Reversal of Fortune: Confirmation of an Increasing Star
  Formation-Density Relation in a Cluster at z=1.62
  We measure the rest-frame colors (dust-corrected), infrared luminosities,
star formation rates, and stellar masses of 92 galaxies in a Spitzer-selected
cluster at z=1.62. By fitting spectral energy distributions (SEDs) to 10-band
photometry (0.4 micron < lambda(obs) <8 micron) and measuring 24 micron fluxes
for the 12 spectroscopically confirmed and 80 photometrically selected members,
we discover an exceptionally high level of star formation in the cluster core
of ~1700 Msun/yr per Mpc^2. The cluster galaxies define a strong blue sequence
in (U-V) color and span a range in color. We identify 17 members with
L(IR)>10^(11) Lsun, and these IR luminous members follow the same trend of
increasing star formation with stellar mass that is observed in the field at
z~2. Using rates derived from both the 24 micron imaging and SED fitting, we
find that the relative fraction of star-forming members triples from the lowest
to highest galaxy density regions, e.g. the IR luminous fraction increases from
~8% at Sigma~10 gal per Mpc^2 to ~25% at Sigma>100 gal per Mpc^2. The observed
increase is a reversal of the well-documented trend at z<1 and signals that we
have reached the epoch when massive cluster galaxies are still forming a
substantial fraction of their stars.
