* Astronomy

Title: RCS2 J232727.6-020437: An Efficient Cosmic Telescope at z=0.6986
Author: Austin Hoag, Marua Bradac, Kuang-Han Huang, Russell E. Ryan Jr., Keren Sharon, Tim Schrabback, Kasper B. Schmidt, Benjamin Cain, Anthony H. Gonzalez, Hendrik Hildebrandt, Joannah L. Hinz, Brian C. Lemaux, Anja von der Linden, Lori M. Lubin, Tommaso Treu, Dennis Zaritsky

We present a detailed gravitational lens model of the galaxy cluster RCS2 J232727.6-020437. Due to cosmological dimming of cluster members and ICL, its high redshift (z=0.6986) makes it ideal for studying background galaxies. Using new ACS and WFC3/IR HST data, we identify 16 multiple images. From MOSFIRE follow up, we identify a strong emission line in the spectrum of one multiple image, likely confirming the redshift of that system to z=2.083. With a highly magnified ( \gtrsim 2) source plane area of ~0.7 arcmin2 at z=7, RCS2 J232727.6-020437 has a lensing efficiency comparable to the Hubble Frontier Fields clusters. We discover four highly magnified z~7 candidate Lyman-break galaxies behind the cluster, one of which may be multiply-imaged. Correcting for magnification, we find that all four galaxies are fainter than 0.5L_*. One candidate is detected at >10sigma in both Spitzer/IRAC [3.6] and [4.5] channels. A spectroscopic follow-up with MOSFIRE does not result in the detection of the Lyman-alpha emission line from any of the four candidates. From the MOSFIRE spectra we place median upper limits on the Lyman-alpha flux of 5 - 14 x 10^-19ergs^-1cm^-2 (5sigma).

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