Focusing Cosmic Telescopes: Exploring Redshift z~5-6 Galaxies with the
  Bullet Cluster 1E0657-56
  The gravitational potential of clusters of galaxies acts as a cosmic
telescope allowing us to find and study galaxies at fainter limits than
otherwise possible and thus probe closer to the epoch of formation of the first
galaxies. We use the Bullet Cluster 1E0657-56 (z = 0.296) as a case study,
because its high mass and merging configuration makes it one of the most
efficient cosmic telescopes we know. We develop a new algorithm to reconstruct
the gravitational potential of the Bullet Cluster, based on a non-uniform
adaptive grid, combining strong and weak gravitational lensing data derived
from deep HST/ACS F606W-F775W-F850LP and ground-based imaging. We exploit this
improved mass map to study z~5-6 Lyman Break Galaxies (LBGs), which we detect
as dropouts. One of the LBGs is multiply imaged, providing a geometric
confirmation of its high redshift, and is used to further improve our mass
model. We quantify the uncertainties in the magnification map reconstruction in
the intrinsic source luminosity, and in the volume surveyed, and show that they
are negligible compared to sample variance when determining the luminosity
function of high-redshift galaxies. With shallower and comparable magnitude
limits to HUDF and GOODS, the Bullet cluster observations, after correcting for
magnification, probe deeper into the luminosity function of the high redshift
galaxies than GOODS and only slightly shallower than HUDF. We conclude that
accurately focused cosmic telescopes are the most efficient way to sample the
bright end of the luminosity function of high redshift galaxies and - in case
they are multiply imaged - confirm their redshifts.
