The European Large Area ISO Survey — II. Mid-infrared extragalactic source counts

Monthly Notices of the Royal Astronomical Society, Aug 2000

We present preliminary source counts at 6.7 and 15 μm from the preliminary analysis of the European Large Area ISO Survey, with limiting flux densities of ∼2 mJy at 15 μm and ∼1 mJy at 6.7 μm. We separate the stellar contribution from the extragalactic using identifications with automated plate measurement sources made with the likelihood ratio technique. We quantify the completeness and reliability of our source extraction using (a) repeated observations over small areas, (b) cross-identifications with stars of known spectral type, (c) detections of the point spread function wings around bright sources and (d) comparison with independent algorithms. Flux calibration at 15 μm was performed using stellar identifications; the calibration does not agree with the pre-flight estimates, probably due to effects of detector hysteresis and photometric aperture correction. The 6.7-μm extragalactic counts are broadly reproduced in the Pearson & Rowan-Robinson model, but the Franceschini et al. model underpredicts the observed source density by ∼0.5–1 dex, although the photometry at 6.7 μm is still preliminary. At 15 μm the extragalactic counts are in excellent agreement with the predictions of the Pearson & Rowan-Robinson, Franceschini et al., Guiderdoni et al. models and the evolving models of Xu et al., over seven orders of magnitude in 15-μm flux density. The counts agree with other estimates from the ISOCAM instrument at overlapping flux densities, provided a consistent flux calibration is used. Luminosity evolution at a rate of (1+z)3, incorporating mid-infrared spectral features, provides a better fit to the 15-μm differential counts than (1+z)4 density evolution. No-evolution models are excluded, implying that below around 10 mJy at 15 μm the source counts become dominated by an evolving cosmological population of dust-shrouded starbursts and/or active galaxies.

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The European Large Area ISO Survey — II. Mid-infrared extragalactic source counts

Stephen Serjeant 6 w Seb Oliver 0 6 Michael Rowan-Robinson 6 Hans Crockett 6 Vasilis Missoulis 6 Tim Sumner 6 Carlotta Gruppioni 5 6 Robert G. Mann 6 8 Nick Eaton 6 David Elbaz 4 David L. Clements 2 11 Amanda Baker 4 11 Andreas Efstathiou 6 Catherine Cesarsky 4 Luigi Danese 1 Alberto Franceschini 10 Reinhardt Genzel 9 Andy Lawrence 8 Dietrich Lemke 13 Richard G. McMahon 12 George Miley 7 Jean-Loup Puget 2 Brigitte Rocca-Volmerange 3 0 Current address: Astronomy Centre, Physics and Astronomy Subject Group, School of CPES, University of Sussex , Falmer, Brighton BN1 9QJ 1 SISSA, International School for Advanced Studies , Via Beirut 2-4, 34014 Trieste , Italy 2 Institut d'Astrophysique Spatiale , Batiment 121 , Universite Paris XI , 91405 Orsay cedex , France 3 Institut d'Astrophysique de Paris , 98bis Boulevard Arago, F 75014 Paris , France 4 CEA / SACLAY , 91191 Gif sur Yvette cedex , France 5 Osservatorio Astronomico di Bologna , via Ranzani 1, 40127 Bologna , Italy 6 Astrophysics Group, Blackett Laboratory, Imperial College of Science Technology & Medicine , Prince Consort Rd, London SW7 2BZ 7 Leiden University , PO Box 9513, NL-2300 RA Leiden , the Netherlands 8 Institute for Astronomy, University of Edinburgh , Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ 9 Max-Planck-Institut fu r extraterrestrische Physik , PO Box 1603, 85740 Garching , Germany 10 Dipartimento di Astronomia, Universita di Padova , Vicolo Osservatorio 5, I-35122 Padova , Italy 11 Dept of Physics & Astronomy, Cardiff University , PO Box 913, Cardiff CF24 3YB 12 Institute of Astronomy , The Observatories, Madingley Road, Cambridge CB3 0HA 13 Max-Planck-Institut fu r Astronomie , Ko nigstuhl 17, D-69117 Heidelburg , Germany A B S T R A C T We present preliminary source counts at 6.7 and 15 mm from the preliminary analysis of the European Large Area ISO Survey, with limiting flux densities of ,2 mJy at 15 mm and ,1 mJy at 6.7 mm. We separate the stellar contribution from the extragalactic using identifications with automated plate measurement sources made with the likelihood ratio technique. We quantify the completeness and reliability of our source extraction using (a) repeated observations over small areas, (b) cross-identifications with stars of known spectral type, (c) detections of the point spread function wings around bright sources and (d) comparison with independent algorithms. Flux calibration at 15 mm was performed using stellar identifications; the calibration does not agree with the pre-flight estimates, probably due to effects of detector hysteresis and photometric aperture correction. The 6.7-mm extragalactic counts are broadly reproduced in the Pearson & Rowan-Robinson model, but the Franceschini et al. model underpredicts the observed source density by ,0.5-1 dex, although the photometry at 6.7 mm is still preliminary. At 15 mm the extragalactic counts are in excellent agreement with the predictions of the Pearson & Rowan-Robinson, Franceschini et al., Guiderdoni et al. models and the evolving models of Xu et al., over seven orders of magnitude in 15-mm flux density. The counts agree with other estimates from the ISOCAM instrument at overlapping flux densities, provided a consistent flux calibration is used. Luminosity evolution at a rate of 1 1 z 3; incorporating mid-infrared spectral features, provides a better fit to the 15-mm differential counts than 1 1 z 4 density evolution. Noevolution models are excluded, implying that below around 10 mJy at 15 mm the source counts become dominated by an evolving cosmological population of dust-shrouded starbursts and/or active galaxies. I N T R O D U C T I O N The IRAS mission enjoyed huge successes, including the sensational discoveries of ultra- and hyper-luminous galaxies and of an enormous population of evolving starbursts. However, the survey had several drawbacks. For example, the bright limiting flux densities restricted the samples to low redshifts z & 0:3 for all but a few ultraluminous objects. Also, only ,1000 galaxies were detected at 12 mm over the whole sky. These deficiencies restricted the study of IR-luminous galaxies at all redshifts. The Infrared Space Observatory (ISO) offered , 1000 improvements in sensitivity in the mid-infrared over IRAS, and the large allocations of guaranteed and discretionary time for deep surveys on ISO will greatly improve on the IRAS surveys in the mid-infrared. For instance, ISO observations of the northern Hubble Deep Field (Serjeant et al. 1997, in preparation; Goldschimdt et al. 1997; Oliver et al. 1997; Aussel et al. 1999; Desert et al. 1999) reached the 15-mm confusion limit (,0.1 mJy) over 17 arcmin2, while the CAM-Deep and CAM-Shallow surveys (Elbaz et al. 1998a,b) were slightly less sensitive but had wider areal coverage (0.5 mJy over 0.3 deg2 and 0.8 mJy over 0.41 deg2). These have also been complemented by deep ISO photometry of selected high-z galaxies (e.g. Flores et al. 1999). The European Large Area ISO S (...truncated)


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Stephen Serjeant, Seb Oliver, Michael Rowan-Robinson, Hans Crockett, Vasilis Missoulis, Tim Sumner, Carlotta Gruppioni, Robert G. Mann, Nick Eaton, David Elbaz, David L. Clements, Amanda Baker, Andreas Efstathiou, Catherine Cesarsky, Luigi Danese, Alberto Franceschini, Reinhardt Genzel, Andy Lawrence, Dietrich Lemke, Richard G. McMahon, George Miley, Jean-Loup Puget, Brigitte Rocca-Volmerange. The European Large Area ISO Survey — II. Mid-infrared extragalactic source counts, Monthly Notices of the Royal Astronomical Society, 2000, pp. 768-778, 316/4, DOI: 10.1046/j.1365-8711.2000.03551.x