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 $R_{ext}=375$ 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 $kT=6.7_{-0.9}^{+1.3}$ keV, and a iron abundance $Z_{Fe}=0.41_{-0.26}^{+0.29}{Z}_{F{e}_{\odot }}$ (error bars correspond to 1 $\ddot{I}ƒ$). We fit the background-subtracted surface brightness profile with a single ${\hat{I}}^2$-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 $\hat{I}›$CDM cosmology.

42 pages, 12 Figures, ApJ in press