The 400d Galaxy Cluster Survey weak lensing programme

III. Evidence for consistent WL and X-ray masses at z ≈ 0.5^⋆

¹ Department of PhysicsDurham University, South Road, Durham DH1 3LE, UK
e-mail: holger.israel@durham.ac.uk
² Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
³ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville VA 22904, USA
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Gard0en Street, Cambridge MA 02138, USA

Received: 17 October 2013
Accepted: 11 February 2014

Abstract

Context. Scaling properties of galaxy cluster observables with cluster mass provide central insights into the processes shaping clusters. Calibrating proxies for cluster mass that are relatively cheap to observe will moreover be crucial to harvest the cosmological information available from the number and growth of clusters with upcoming surveys like eROSITA and Euclid. The recent Planck results led to suggestions that X-ray masses might be biased low by ~40%, more than previously considered.

Aims. We aim to extend knowledge of the weak lensing – X-ray mass scaling towards lower masses (as low as 1 × 10¹⁴M_⊙) in a sample representative of the z ~ 0.4–0.5 population. Thus, we extend the direct calibration of cluster mass estimates to higher redshifts.

Methods. We investigate the scaling behaviour of MMT/Megacam weak lensing (WL) masses for 8 clusters at 0.39 ≤ z ≤ 0.80 as part of the 400d WL programme with hydrostatic Chandra X-ray masses as well as those based on the proxies, e.g. Y_X = T_XM_gas.

Results. Overall, we find good agreement between WL and X-ray masses, with different mass bias estimators all consistent with zero. When subdividing the sample into a low-mass and a high-mass subsample, we find the high-mass subsample to show no significant mass bias while for the low-mass subsample, there is a bias towards overestimated X-ray masses at the ~2σ level for some mass proxies. The overall scatter in the mass-mass scaling relations is surprisingly low. Investigating possible causes, we find that neither the greater range in WL than in X-ray masses nor the small scatter can be traced back to the parameter settings in the WL analysis.

Conclusions. We do not find evidence for a strong (~40%) underestimate in the X-ray masses, as suggested to reconcile recent Planck cluster counts and cosmological constraints. For high-mass clusters, our measurements are consistent with other studies in the literature. The mass dependent bias, significant at ~2σ, may hint at a physically different cluster population (less relaxed clusters with more substructure and mergers); or it may be due to small number statistics. Further studies of low-mass high-z lensing clusters will elucidate their mass scaling behaviour.