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Published August 20, 1995 | Published
Journal Article Open

Deep Optical Galaxy Counts with the Keck Telescope

Abstract

We present faint galaxy counts from deep VRI images obtained with the Keck Telescope. These images reach R ~ 27 in median seeing FWHM ~ 0."5-0."6, and we detect a integrated galaxy number density of 7 × 105 deg-2, equivalent to 3 × 10^(10) galaxies in the observable universe. In addition we present median galaxy colors as a function of magnitude; bluing trends are visible in all colors to R ~ 24.5. Fainter than R ~ 24.5, however, the typical V - R color becomes redder again, V - I remains constant, and R - I becomes yet bluer. These trends are consistent with the VRI count slopes, implying a decrease in the V slope at the faintest levels, which our data support. Taking advantage of our good seeing we also present median half-light radii for faint galaxies; these show a steady decline at fainter magnitudes, leading to an intrinsic half-light radius of ~0 2 for a typical R ~ 26 galaxy. Irrespective of the redshift distribution, the extremely high galaxy surface densities and their small intrinsic sizes are consistent with a scenario in which the majority of the very faint field population are dwarf galaxies or subgalactic units.

Additional Information

© 1995 The American Astronomical Society. Received 1995 April 28; accepted 1995 June 15. Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California. First, we thank Shri Kulkarni for his great generosity in allowing us to use these data. We acknowledge useful discussions and encouragement from Rebecca Bernstein, Roger Blandford, and Nigel Metcalfe. We also thank the referee, Richard Kron, for many helpful comments. Support via a NATO Advanced Fellowship and a Carnegie Fellowship (I. R. S.) and an NSF Graduate Fellowship (D. W. H.) is gratefully acknowledged. Finally, it is a pleasure to thank the W. M. Keck Foundation and its president, Howard B. Keck, for the generous grant that made the W. M. Keck Observatory possible.

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August 20, 2023
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