Discovering the First Galaxies with a New Kind of Gravitational-Lens Telescope

Ann Zabludoff,


Observing the most distant galaxies, which are also the earliest galaxies, is key to understanding how the first stars and galaxies formed. Yet detecting these ultra-faint sources is extremely challenging. Attempts to do so require large allocations of telescope time. Some efforts employ gravitational lensing, in which a foreground massive cluster of galaxies magnifies the light of distant galaxies behind it. Lensing can brighten a background galaxy into detectability, and recent successes include Hubble’s spectacular detection of a redshift z =10.7 source (the CLASH project; Postman et al. 2012, Coe et al. 2012). Large magnifications also reduce the area of the distant sky that is observed, however, which lowers the chance of any background galaxy being lensed. To progress further, we must imagine a different kind of gravitational lens, where mass is optimally distributed in space to produce significant magnification over the largest possible area. My collaborators—Ken Wong, Mark Ammons, Decker French, Curtis McCully, Chuck Keeton, and David Hogg—and I are developing a new theoretical formalism and an extensive observational program to identify those directions in the sky most likely to produce detections of distant, faint galaxies.

The Most Powerful Telescopes

The best gravitational telescopes are lines-of-sight, or “beams,” whose integrated mass is large and ideally distributed to produce the highest étendue, or the greatest possible area in the source plane with significant magnification. Multiple cluster-scale lenses in a given beam can boost the lensing cross-section by up to about three times relative to the single-cluster lenses commonly used to detect the most distant galaxies (Wong et al. 2012a). This improvement arises from interactions among the lensing potentials, and it can translate into an order-of-magnitude increase over blank-field surveys in the number of intrinsically faint, z ~ 10 galaxy detections, depending on the (very uncertain) faint-end slope of the galaxy luminosity function at high z. It is precisely these special lines of sight that could provide real constraints on the faint-end slope and the properties of the earliest galaxies.