A far-infrared survey at the North Galactic Pole I: Nearby star-forming
  galaxies and the effect of confused sources on source counts
  We present follow-up observations of the far-infrared (FIR) sources at 90,
150 and 180 mu detected as part of the ISOPHOT EBL project, which has recently
measured the absolute surface brightness of the cosmic infrared background
radiation (CIRB) for the first time independently from COBE data. We have
observed the fields at the North Galactic Pole region in the optical and
near-IR, and complement these data with SDSS photometry, and spectroscopy where
available, and present identifications of the 25 FIR sources which reach down
to ~150 mJy in all three ISOPHOT bands. Identifications are done by means of
full spectral energy density fitting to all sources in the FIR error circle
areas. Approximately 80 per cent are identified as star-forming or
star-bursting galaxies at z<0.3. We also find that more than half of the
counterparts have disturbed morphologies, with signs of past or present
interactions. However, only 20 per cent of all the sources are uniquely matched
with a single galaxy -- 40 per cent are blends of two or more of these nearby
star-forming galaxies, while another 20 per cent are likely blends of nearby
and fainter galaxies. The final 20 per cent are likely to be more luminous IR
galaxies at higher redshifts. The blended sources have an effect on the FIR
source counts. In particular, taking into account realistic confusion or
blending of sources, the differential FIR counts move down by a factor of ~1.5
and steepen in the 100 to 400 mJy range.
