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Register a new userFIRBACK: III. Catalog, Source counts, and cosmological implications of the 170 micron ISO Deep survey
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The FIRBACK (Far Infrared BACKground) survey is one of the deepest imaging surveys carried out at 170 microns with ISOPHOT onboard ISO, and is aimed at the study of the structure of the Cosmic Far Infrared Background. This paper provides the analysis of resolved sources. After a validated process of data reduction and calibration, we perform intensive simulations to optimize the source extraction, measure the confusion noise (sigma_c = 45 mJy), and give the photometric and astrometric accuracies. 196 galaxies with flux S > 3 sigma_c are detected in the area of 3.89 square degrees. Counts of sources with flux S > 4 sigma_c present a steep slope of 3.3 +/- 0.6 on a differential logN-logS plot between 180 and 500 mJy. As a consequence, the confusion level is high and will impact dramatically on future IR deep surveys. This strong evolution, compared with a slope of 2.5 from Euclidian geometry, is in line with models implying a strongly evolving Luminous Infrared Galaxy population. The resolved sources account for less than 10% of the Cosmic Infrared Background at 170 microns, which is expected to be resolved into sources in the 1 to 10 mJy range.
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FIRBACK is a one of the deepest surveys performed at 170 microns with ISOPHOT onboard ISO, and is aimed at the study of cosmic far infrared background sources. About 300 galaxies are detected in an area of four square degrees, and source counts present a strong slope of 2.2 on an integral logN-logS plot, which cannot be due to cosmological evolution if no K-correction is present. The resolved sources account for less than 10% of the Cosmic Infrared Background at 170 microns. In order to understand the nature of the sources contributing to the CIB, and to explain deep source counts at other wavelengths, we have developed a phenomenological model, which constrains in a simple way the luminosity function evolution with redshift, and fits all the existing deep source counts from the mid-infrared to the submillimetre range. Images, materials and papers available on the FIRBACK web: http://wwwfirback.ias.u-psud.fr wwwfirback.ias.u-psud.fr
We present a re--analysis of the European Large Area ISO Survey (ELAIS) 90mum observations carried out with ISOPHOT, an instrument on board the ESAs Infrared Space Observatory (ISO). With more than 12 sq. deg., the ELAIS survey is the largest area covered by ISO in a single program and is about one order of magnitude deeper than the IRAS 100mum survey. The data analysis is presented and was mainly performed with the Phot Interactive Analysis software but using the pairwise method of Stickel et al. (2003) for signal processing from ERD (Edited Raw Data) to SCP (Signal per Chopper Plateau). The ELAIS 90mum catalogue contains 229 reliable sources with fluxes larger than 70 mJy and is available at www.blackwell-synergy.com. Number counts are presented and show an excess above the no-evolution model prediction. This confirms the strong evolution detected at shorter(15mum) and longer (170mum) wavelengths in other ISO surveys. The ELAIS counts are in agreement with previous works at 90mum and in particular with the deeper counts extracted from the Lockman hole observations. Comparison with recent evolutionary models show that the models of Franceschini et al. and Guiderdoni et al. which includes a heavily-extinguished population of galaxies give the best fit to the data. Deeper observations are nevertheless required to better discriminate between the model predictions in the far-infrared and are scheduled with the Spitzer Space Telescope which already started operating and will also be performed by ASTRO-F.
We present Keck spectroscopy and UKIRT near-IR imaging observations of two 170micron-selected sources from the ISO-FIRBACK survey which have faint counterparts in the optical, and r-K~5. Both sources were expected to lie at z>1 based on their far-infrared, submillimeter and radio fluxes, assuming a similar spectral energy distribution to the local ultra-luminous infrared galaxy (ULIRG) Arp220. However, our spectroscopy indicates that the redshifts of these galaxies are z<1: z=0.91 for FN1-64 and z=0.45 for FN1-40. While the bolometric luminosities of both galaxies are similar to Arp220, it appears that the dust emission in these systems has a characteristic temperature of 30K much cooler than the ~50K seen in Arp220. Neither optical spectrum shows evidence of AGN activity. If these galaxies are characteristic of the optically faint FIRBACK population, then evolutionary models of the far-infrared background must include a substantial population of cold, luminous galaxies. These galaxies provide an important intermediate comparison between the local luminous IR galaxies, and the high redshift submillimeter-selected galaxies, for which there is very little information available.
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 micron imaging from Spitzer surveys, which include approximately 50,000 sources to an 80% completeness of 60 uJy. The 24 micron counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 - 4 mJy, and converge below 0.3 mJy. The 24 micron counts exceed expectations from non-evolving models by a factor >10 at 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 micron counts from ISO. 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 uJy, we derive a lower limit on the 24 micron 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 uJy contribute 0.8 {+0.9/-0.4} nW m-2 sr-1 to the background, which provides an estimate of the total 24 micron background of 2.7 {+1.1/-0.7} nW m-2 sr-1.
We present preliminary source counts at 6.7um and 15um from the Preliminary Analysis of the European Large Area ISO survey, with limiting flux densities of ~2mJy at 15um & ~1mJy at 6.7um. We separate the stellar contribution from the extragalactic using identifications with APM sources made with the likelihood ratio technique. We quantify the completeness & reliability of our source extraction using (a) repeated observations over small areas, (b) cross-IDs with stars of known spectral type, (c) detections of the PSF wings around bright sources, (d) comparison with independent algorithms. Flux calibration at 15um was performed using stellar IDs; the calibration does not agree with the pre-flight estimates, probably due to effects of detector hysteresis and photometric aperture correction. The 6.7um extragalactic counts are broadly reproduced in the Pearson & Rowan-Robinson model, but the Franceschini et al. (1997) model underpredicts the observed source density by ~0.5-1 dex, though the photometry at 6.7um is still preliminary. At 15um the extragalactic counts are in excellent agreement with the predictions of the Pearson & Rowan-Robinson (1996), Franceschini et al. (1994), Guiderdoni et al. (1997) and the evolving models of Xu et al. (1998), over 7 orders of magnitude in 15um flux density. The counts agree with other estimates from the ISOCAM instrument at overlapping flux densities (Elbaz et al. 1999), provided a consistent flux calibration is used. Luminosity evolution at a rate of (1+z)^3, incorporating mid-IR spectral features, provides a better fit to the 15um differential counts than (1+z)^4 density evolution. No-evolution models are excluded, and implying that below around 10mJy at 15um the source counts become dominated by an evolving cosmological population of dust-shrouded starbursts and/or active galaxies.
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