Astron. Astrophys. 349, 751-764 (1999)

IR colors and sizes of faint galaxies ^*

P. Saracco ¹, S. D'Odorico ², A. Moorwood ², A. Buzzoni ¹, J.-G. Cuby ³ and C. Lidman <sup>3

1</sup> Osservatorio Astronomico di Brera, via E. Bianchi 46, I-22055 Merate (LC), Italy
² European Southern Observatory, ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
³ European Southern Observatory, ESO, Casilla 19001, Santiago 19, Chile

Received 19 April 1999 / Accepted 3 August 1999

Abstract

We present J and Ks band galaxy counts down to J=24 and Ks=22.5 obtained with the new infrared imager/spectrometer, SOFI, at the ESO New Technology Telescope. The co-addition of short, dithered, images led to a total exposure time of 256 and 624 minutes respectively, over an area of arcmin² centered on the NTT Deep Field. The total number of sources with S/N 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 in J and 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 and Ks, small sources dominate the number counts. Galaxies get redder in J-K down to J and Ks. 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 compatible with a high redshift () 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- galaxies when going to fainter apparent magnitudes. Such evidence strongly supports the existence of a steeply rising () faint end of the local infrared luminosity function of galaxies - at least for luminosities .

Key words: cosmology: observations galaxies: evolution galaxies: luminosity function, mass function galaxies: photometry galaxies: statistics infrared: galaxies

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: Paolo Saracco

© European Southern Observatory (ESO) 1999

Online publication: September 13, 1999

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