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Astron. Astrophys. 342, 665-670 (1999)


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Discovery of distant high luminosity infrared galaxies

Paul P. van der Werf 1, D.L. Clements 2,3, P.A. Shaver 2 and M.R.S. Hawkins 4

1 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
2 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
3 Institut d'Astrophysique Spatiale, Université Paris XI, Batiment 121, F-91405 Orsay Cedex, France
4 Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, Scotland

Received 14 April 1997 / Accepted 18 September 1998

Abstract

We have developed a method for selecting the most luminous galaxies detected by IRAS based on their extreme values of R, the ratio of 60 µm and B-band luminosity. These objects have optical counterparts that are close to or below the limits of Schmidt surveys. We have tested our method on a [FORMULA] region of sky, where we have selected a sample of IRAS sources with 60 µm flux densities greater than [FORMULA], corresponding to a redshift limit [FORMULA] for objects with far-IR luminosities of [FORMULA]. Optical identifications for these were obtained from the UK Schmidt Telescope plates, using the likelihood ratio method. Optical spectroscopy has been carried out to reliably identify and measure the redshifts of six objects with very faint optical counterparts, which are the only objects with [FORMULA] in the sample. One object is a hyperluminous infrared galaxy (HyLIG) at [FORMULA]. Of the remaining, fainter objects, five are ultraluminous infrared galaxies (ULIGs) with a mean redshift of 0.45, higher than the highest known redshift of any non-hyperluminous ULIG prior to this study. High excitation lines reveal the presence of an active nucleus in the HyLIG, just as in the other known infrared-selected HyLIGs. In contrast, no high excitation lines are found in the non-hyperluminous ULIGs. We discuss the implications of our results for the number density of HyLIGs at [FORMULA] and for the evolution of the infrared galaxy population out to this redshift, and show that substantial evolution is indicated. Our selection method is robust against the presence of gravitational lensing if the optical and infrared magnification factors are similar, and we suggest a way of using it to select candidate gravitationally lensed infrared galaxies.

Key words: infrared: galaxies – galaxies: starburst – galaxies: evolution – galaxies: distances and redshifts

Send offprint requests to: Paul van der Werf

Correspondence to: pvdwerf@strw.leidenuniv.nl

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999

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