Chandra deep observation of XDCP J0044.0-2033, a massive galaxy cluster
  at z>1.5
  We report the analysis of the Chandra observation of XDCP J0044.0-2033, a
massive, distant (z=1.579) galaxy cluster discovered in the XDCP survey. The
total exposure time of 380 ks with Chandra ACIS-S provides the deepest X-ray
observation currently achieved on a massive, high redshift cluster. Extended
emission from the Intra Cluster Medium (ICM) is detected at a very high
significance level (S/N~20) on a circular region with a 44" radius,
corresponding to $R_{ext}=375$ kpc at the cluster redshift. We perform an X-ray
spectral fit of the ICM emission modeling the spectrum with a
single-temperature thermal mekal model. Our analysis provides a global
temperature $kT=6.7^{+1.3}_{-0.9}$ keV, and a iron abundance $Z_{Fe} =
0.41_{-0.26}^{+0.29}Z_{Fe_\odot}$ (error bars correspond to 1 $\sigma$). We fit
the background-subtracted surface brightness profile with a single
$\beta$-model out to 44", finding a rather flat profile with no hints of a cool
core. We derive the deprojected electron density profile and compute the ICM
mass within the extraction radius $R_{ext}=375$ kpc to be $M_{ICM}(r<R_{ext}) =
(1.48 \pm 0.20) \times 10^{13} M_\odot$. Under the assumption of hydrostatic
equilibrium and assuming isothermality within $R_{ext}$, the total mass is
$M_{2500}= 1.23_{-0.27}^{+0.46} \times 10 ^{14} M_\odot$ for $R_{2500} =
240_{-20}^{+30}$ kpc. Extrapolating the profile at radii larger than the
extraction radius $R_{ext}$ we find $M_{500} = 3.2_{-0.6}^{+0.9} \times 10
^{14}M_\odot$ for $R_{500} = 562_{-37}^{+50}$ kpc. This analysis establishes
the existence of virialized, massive galaxy clusters at redshift $z\sim 1.6$,
paving the way to the investigation of the progenitors of the most massive
clusters today. Given its mass and the XDCP survey volume, XDCP J0044.0-2033
does not create significant tension with the WMAP-7 $\Lambda$CDM cosmology.
