The quantity Y _ { X } , the product of the X-ray temperature T _ { X } and gas mass M _ { g } , has recently been proposed as a robust low-scatter mass indicator for galaxy clusters .
Using precise measurements from XMM-Newton data of a sample of 10 relaxed nearby clusters , spanning a Y _ { X } range of 10 ^ { 13 } – 10 ^ { 15 } M _ { \odot } keV , we investigate the M _ { 500 } – Y _ { X } relation .
The M _ { 500 } – Y _ { X } data exhibit a power law relation with slope \alpha = 0.548 \pm 0.027 , close to the self-similar value ( 3/5 ) and independent of the mass range considered .
However , the normalisation is \sim 20 \% below the prediction from numerical simulations including cooling and galaxy feedback .
We discuss two effects that could contribute to the normalisation offset : an underestimate of the true mass due to the HE assumption used in X-ray mass estimates , and an underestimate of the hot gas mass fraction in the simulations .
A comparison of the functional form and scatter of the relations between various observables and the mass suggest that Y _ { X } may indeed be a better mass proxy than T _ { X } or M _ { g, 500 } .