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Photometric redshifts in the SWIRE survey

Rowan-Robinson, Michael and Babbedge, Tom and Oliver, Seb and Trichas, Markos and Berta, Stefano and Lonsdale, Carol and Smith, Gene and Shupe, David L. and Surace, Jason and Arnouts, Stephane and Ilbert, Olivier and Le Fèvre, Olivier and Afonso-Luis, Alejandro and Pérez-Fournon, Ismael and Hatziminaoglou, Evanthia and Polletta, Mari and Farrah, Duncan and Vaccari, Mattia (2008) Photometric redshifts in the SWIRE survey. Monthly Notices of the Royal Astronomical Society, 386 (2). pp. 697-714. ISSN 0035-8711.

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We present the SWIRE Photometric Redshift Catalogue 1 025 119 redshifts of unprecedented reliability and of accuracy comparable with or better than previous work. Our methodology is based on fixed galaxy and quasi-stellar object templates applied to data at 0.36–4.5 μm, and on a set of four infrared emission templates fitted to infrared excess data at 3.6–170 μm. The galaxy templates are initially empirical, but are given greater physical validity by fitting star formation histories to them, which also allows us to estimate stellar masses. The code involves two passes through the data, to try to optimize recognition of active galactic nucleus (AGN) dust tori. A few carefully justified priors are used and are the key to supression of outliers. Extinction, A_V , is allowed as a free parameter. The full reduced χ^2_ν (z) distribution is given for each source, so the full error distribution can be used, and aliases investigated. We use a set of 5982 spectroscopic redshifts, taken from the literature and from our own spectroscopic surveys, to analyse the performance of our method as a function of the number of photometric bands used in the solution and the reduced χ^2_ν . For seven photometric bands (5 optical + 3.6, 4.5 μm), the rms value of (z_(phot)−z_(spec)/(1 +z_(spec) is 3.5 per cent, and the percentage of catastrophic outliers [defined as >15 per cent error in (1 +z)], is ∼1 per cent. These rms values are comparable with the best achieved in other studies, and the outlier fraction is significantly better. The inclusion of the 3.6- and 4.5-μm IRAC bands is crucial in supression of outliers. We discuss the redshift distributions at 3.6 and 24 μm. In individual fields, structure in the redshift distribution corresponds to clusters which can be seen in the spectroscopic redshift distribution, so the photometric redshifts are a powerful tool for large-scale structure studies. 10 per cent of sources in the SWIRE photometric redshift catalogue have z > 2, and 4 per cent have z > 3, so this catalogue is a huge resource for high-redshift galaxies. A key parameter for understanding the evolutionary status of infrared galaxies is L_(ir)/L_(opt) . For cirrus galaxies this is a measure of the mean extinction in the interstellar medium of the galaxy. There is a population of ultraluminous galaxies with cool dust and we have shown SEDs for some of the reliable examples. For starbursts, we estimate the specific star formation rate, φ_*/M_* . Although the very highest values of this ratio tend to be associated with Arp220 starbursts, by no means all ultraluminous galaxies are. We discuss an interesting population of galaxies with elliptical-like spectral energy distributions in the optical and luminous starbursts in the infrared. For dust tori around type 1 AGN, L_(tor)/L_(opt) is a measure of the torus covering factor and we deduce a mean covering factor of 40 per cent. Our infrared templates also allow us to estimate dust masses for all galaxies with an infrared excess.

Item Type:Article
Related URLs:
URLURL TypeDescription
Oliver, Seb0000-0001-7862-1032
Berta, Stefano0000-0002-0320-1532
Lonsdale, Carol0000-0003-0898-406X
Shupe, David L.0000-0003-4401-0430
Surace, Jason0000-0001-7291-0087
Ilbert, Olivier0000-0002-7303-4397
Le Fèvre, Olivier0000-0001-5891-2596
Polletta, Mari0000-0001-7411-5386
Farrah, Duncan0000-0003-1748-2010
Vaccari, Mattia0000-0002-6748-0577
Additional Information:© 2008 The Authors. Journal compilation © 2009 RAS. Accepted 2008 February 14. Received 2008 February 13; in original form 2007 August 10. Published Online: 10 April 2008. Analysis of the VVDS area was based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de L'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. The work is based in part on data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada–France–Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS. We are grateful for financial support from NASA and from PPARC/STFC. We thank Max Pettini for helpful comments.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Particle Physics and Astronomy Research Council (PPARC)UNSPECIFIED
Science and Technology Facilities Council (STFC)UNSPECIFIED
Subject Keywords:stars: formation, galaxies: evolution, galaxies: starburst, cosmology: observations, infrared: galaxies.
Issue or Number:2
Record Number:CaltechAUTHORS:20090723-082907868
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14647
Deposited By: Ruth Sustaita
Deposited On:12 Aug 2009 21:32
Last Modified:30 Sep 2020 18:11

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