IR COLORS AND SIZES OF FAINT GALAXIES

We present J and Ks band galaxy counts down to J=24 and Ks=22.5 obtained with the new infrared im- ager/spectrometer, SOFI, at the ESO New Technology Tele- scope. The co-addition of short, dithered, images led to a total exposure time of 256 and 624 minutes respectively, over an area of 20 arcmin 2 centered on the NTT Deep Field. The to- tal number of sources with S/N> 5 is 1569 in the J sample and 1025 in the Ks-selected sample. These are the largest samples currently available at these depths. A dlogN/dm relation with slope of 0:36 in J and 0:38 in Ks is found with no evident sign of a decline at the magnitude limit. The observed surface density of "small" sources is much lower than "large" ones at bright magnitudes and rises more steeply than the large sources to fainter magnitudes. Fainter than J 22:5 and Ks 21:5, small sources dominate the number counts. Galaxies get redder in J-K down to J 20 and Ks 19. At fainter magnitudes, the median color becomes bluer with an accompanying increase in the compactness of the galaxies. We show that the blue, small sources which dominate the faint IR counts are not compati- ble with a high redshift ( z> 1) population. On the contrary, the observed color and compactness trends, together with the absence of a turnover at faint magnitudes and the dominance of small sources, can be naturally explained by an increasing contribution of sub-L galaxies when going to fainter apparent magnitudes. Such evidence strongly supports the existence of a steeply rising ( 1) faint end of the local infrared luminos- ity function of galaxies - at least for luminosities L< 0:01L.