The contribution of faint blue galaxies to the submillimetre counts and background

Observations in the submillimetre (submm) waveband have recently revealed a new population of luminous sources. These are proposed to lie at high reds hift and to be optically faint because of their high intrinsic dust obscura tion. The presence of dust has been previously invoked in optical galaxy co unt models which use the Bruzual & Chariot evolution models with an exponen tial tau = 9 Gyr star formation rate (SFR) for spirals, and these fit the c ount data well from U to K. We now show that by using either a 1/lambda or Calzetti absorption law for the dust and re-distributing the evolved spiral galaxy ultraviolet (UV) radiation into the far-infrared (FIR), these model s can account for all of the 'faint' (less than or equal to1 mJy) 850-mum g alaxy counts, but fail to fit 'bright' (greater than or equal to2 mJy) sour ces, indicating that another explanation for the submm counts may apply at brighter fluxes, e.g., quasi-stellar objects (QSOs) or ultraluminous infrar ed galaxies (ULIRGs). We find that the main contribution to the faint, subm m number counts is in the redshift range 0.5 < z < 3, peaking at z = 1.8. T he above model, using either dust law, can also explain a significant propo rtion of the extragalactic background at 850 mum, as well as producing a re asonable fit to the bright 60-mum IRAS counts.