2. Sample selection

Strauss et al. (1990) selected 5014 objects from the IRAS database(IRAS Point Source Catalog, Version 2, 1988) according to the criteria:

1. 
2. f₆₀² > f₁₂ f₂₅
   a color criterion distinguishing galaxies from objects in the Galaxy.
3. Galactic latitude .

Strauss et al. (1992) then went on to publish the redshifts of the 2658 objects which were galaxies (here after the 2 Jy samples).

Our VLIRG sample is a subset of the 2 Jy sample. Considering the observatory site, instruments and possible observation times, we selected galaxies according to the following criteria:

1. 
   
2. 
   
   
3. .
   

Here the magnitudes are from the 2 Jy catalog. These criteria guarantee S/N ratios good enough for spectroscopic classification. 73 of 2 Jy-catalogue VLIRGs met these criteria.

On account of the low S/N at and for some of sources in 2 Jy sample, we used Dennefeld et al.'s (1986) formula for calculating the infrared flux, between and :

where, S₆₀ and S₁₀₀ are the fluxes at and respectively.

The infrared luminosity is therefore:

for each object, where D is the distance from the Galactic center.