NICMOS Imaging of the Dusty Microjansky Radio Source VLA J123642+621331 at z = 4.424

Abstract

We present the discovery of a radio galaxy at a likely redshift of z=4.424 in one of the flanking fields of the Hubble Deep Field (HDF). Radio observations with the Very Large Array and MERLIN centered on the HDF yielded a complete sample of microjansky radio sources, of which about 20% have no optical counterpart to I<=25 mag. In this Letter, we address the possible nature of one of these sources through deep Hubble Space Telescope near-infrared camera and multiobject spectrometer (NICMOS) images in the F110W (J₁₁₀) and F160W (H₁₆₀) filters. VLA J123642+621331 has a single emission line at 6595 Å, which we identify with Lyα at z=4.424. We argue that this faint (H₁₆₀=23.9 mag), compact (r_e~=0.2"), red (I₈₁₄-K=2.0) object is most likely a dusty, star-forming galaxy with an embedded active nucleus.

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., under NASA contract NAS5-26555 and with the W. M. Keck Observatory, which is operated as a scientific partnership among the University of California, the California Institute of Technology, and NASA, made possible by the generous financial support of the W. M. Keck Foundation.