  We have obtained deep optical spectroscopic data of the highest-redshift
cluster candidate (z ~ 1.4, CVB13) selected by Van Breukelen et al. (2006) in a
photometric optical/infrared catalogue of the Subaru XMM-Newton Deep Field. The
data, which comprise 104 targeted galaxies, were taken with the DEep Imaging
Multi-Object Spectrograph (DEIMOS) on the Keck 2 telescope and yielded 31
secure redshifts in the range 1.25 < z < 1.54 within a 7' x 4' field centred on
CVB13. Instead of one massive cluster at z = 1.4, we find evidence for three
projected structures at z = 1.40, z = 1.45, and z = 1.48. The most
statistically robust of these structures, at z = 1.454, has six
spectroscopically confirmed galaxies. Its total mass is estimated at > 10^14
M_sun and it may therefore be termed a cluster. There is an X-ray source at the
cluster position which is marginally spatially resolved but whose X-ray
spectrum is too hard to be thermal cluster emission. Its origin could be the
summed X-ray emission from active galaxies in, and projected onto, the cluster.
Serendipitously we have discovered a cluster at z = 1.28 with a mass of > 10^14
M_sun at the same position on the sky, comprising six spectroscopically
confirmed cluster galaxies and at least one additional radio source. The
selection of CVB13 for the cluster catalogue was evidently aided by the
superposition of other, presumably lower-mass, structures, whereas the single
cluster at z = 1.28 contained too few galaxies to be isolated by the same
algorithm. Given the complicated nature of such structures, caution must be
employed when measuring the mass function of putative high-redshift clusters
with photometric techniques alone.
