTIBAND IMAGING PHOTOMETER FOR SPITZER (MIPS)

The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 ?m and measurements of spectral energy distributions between 52 and 100 ?m at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point-spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The 24 ?m array has excellent photometric properties, and measurements with rms relative errors of about 1% can be obtained. The two longer-wavelength arrays use detectors with poor photometric stability, but a system of onboard stimulators used for relative calibration, combined with a unique data pipeline, produce good photometry with rms relative errors of less than 10%.

DETERMINING STAR FORMATION RATES FOR INFRARED GALAXIES

We show that measures of star formation rates (SFRs) for infrared galaxies using either single-band 24 μm or extinction-corrected Paα luminosities are consistent in the total infrared luminosity = L(TIR) ~ 1010 L ☉ range. MIPS 24 μm photometry can yield SFRs accurately from this luminosity upward: SFR(M ☉ yr–1) = 7.8 × 10–10 L(24 μm, L ☉) from L(TIR) = 5× 109 L ☉ to 1011 L ☉ and SFR = 7.8 × 10–10 L(24 μm, L ☉)(7.76 × 10–11 L(24))0.048 for higher L(TIR). For galaxies with L(TIR) ≥ 1010 L ☉, these new expressions should provide SFRs to within 0.2 dex. For L(TIR) ≥ 1011 L ☉, we find that the SFR of infrared galaxies is significantly underestimated using extinction-corrected Paα (and presumably using any other optical or near-infrared recombination lines). As a part of this work, we constructed spectral energy distribution templates for eleven luminous and ultraluminous purely star forming infrared galaxies and over the spectral range 0.4 μm to 30 cm. We use these templates and the SINGS data to construct average templates from 5 μm to 30 cm for infrared galaxies with L(TIR) = 5× 109 to 1013 L ☉. All of these templates are made available online.

SPITZER VIEW ON THE EVOLUTION OF STAR-FORMING GALAXIES FROM z = 0 TO z ~ 3

We use a 24 ?m-selected sample containing more than 8000 sources to study the evolution of star-forming galaxies in the redshift range from z = 0 to z ~ 3. We obtain photometric redshifts for most of the sources in our survey using a method based on empirically built templates spanning from ultraviolet to mid-infrared wavelengths. The accuracy of these redshifts is better than 10% for 80% of the sample. The derived redshift distribution of the sources detected by our survey peaks at around z = 0.6-1.0 (the location of the peak being affected by cosmic variance) and decays monotonically from z ~ 1 to z ~ 3. We have fitted infrared luminosity functions in several redshift bins in the range 0 1011 L?) to the total SFR density increases steadily from z ~ 0 up to z ~ 2.5, forming at least half of the newly born stars by z ~ 1.5. Ultraluminous infrared galaxies (LTIR > 1012 L?) play a rapidly increasing role for z 1.3.

Infrared power-law galaxies in the chandra deep field-south: Active galactic nuclei and ultraluminous infrared galaxies

We investigate the nature of a sample of 92 Spitzer MIPS 24 � m–selected galaxies in the CDF-S, showing powerlaw–like emission in the Spitzer IRAC 3.6–8 � m bands. The main goal is to determine whether the galaxies not detectedinX-rays (47%ofthesample)arepartofthehypotheticalpopulationofobscuredAGNsnotdetectedevenin deep X-ray surveys. The majority of the IR power-law galaxies are ULIRGs at z > 1, and those with LIRG-like IR luminosities are usually detected in X-rays. The optical-to-IR SEDs of the X-ray–detected galaxies are almost equally divided between aBLAGN SED class (similar to anopticallyselected QSO) and an NLAGN SED (similar to the BLAGN SED but with an obscured UV/optical continuum). A small fraction of SEDs resemble warm ULIRGs (e.g., Mrk 231). Most galaxies not detected in X-rays have SEDs in the NLAGN+ULIRG class as they tend to be optically fainter and possibly more obscured. Moreover, the IR power-law galaxies have SEDs significantly different from those of high-z (zsp > 1) IR (24 � m) selected and optically bright (VVDS IAB � 24) star-forming galaxies whoseSEDsshow averyprominent stellar bumpat1.6 � m.ThegalaxiesdetectedinX-rays have2–8keVrest-frame luminosities typical ofAGNs. Thegalaxies notdetectedinX-rayshave global X-ray–to–mid-IR SED properties that make them good candidates to contain IR-bright X-ray–absorbed AGNs. If all these sources are actually obscured AGNs, we would observe a ratio of obscured to unobscured 24 � m–detected AGNs of 2:1, whereas models predict a ratio of up to 3:1. Additional studies using Spitzer to detect X-ray–quiet AGNs are likely to find more such obscured sources. Subject headings: galaxies: active — galaxies: high-redshift — infrared: galaxies — X-rays: galaxies Online material: color figuresWe investigate the nature of a sample of 92 Spitzer/MIPS 24 μm selected galaxies in the CDFS, showing power law-like emission in the Spitzer/IRAC 3.6– 8μm bands. The main goal is to determine whether the galaxies not detected in X-rays (47% of the sample) are part of the hypothetical population of obscured AGN not detected even in deep X-ray surveys. The majority of the IR powerlaw galaxies are ULIRGs at z > 1, and those with LIRG-like IR luminosities are usually detected in X-rays. The optical to IR spectral energy distributions (SEDs) of the X-ray detected galaxies are almost equally divided between a BLAGN SED class (similar to an optically selected QSO) and a NLAGN SED (similar to the BLAGN SED but with an obscured UV/optical continuum). A small fraction of SEDs resemble warm ULIRG galaxies (e.g., Mrk 231). Most galaxies not detected in X-rays have SEDs in the NLAGN+ULIRG class as they tend to be optically fainter, and possibly more obscured. Moreover, the IR powerlaw galaxies have SEDs significantly different from those of high-z (zsp > 1) IR (24 μm) selected and optically bright (VVDS IAB ≤ 24) star-forming galaxies Departamento de Astrof́ısica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, E28006 Madrid, Spain; e-mail: aalonso@damir.iem.csic.es Steward Observatory, The University of Arizona, 933 N. Cherry, Tucson, AZ 85721 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK Department of Astrophysics, Oxford University, Keble Rd, Oxford, OX1 3RH, UK Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 Columbia Astrophysics Laboratory, Columbia University Pupin Laboratories, 550 W. 120th St., Rm 1418, NY, 10027 Department of Astronomy and Astrophysics; The Pennsylvania State University; 525 Davey Lab; University Park, PA 16802 Institut d’Astrophysique Spatiale, bât 121, Université Paris Sud, F-91405 Orsay Cedex, France

The 24 Micron Source Counts in Deep Spitzer Space Telescope Surveys

Galaxy source counts in the infrared provide strong constraints on the evolution of the bolometric energy output from distant galaxy populations. We present the results from deep 24 μm imaging from Spitzer surveys, which include ≈5 × 10^4 sources to an 80% completeness of ≃ 60 μJy. The 24 μm counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 and 4 mJy, and converge below ~0.3 mJy. The 24 μm counts exceed expectations from nonevolving models by a factor of ≳10 at S_ν ~ 0.1 mJy. The peak in the differential number counts corresponds to a population of faint sources that is not expected from predictions based on 15 μm counts from the Infrared Space Observatory. We argue that this implies the existence of a previously undetected population of infrared-luminous galaxies at z ~ 1-3. Integrating the counts to 60 μJy, we derive a lower limit on the 24 μm background intensity of 1.9 ± 0.6 nW m^(-2) sr^(-1) of which the majority (~60%) stems from sources fainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60 μJy contribute 0.8^(+0.9)_(-0.4) nW m^(-2) sr^(-1) to the background, which provides an estimate of the total 24 μm background of 2.7^(+1.1)_(-0.7) nW m^(-2) sr^(-1).

The Anatomy of Star Formation in NGC 300

The Spitzer Space Telescope was used to study the mid- to far-infrared properties of NGC 300 and to compare dust emission to Hα to elucidate the heating of the interstellar medium (ISM) and the star formation cycle at scales smaller than 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 μm dust emission with respect to 24 μm dust and to H II regions traced by Hα light. The 8 μm emission highlights the rims of H II regions, and the 24 μm emission is more strongly peaked in star-forming regions than 8 μm. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 μm, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star-forming regions. When averaging over regions larger than ~1 kpc, the ratio of Hα to aromatic feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid- to far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.

Spitzer Power-Law Active Galactic Nucleus Candidates in the Chandra Deep Field-North

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

Polycyclic Aromatic Hydrocarbon Contribution to the Infrared Output Energy of the Universe at z 2

We present an updated phenomenological galaxy evolution model to fit the Spitzer 24, 70, and 160 μm number counts, as well as all the previous mid- and far-infrared observations. Only a minor change of the comoving luminosity density distribution in the previous model (Lagache, Dole, & Puget), combined with a slight modification of the starburst template spectra mainly between 12 and 30 μm, are required to fit all the data available. We show that the peak in the Spitzer Multiband Imaging Photometer 24 μm counts is dominated by galaxies with redshift between 1 and 2, with a nonnegligible contribution from the z ≥ 2 galaxies (~30% at S = 0.2 mJy). The very close agreement between the model and number counts at 15 and 24 μm strikingly implies that (1) the polycyclic aromatic hydrocarbon features remain prominent in the redshift band 0.5-2.5 and (2) the IR energy output has to be dominated by ~3 × 1011 L⊙ to ~3 × 1012 L⊙ galaxies from redshift 0.5 to 2.5. Combining Spitzer with Infrared Space Observatory deep cosmological surveys gives for the first time an unbiased view of the infrared universe from z = 0 to 2.5.

Near-infrared and star-forming properties of local luminous infrared galaxies

We use Hubble Space Telescope (HST) NICMOS continuum and Paα observations to study the near-infrared and star formation properties of a representative sample of 30 local (d ~ 35-75 Mpc) luminous infrared galaxies (LIRGs, infrared [8-1000 μm] luminosities of log L_IR = 11-11.9 L_☉). The data provide spatial resolutions of 25-50 pc and cover the central ~3.3-7.1 kpc regions of these galaxies. About half of the LIRGs show compact (~1-2 kpc) Paα emission with a high surface brightness in the form of nuclear emission, rings, and minispirals. The rest of the sample show Paα emission along the disk and the spiral arms extending over scales of 3-7 kpc and larger. About half of the sample contains H II regions with Hα luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24 μm and hydrogen recombination (extinction-corrected Paα) luminosity for these LIRGs, and the H II 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 R. Kennicutt, we derive an empirical calibration of the SFR in terms of the monochromatic 24 μm luminosity that can be used for luminous, dusty galaxies.

TORUS AND ACTIVE GALACTIC NUCLEUS PROPERTIES OF NEARBY SEYFERT GALAXIES: RESULTS FROM FITTING INFRARED SPECTRAL ENERGY DISTRIBUTIONS AND SPECTROSCOPY

We used the CLUMPY torus models and a Bayesian approach to fit the infrared spectral energy distributions and ground-based high angular resolution mid-infrared spectroscopy of 13 nearby Seyfert galaxies. This allowed us to put tight constraints on torus model parameters such as the viewing angle i, the radial thickness of the torus Y, the angular size of the cloud distribution σtorus, and the average number of clouds along radial equatorial rays N 0. We found that the viewing angle i is not the only parameter controlling the classification of a galaxy into type 1 or type 2. In principle, type 2s could be viewed at any viewing angle i as long as there is one cloud along the line of sight. A more relevant quantity for clumpy media is the probability for an active galactic nucleus (AGN) photon to escape unabsorbed. In our sample, type 1s have relatively high escape probabilities, P esc ~ 12%-44%, while type 2s, as expected, tend to have very low escape probabilities. Our fits also confirmed that the tori of Seyfert galaxies are compact with torus model radii in the range 1-6 pc. The scaling of the models to the data also provided the AGN bolometric luminosities L bol(AGN), which were found to be in good agreement with estimates from the literature. When we combined our sample of Seyfert galaxies with a sample of PG quasars from the literature to span a range of L bol(AGN) ~ 1043-1047 erg s–1, we found plausible evidence of the receding torus. That is, there is a tendency for the torus geometrical covering factor to be lower (f 2 ~ 0.1-0.3) at high AGN luminosities than at low AGN luminosities (f 2 ~ 0.9-1 at ~1043-1044 erg s–1). This is because at low AGN luminosities the tori appear to have wider angular sizes (larger σtorus) and more clouds along radial equatorial rays. We cannot, however, rule out the possibility that this is due to contamination by extended dust structures not associated with the dusty torus at low AGN luminosities, since most of these in our sample are hosted in highly inclined galaxies.