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

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6. Conclusions

  1. We have identified the most luminous infrared galaxies in an unbiased sample of 313 reliable extragalactic IRAS FSC sources with [FORMULA]. Our method is based on the bivariate B-60 µm luminosity function of infrared galaxies, which implies that the most luminous objects have the highest values of R (as defined by Eq. (1)), and have optical counterparts that are so faint that they cannot be reliably identified (or are undetected) in typical Schmidt surveys. Using optical spectroscopy, we have systematically identified the optical counterparts of all of the 6 sources in our 60 µm sample that were too faint in [FORMULA] to be reliably identified on the UKST plates. Our results confirm that this method selects the galaxies with the largest values of R, so that these galaxies are indeed the 6 most luminous infrared galaxies in our sample. Five of these are non-hyperluminous ULIGs with a mean redshift of 0.45, higher than any previously known non-hyperluminous ULIG; the remaining source is a HyLIG at [FORMULA].

  2. The HyLIG in our sample ([FORMULA]) contains an AGN, as shown by the presence of [[FORMULA]] emission. Hence all infrared-selected HyLIGs discovered so far unambiguously show the presence of AGNs. In contrast, none of the non-hyperluminous ULIGs in our sample show evidence for the presence of AGNs, and such evidence is rare among non-hyperluminous ULIGs in general.

  3. Our method is robust against the effects of gravitational lensing if the optical and infrared magnification factors are similar. Under this assumption this method may be useful for selecting candidate gravitationally lensed sources by comparing an intrinsic luminosity (estimated from R) with the apparent luminosity (calculated from [FORMULA] and z).

  4. Our survey constitutes an unbiased, sparse (approximately 1 in 8) survey of infrared galaxies with [FORMULA] over a 1079deg2 area, and the results allow an estimate of the number density of HyLIGs at [FORMULA] of approximately 7[FORMULA]10-3deg-2, with considerable uncertainty due to the small numbers involved. Compared to the local luminosity function of infrared galaxies, this estimate indicates substantial evolution at the highest luminosities, except in the unlikely case that the HyLIG found in our sparse survey is the only HyLIG at [FORMULA] in the entire 1079deg2 survey area.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999