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H. Spinrad (UCB), D. Stern (IPAC/JPL), A. Dey (NOAO), S. Dawson (UCB), W. van Breugel, W. de Vries, M. Reulend (IGPP/LLNL)
We report the serendipitous detection in high--resolution spectroscopy of an emission--line galaxy (hereafter ES1) with a strong, asymmetric Ly\alpha emission line at z = 5.189. The spectroscopic observations of ES1 were obtained on 25 February 2001 with the Echelle Spectrograph and Imager on the Keck II telescope. The echellogram has a resolution of ~75 km/s; the position angle from the nearby target galaxy (D16 in the HDF North West Flanking Field) was set at 150\circ. As the spectrograph configuration only covered 20 square arcseconds, the detection of ES1 was highly providential; we estimate the likelihood of such discoveries to be less than one chance in 100. ES1 lies in the HDF North West Flanking Field, for which a single--orbit I814 HST Wide Field Planetary Camera image is available. The faint galaxy responsible for the Ly\alpha emission line is quite compact: the full width at half maximum on its I814 flanking field image is only 0\stackrel{\prime\prime}{.}3. Hence, the emitting region and continuum of the galaxy are observable over only ~2 kpc. One very intriguing aspect of this unlikely discovery is the spectral extent of the red wing of the Ly\alpha line. A 3--parameter Gaussian fit suggests an outflow velocity of over 400 km/s. This young galaxy may therefore present evidence for both a high star formation rate and a starburst--driven galactic wind.
The author(s) of this abstract have provided an email address for comments about the abstract: spinrad@bigz.berkeley.edu