We introduce a galaxy cluster mass observable , \mu _ { \star } , based on the stellar masses of cluster members , and we present results for the Dark Energy Survey ( DES ) Year 1 observations .
Stellar masses are computed using a Bayesian Model Averaging method , and are validated for DES data using simulations and COSMOS data .
We show that \mu _ { \star } works as a promising mass proxy by comparing our predictions to X–ray measurements .
We measure the X–ray temperature– \mu _ { \star } relation for a total of 129 clusters matched between the wide–field DES Year 1 redMaPPer catalogue and Chandra and XMM archival observations , spanning the redshift range 0.1 < z < 0.7 .
For a scaling relation which is linear in logarithmic space , we find a slope of \alpha = 0.488 \pm 0.043 and a scatter in the X–ray temperature at fixed \mu _ { \star } of \sigma _ { { ln } T _ { X } | \mu _ { \star } } = 0.266 ^ { +0.019 } _ { -0.020 } for the joint sample .
By using the halo mass scaling relations of the X–ray temperature from the Weighing the Giants program , we further derive the \mu _ { \star } –conditioned scatter in mass , finding \sigma _ { { ln } M| \mu _ { \star } } = 0.26 ^ { +0.15 } _ { -0.10 } .
These results are competitive with well–established cluster mass proxies used for cosmological analyses , showing that \mu _ { \star } can be used as a reliable and physically motivated mass proxy to derive cosmological constraints .