Modelling the deep counts: luminosity evolution, dust and faint galaxies

In this paper we analyse the deep number counts problem, taking account ofnew observational and theoretical developments. First we show that the newBruzual and Charlot (1993) models allow a new class of spiral dominatedluminosity evolution (LE) model where significant amounts of the luminosityevolution needed to fit faint count data are due to spiral rather thanearly-type galaxies. Second we show that the inclusion of dust may be a vitalingredient for obtaining fits with any LE model. Third we compare the qualityof fit of both the spiral and early-type LE models, including dust, for a widevariety of observational data. We find that parameters can be found for both LEmodels which allow a good fit to all data with the exception of the faintestB>25 counts in the case of q0=0.5 cosmologies, where some luminosity dependentevolution may be needed (see also Metcalfe et al 1995). Otherwise both theseclasses of LE model, with the inclusion of dust, provide an excellentfoundation for understanding the B<25 galaxy counts and galaxy counts andredshift distributions in a variety of other wavebands.Comment: 11 pages, uuencoded compressed postscript file uncluding text and figures. Submitted for publication to MNRAS (main journal)