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

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1. Introduction

One of the most fundamental results of the IRAS survey was the discovery of a class of galaxies that emit the bulk of their energy at far-infrared wavelengths and that have 8-1000 µm luminosities [FORMULA] (for H0 = 75kms-1 Mpc-1 and [FORMULA], as assumed throughout this paper). As shown by Soifer et al. (1986, 1987), these ultraluminous infrared galaxies (ULIGs) are a very significant population in the local universe, dominating the high luminosity end of the local luminosity function, and outnumbering local optically selected quasars by a factor of at least 2 (see Sanders & Mirabel 1996 and references therein). More recently, a number of objects with [FORMULA] have been identified, for which the term hyperluminous infrared galaxies (HyLIGs) has been proposed. The first object of this type to be discovered in an infrared-selected sample was [FORMULA] at [FORMULA] Rowan-Robinson et al. 1991a. Although this object is now known to be gravitationally lensed (Broadhurst & Lehár 1995; Close et al. 1995; Serjeant et al. 1995; Eisenhardt et al. 1996), it is a HyLIG even after the gravitational magnification has been accounted for Downes et al. 1995. Furthermore, of the now more than 30 known objects in this class, most do not show any evidence of gravitational lensing.

The nature of the energy source powering the high bolometric luminosities of ULIGs and HyLIGs is a subject of intense debate (see e.g., Rowan-Robinson 1996 for a recent review). Since ULIGs and HyLIGs have luminosities in the range of the most powerful active galactic nuclei (AGNs), it has been proposed that they are powered by dust-embedded AGNs. Alternatively, the far-infrared (FIR) luminosity may be provided by a burst of intense star formation, with implied star formation rates [FORMULA][FORMULA]102-103[FORMULA]yr-1, or even higher for HyLIGs. At the high end of this interval, a starburst lasting a typical [FORMULA] would produce a stellar mass of [FORMULA], comparable to the luminous mass of a typical galaxy. Therefore, if powered only by star formation, the HyLIGs form stars at the rate expected for high redshift galaxies in their formation process, undergoing an initial starburst that builds up the bulk of their stellar population. AGNs are known to be present in most of the known HyLIGs, but since the fraction of the FIR luminosity that they provide is unknown, the presence of intense star formation is not ruled out. Indeed, several lines of evidence indicate the presence of vigorous star formation in [FORMULA] Rowan-Robinson et al. 1993; Kroker et al. 1996; Green & Rowan-Robinson 1996, in addition to an embedded AGN Elston et al. 1994; Soifer et al. 1995; Goodrich et al. 1996. Furthermore, since most of the known HyLIGs have been found in surveys of AGNs, the ubiquity of AGNs in known HyLIGs may be entirely a selection effect. Thus an unbiased survey for HyLIGs based on a sample of faint far-IR sources would be valuable for a reliable assessment of the nature of these objects. In addition, since HyLIGs can be observed to very significant redshifts, a determination of their number densities will strongly constrain the evolution of IRAS galaxies at these redshifts, as far as the most luminous part of the luminosity function is concerned.

In this paper we present a survey for the most luminous galaxies detected by IRAS in a [FORMULA] area of sky. We discuss the sample selection process in Sect. 2 and our observation and reduction procedures in Sect. 3. The results are presented and discussed in Sects. 4 and 5, while our conclusions are summarized in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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