RCS2 J232727.6-020437: An Efficient Cosmic Telescope at $z=0.6986$

Abstract

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 ($\hat{I}¼\gtrsim 2$) source plane area of $\sim 0.7$ arcmin${}^2$ at $z=7$, RCS2 J232727.6-020437 has a lensing efficiency comparable to the Hubble Frontier Fields clusters. We discover four highly magnified $z\sim 7$ 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 $0.5L_{\star }$. One candidate is detected at $>10\ddot{I}ƒ$ 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 $5-14\times 10^{-19}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}$ ($5\ddot{I}ƒ$).

14 pages, 9 figures, submitted to ApJ on 3/06/2015