rmed at z = 7.73 by O15. Normally such objects would
be selected as Y -band dropouts but, given Y -band observations
are not yet available over the full CANDELS
field, an alternative selection criterion was adopted that
takes advantage of red IRAC [3.6]-[4.5] colors, indicating
prominent [O III]+Hβ emission within the 4.5 µm band
(see also Labb´e et al. 2013; Smit et al. 2015). Of the objects
listed by RB15, EGSY8p7 is the only one for which
Y-band data is not available. As a result, its photometric
redshift derived from HST data alone is fairly uncertain
(5.6 < zphot < 9.2; Fig. 5 of RB15) but including its
IRAC [3.6]-[4.5] color of 0.76±0.14 (and blue JH140-[3.6]
color) narrows the range to zphot = 8.57+0.22
−0.43.
We observed EGSY8p7, the highest-redshift candidate
in RB15’s list, on June 10 and 11 2015 with MOSFIRE
on the Keck 1 telescope. Observations in the J-band
spanned the wavelength range 11530˚A < λ < 13520˚A
using an AB dithering pattern of ±1.2500 along the slit
with individual frames of 120s. The slit masks on the
two nights differed by 120◦
in orientation both with a
slit width of 0.7
00. In each mask we allocated one slit
to a nearby star to monitor changes in seeing, transparency,
and possible positional drifts. Conditions were
clear throughout, with an average seeing of 0.6000 for the
first night and 0.7600 for the second night. The total
nightly exposure times were 158 minutes and 128 minutes.
Excluding exposures where the seeing was significantly
(& 1σ) worse than average, the useful exposure
time comprises all frames from the first night and 80% of
those acquired on the second night (where the conditions
were less stable) culminating in a total exposure-time of
4.33 hours. Calibrations were obtained via long-slit ob-
4 Zitrin et al.
servations of standard A0V stars.