An Extreme Starburst in Close Proximity to the Central Galaxy of a Rich
  Galaxy Cluster at z=1.7
  We have discovered an optically rich galaxy cluster at z=1.7089 with star
formation occurring in close proximity to the central galaxy. The system,
SpARCS104922.6+564032.5, was detected within the Spitzer Adaptation of the
red-sequence Cluster Survey, (SpARCS), and confirmed through Keck-MOSFIRE
spectroscopy. The rest-frame optical richness of Ngal(500kpc) = 30+/-8 implies
a total halo mass, within 500kpc, of ~3.8+/-1.2 x 10^14 Msun, comparable to
other clusters at or above this redshift. There is a wealth of ancillary data
available, including Canada-France-Hawaii Telescope optical, UKIRT-K,
Spitzer-IRAC/MIPS, and Herschel-SPIRE. This work adds submillimeter imaging
with the SCUBA2 camera on the James Clerk Maxwell Telescope and near-infrared
imaging with the Hubble Space Telescope (HST). The mid/far-infrared (M/FIR)
data detect an Ultra-luminous Infrared Galaxy spatially coincident with the
central galaxy, with LIR = 6.2+/-0.9 x 10^12 Lsun. The detection of polycyclic
aromatic hydrocarbons (PAHs) at z=1.7 in a Spitzer-IRS spectrum of the source
implies the FIR luminosity is dominated by star formation (an Active Galactic
Nucleus contribution of 20%) with a rate of ~860+/-30 Msun/yr. The optical
source corresponding to the IR emission is likely a chain of of > 10 individual
clumps arranged as "beads on a string" over a linear scale of 66 kpc. Its
morphology and proximity to the Brightest Cluster Galaxy imply a gas-rich
interaction at the center of the cluster triggered the star formation. This
system indicates that wet mergers may be an important process in forming the
stellar mass of BCGs at early times.
