The unusually high dark matter concentration of the galaxy group NGC
  1600
  We investigate the properties of the dark matter (DM) halo surrounding the
nearby galaxy group NGC 1600. Through the use of deep (252 ks) Chandra
observations and 64.3 ks of XMM-Newton observations, we construct surface
brightness profiles in multiple energy bands in order to perform hydrostatic
equilibrium analysis of the hot plasma within NGC 1600. Regardless of the DM
model profile assumed, we measure a halo concentration (c$_{200}$) that is an
extreme, positive outlier of the $\Lambda$CDM c$_{200}$-M$_{200}$ relation. For
a typical NFW DM profile, we measure c$_{200}\!=\!26.7\pm1.4$ and
M$_{200}\!=\!(2.0\pm0.2)\times10^{13}$ M$_\odot$; assuming a similar halo mass,
the average concentration expected is c$_{200}=6-7$ for the theoretical
$\Lambda$CDM c-M relation. Such a high concentration is similar to that of
well-known fossil groups MRK 1216 and NGC 6482. While NGC 1600 exhibits some
properties of a fossil group, it fails to meet the X-ray luminosity threshold
of L$_X>5\times10^{41}$ erg s$^{-1}$. Whether or not it is considered a fossil
group, the high concentration value makes it part of a select group of galaxy
groups.
