Chandra Observations of the Planck Early Sunyaev–Zeldovich Sample: A Reexamination of Masses and Mass Proxies

Using Chandra observations, we derive the Y _X proxy and associated total mass measurement, $M_{500}^{Y_X}$, for 147 clusters with z < 0.35 from the Planck early Sunyaev–Zeldovich catalog, and for 80 clusters with z < 0.22 from an X-ray flux-limited sample. We reextract the Planck Y _SZ measurements and obtain the corresponding mass proxy, $M_{500}^{SZ}$, from the full Planck mission maps, minimizing Malmquist bias due to observational scatter. The masses reextracted using the more precise X-ray position and characteristic size agree with the published PSZ2 values, but yield a significant reduction in the scatter (by a factor of two) in the $M_{500}^{SZ}$–$M_{500}^{Y_X}$ relation. The slope is 0.93 ± 0.03, and the median ratio, $M_{500}^{SZ}/M_{500}^{Y_X}=0.91\pm 0.01$, is within the expectations from known X-ray calibration systematics. Y _SZ/Y _X is 0.88 ± 0.02, in good agreement with predictions from cluster structure, and implying a low level of clumpiness. In agreement with the findings of the Planck Collaboration, the slope of the Y _SZ–$D_A^{-2}{Y}_X$ flux relation is significantly less than unity (0.89 ± 0.01). Using extensive simulations, we show that this result is not due to selection effects, intrinsic scatter, or covariance between quantities. We demonstrate analytically that changing the Y _SZ–Y _X relation from apparent flux to intrinsic properties results in a best-fit slope that is closer to unity and increases the dispersion about the relation. The redistribution resulting from this transformation implies that the best-fit parameters of the $M_{500}^{SZ}$–$M_{500}^{Y_X}$ relation will be sample-dependent.