X-ray flux–mass relation for z ≳ 0.7 galaxy clusters

¹ Space Research Institute (IKI), Profsoyuznaya 84/32, Moscow 117997, Russia
² Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany
³ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany

^⋆ Corresponding author: lyskova@cosmos.ru

Received: 15 June 2025
Accepted: 5 August 2025

Abstract

We use a subsample of co-detections of the ACT and MaDCoWS cluster catalogs to verify the predicted relation between the observed X-ray flux F_X in the 0.5−2 keV band and the cluster mass M_500c for halos at z > 0.6 − 0.7. We modified this relation by introducing a correction coefficient, η, which is supposed to encapsulate factors associated with a particular method of flux estimation, the sample selection function, the definition of the cluster mass. We show that the X-ray flux, being the most basic X-ray observable, serves as a convenient and low-cost mass indicator for distant galaxy clusters with photometric or even missing redshifts (by setting z = 1) as long as it is known that z ≳ 0.6 − 0.7. The correction coefficient η is ≈0.8 if M^UPP_500c from the ACT-DR5 catalog are used as cluster masses, whereas η ≈ 1.1 if weak-lensing-calibrated masses M^Cal_500c are used instead.