Chandra Deep Observation of XDCP J0044.0-2033, a Massive Galaxy Cluster at z > 1.5

Abstract

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 <SUB>ext</SUB> = 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<SUP>+1.3</SUP><SUB>-0.9</SUB> keV, and a iron abundance Z<SUB>Fe</SUB> = 0.41<SUB>-0.26</SUB><SUP>+0.29</SUP>Z<SUB>Fe_☉ </SUB> (error bars correspond to 1σ). We fit the background-subtracted surface brightness profile with a single β-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 <SUB>ext</SUB> = 375 kpc to be M <SUB>ICM</SUB>(r < R <SUB>ext</SUB>) = (1.48 ± 0.20) × 10<SUP>13</SUP> M <SUB>☉</SUB>. Under the assumption of hydrostatic equilibrium and assuming isothermality within R <SUB>ext</SUB>, the total mass is M<SUB>2500</SUB>= 1.23<SUB>-0.27</SUB><SUP>+0.46</SUP> × 10 <SUP>14</SUP> M_☉ for R<SUB>2500</SUB> = 240<SUB>-20</SUB><SUP>+30</SUP> kpc. Extrapolating the profile at radii larger than the extraction radius R <SUB>ext</SUB> we find M<SUB>500</SUB> = 3.2<SUB>-0.6</SUB><SUP>+0.9</SUP> × 10 <SUP>14</SUP> M_☉ for R<SUB>500</SUB> = 562<SUB>-37</SUB><SUP>+50</SUP> kpc. This analysis establishes the existence of virialized, massive galaxy clusters at redshift z ~ 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 ΛCDM cosmology.