Keck Spectroscopy of Objects with Lens-like Morphologies in the Hubble Deep Field

Abstract

We present spectroscopy from the Keck telescope of three sets of objects in the Hubble Deep Field which have lens-like morphologies. In the case of J123641+621204, which is composed of four objects with similar colors and a mean separation of <= 0.8", we find at least two distinct components at redshifts of z=3.209 and z=3.220 which are separated by 0.5" spatially. Each of these components has narrow Ly-alpha emission, and possibly NV emission and SiIV and CIV in absorption or with a P-Cygni profile. The second case is J123652+621227, which has an arc-like feature offset by 1.8" to the southwest of a red elliptical-like galaxy, and a ``counterimage'' offset 1.4" on the opposite side. We tentatively find a single line at 5301 AA at the spatial position of the counterimage, and no corresponding emission line at the position of the arc. The colors of the counterimage are consistent with the identification of this line as Ly-alpha at z=3.36. The colors of the arc are different than those of the counterimage, and thus both the colors and spectra indicate that this object is unlikely to be a gravitational lens. For a third lensing candidate (J123656+621221), which is a blue arc offset by 0.9" from a red, elliptical-like galaxy, our spectroscopy does not clearly resolve the system spatially, complicating the interpretation of the spectrum. We discuss possible identifications of features in the spectrum, and find that gravitational lensing remains a possibility in this case. We conclude that the frequency of strong gravitational lensing by galaxies in the HDF appears to be very low. This result is difficult to reconcile with the introduction of a cosmological constant to account for the large number of faint blue galaxies via a large volume element at high redshift.