ST ScI Preprint #1379

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The Unusual Infrared Object HDF-N J123656.3+621322

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS5-26555.

Partially based on data obtained at the W. M. Keck Observatory, operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and made possible by the generous financial support of the W. M. Keck Foundation.

Authors: Mark Dickinson,^1,2 Christopher Hanley,¹ Richard Elston,^2,3 Peter R. Eisenhardt,^2,4 S. A. Stanford,^2,5,6 Kurt L. Adelberger,⁷ Alice Shapley,⁷ Charles C. Steidel,⁷ Casey Papovich,⁸ Alexander S. Szalay,⁸ Matthew A. Bershady,^2,9 Christopher J. Conselice,⁹ Henry C. Ferguson,¹ Andrew S. Fruchter¹

We describe an object in the Hubble Deep Field North with very unusual near-infrared properties. It is readily visible in Hubble Space Telescope NICMOS images at 1.6m and from the ground at 2.2m, but it is undetected (with S/N 2) in very deep WFPC2 and NICMOS data from 0.3 to 1.1m. The f flux density drops by a factor 8.3 (97.7% confidence) from 1.6 to 1.1m. The object is compact but may be slightly resolved in the NICMOS 1.6m image. In a low-resolution, near-infrared spectrogram, we find a possible emission line at 1.643m, but a reobservation at higher spectral resolution failed to confirm the line, leaving its reality in doubt. We consider various hypotheses for the nature of this object. Its colors are unlike those of known Galactic stars, except perhaps the most extreme carbon stars or Mira variables with thick circumstellar dust shells. It does not appear to be possible to explain its spectral energy distribution as that of a normal galaxy at any redshift without additional opacity from either dust or intergalactic neutral hydrogen. The colors can be matched by those of a dusty galaxy at z 2, by a maximally old elliptical galaxy at z 3 (perhaps with some additional reddening), or by an object at z 10 whose optical and 1.1m light have been suppressed by the intergalactic medium. Under the latter hypothesis, if the luminosity results from stars and not an AGN, the object would resemble a classical, unobscured protogalaxy, with a star formation rate 100 M yr^-1. Such UV-bright objects are evidently rare at 2 z 12.5, however, with a space density several hundred times lower than that of present-day L^* galaxies.

Status:: Appeared in: The Astronomical Journal
Affiliations:

1) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
2) Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.
3) Department of Astronomy, University of Florida, Gainesville, FL 32611
4) MS 169-327, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109
5) Physics Department, University of California, Davis, CA 95616
6) Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratories, Livermore, CA 94550
7) Palomar Observatory, Caltech 105-24, Pasadena, CA 91125
8) Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218
9) Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706

toolan@stsci.edu
Last updated: January 22, 2001