Galaxy cluster thermodynamics at z=1.7: Resolved tSZ and X-ray analysis of CL J0459.6-4946

Abstract

Measurements of the entropy and temperature distribution of the intracluster medium (ICM) of local clusters have proven pivotal for the study of cluster astrophysics. Here, we extend these measurements to $z=1.7$ with a resolved analysis of the ICM in ACT/SPT-CL J0459.6-4946. By combining high-resolution thermal Sunyaev-Zeldovich measurements using new, deep ALMA Band 1 and 3 observations, with archival X-ray data, we conclude that the cluster has (i) an ICM with a flat and relatively high ($\approx 200$ keV cm${}^2$) entropy distribution out to $\approx 0.3r_{500,\mathrm{c}}$, (ii) a hot core ($T_{\mathrm{e}}\simeq 14$ keV) with a declining temperature profile, (iii) a short effective path length along the line of sight of $l_{\mathrm{eff}}\simeq (0.75\pm 0.05)\times r_{500,\mathrm{c}}$, and (iv) a clumpiness factor $C=⟨n_{\mathrm{e}}^2⟩/⟨n_{\mathrm{e}}{⟩}^2\simeq 1.17\pm 0.05$ within $r_{500,\mathrm{c}}$. Its dynamic nature and high redshift suggest that the cluster is still thermalising its ICM, consistent with a relatively large hydrostatic mass bias of $b=1-M_{500,\mathrm{c},\mathrm{HE}}/M_{500,\mathrm{c},\mathrm{WL}}=0.27_{-0.67}^{+0.32}$ that we measure with respect to the weak lensing mass. Furthermore, a region with low entropy and high density to the south of the cluster centre is observed in the X-rays yet absent in the Compton-$y$ map, indicating pressure equilibrium; we interpret this as a sloshing cold front -- the first such detection at high redshifts. This case study demonstrates that $\approx 3$h-long ALMA observations, combined with comparatively "short" X-ray exposures ($\approx 122$ ks), can fully characterise the ICM at high redshift, opening a path towards the thermodynamics of young and distant galaxy clusters for larger samples.