Deprojection of X-ray data in galaxy clusters: confronting simulations with observations

Abstract

Numerical simulations with varying realism indicate an emergent principle -- multiphase condensation and large cavity power occur when the ratio of the cooling time to the free-fall time ($t_{\mathrm{cool}}/t_{\mathrm{ff}}$) falls below a threshold value close to 10. Observations indeed show cool-core signatures when this ratio falls below 20-30, but the prevalence of cores with $t_{\mathrm{cool}}/t_{\mathrm{ff}}$ ratio below 10 is rare as compared to simulations. In X-ray observations, we obtain projected spectra from which we have to infer radial gas density and temperature profiles. Using idealized models of X-ray cavities and multiphase gas in the core and 3-D hydro jet-ICM simulations, we quantify the biases introduced by deprojection based on the assumption of spherical symmetry in determining $t_{\mathrm{cool}}/t_{\mathrm{ff}}$. We show that while the used methods are able to recover the $t_{\mathrm{cool}}/t_{\mathrm{ff}}$ ratio for relaxed clusters, they have an uncertainty of a factor of $2-3$ in systems containing large cavities ($\gtrsim 20$ kpc). We also show that the mass estimates from these methods, in the absence of X-ray spectra close to the virial radius, suffer from a degeneracy between the virial mass ($M_{200}$) and the concentration parameter ($c$) in the form of $M_{200}{c}^2\approx$ constant. Additionally, lack of soft-X-ray ($\lesssim 0.5$ keV) coverage and poor spatial resolution make us overestimate min($t_{\mathrm{cool}}/t_{\mathrm{ff}}$) by a factor of few in clusters with min($t_{\mathrm{cool}}/t_{\mathrm{ff}}$) $\lesssim 5$. This bias can largely explain the lack of cool-core clusters with min($t_{\mathrm{cool}}/t_{\mathrm{ff}}$) $\lesssim 5$.

Accepted for publication in MNRAS. Comments are welcome

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