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

TL;DR

This study investigates biases in X-ray deprojection methods used to infer the cooling-to-free-fall time ratio in galaxy clusters, highlighting uncertainties and their impact on understanding cool-core prevalence.

Contribution

It quantifies the biases introduced by spherical deprojection assumptions and observational limitations, improving interpretation of cluster core properties.

Findings

Deprojection methods recover $t_{cool}/t_{ff}$ for relaxed clusters within a factor of 2-3.

Mass estimates suffer from a degeneracy between virial mass and concentration parameter.

Poor spectral coverage and resolution lead to overestimation of $t_{cool}/t_{ff}$ in certain clusters.

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 ($t_{\rm cool}/t_{\rm ff}$) 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 $t_{\rm cool}/t_{\rm ff}$ 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 $t_{\rm cool}/t_{\rm ff}$. We show that while the used methods are able to recover the $t_{\rm cool}/t_{\rm ff}$ 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($t_{\rm cool}/t_{\rm ff}$) by a factor of few in clusters with min($t_{\rm cool}/t_{\rm ff}$) $\lesssim 5$. This bias can largely explain the lack of cool-core clusters with min($t_{\rm cool}/t_{\rm ff}$) $\lesssim 5$.