An Extremely Red r^{1/4} Galaxy in the Test Image of the Hubble Deep Field South

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 r^{1/4} law with effective radius r_e=0."20+-0."05 and H_{AB}=21.7+-0.1 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 (R-H)_{AB}>3.9 and (I-H)_{AB}>3.5 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 z>~1.7. Alternatively the colors can be reproduced by an ``old'' elliptical galaxy at somewhat lower redshift (z>~1) with significant amount of dust, or by a younger galaxy at higher redshift. This object represents a very interesting target for future VLT observations.