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Astron. Astrophys. 340, L10-L14 (1998)

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Letter to the Editor

An extremely red r1/4 galaxy in the test image of the Hubble Deep Field South *

T. Treu 12, M. Stiavelli ** 1, 2, A.R. Walker 3, R.E.Williams 2, S.A. Baum 2, G. Bernstein 4, B.S. Blacker 2, C.M. Carollo *** 5, S. Casertano 2, M.E. Dickinson 2, D.F. Demello 2, H.C. Ferguson 2, A.S. Fruchter 2, R.A. Lucas 2, J. MacKenty 2, P. Madau 2 and M. Postman 2

1 Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126, Pisa, Italy (e-mail: treu@cibs.sns.it; mstiavel@astro.sns.it)
2 Space Telescope Science Institute, 3700 San Martin Dr., 21218 MD, USA (e-mail: treu@stsci.edu; mstiavel@stsci.edu; wms@stsci.edu)
3 Cerro Tololo Inter-American Observatory, NOAO, Casilla 603, La Serena, Chile (e-mail: awalker@noao.edu)
4 University of Michigan, Department of Astronomy, 830 Dennison Building, Ann Arbor, MI 48109, USA
5 Johns Hopkins University, 3701 San Martin Dr., 21218 MD, USA

Received 26 August 1998 / Accepted 5 October 1998

Abstract

We report the serendipitous discovery of an extremely red object in the Hubble Deep Field South (HDFS) Test NICMOS (Near Infrared Camera and Multi Object Spectrograph) field of view. The object is resolved in the NICMOS image and has a light profile very well described by an [FORMULA] law with effective radius [FORMULA] and [FORMULA] magnitudes. In contrast, the galaxy is undetected in the R and I band ground based images taken at the CTIO 4 m Blanco Telescope, giving a lower limit to the color of [FORMULA] and [FORMULA] at the 95% confidence level. The colors of a range of synthetic galactic spectra are computed, showing that the object is likely to be an "old" elliptical galaxy at redshift [FORMULA]. Alternatively the colors can be reproduced by an "old" elliptical galaxy at somewhat lower redshift ([FORMULA]) with significant amount of dust, or by a younger galaxy at higher redshift. This object represents a very interesting target for future VLT observations.

Key words: galaxies: elliptical and lenticular, cD – infrared: galaxies – galaxies: formation – cosmology: observations – early universe

* Based on observations collected with the NASA/ESA HST, obtained at the STScI, which is operated by AURA, under NASA contract NAS5-26555 and at the CTIO, NOAO, which is operated by AURA, under cooperative agreement with the NSF.
** On assignment from the Space Science Department of the European Space Agency
*** Hubble Fellow

Send offprint requests to: T. Treu
Correspondence to: T. Treu

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© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998
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