Abstract

We discuss the observational properties of a remarkably faint triply imaged galaxy revealed in a deep z^'-band Advanced Camera for Surveys observation of the lensing cluster A2218 (z=0.175). A well-constrained mass model for the cluster, which incorporates the outcome of recent Keck spectroscopic campaigns, suggests that the triple system arises via a high-redshift (z>6) source viewed at high magnification (~=25 times). Optical and infrared photometry from Hubble Space Telescope and the Keck Observatory confirms the lensing hypothesis and suggests that a significant discontinuity occurs in the spectral energy distribution within the wavelength interval 9250-9850 Å. If this break is associated with Gunn-Peterson absorption from neutral hydrogen, a redshift of 6.6<z<7.1 is inferred. Deep Keck spectroscopy conducted using both optical and infrared spectrographs fails to reveal any prominent emission lines in this region. However, an infrared stellar continuum is detected whose decline below 9800 Å suggests a spectroscopic redshift toward the upper end of the range constrained photometrically, i.e., z~=7. Regardless of the precise redshift, the source is remarkably compact (<~1 h^-1₇₀ kpc) and faint (z_F850LP=28.0) yet is undergoing vigorous star formation at a rate ~=2.6 M_solar yr^-1. An intriguing property is the steep slope of the ultraviolet continuum implied by the photometry, which may suggest that the source is representative of an early population of galaxies responsible for cosmic reionization. Independent verification of these results is highly desirable, but our attempts highlight the difficulty of studying such sources with present facilities and the challenges faced in pushing back the frontiers of the observable universe beyond z~6.5.

Using data obtained with the Hubble Space Telescope, operated by AURA, Inc., for NASA and the W. M. Keck Observatory on Mauna Kea, Hawaii. The W.M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W.M. Keck Foundation.