Chandra and XMM-Newton observations of the merging cluster of galaxies PLCK G036.7+14.9

We present Chandra and XMM-Newton observations of PLCK G036.7+14.9 from the Chandra–Planck Legacy Program. The high resolution X-ray observations reveal two close (∼72″ = 193 kpc in projection) subclusters, G036N and G036S, which were not resolved by previous ROSAT, optical, or recent Planck observations. We perform detailed imaging and spectral analyses and use a simplified model to study the kinematics of this system. The basic picture is that PLCK G036.7+14.9 is undergoing a major merger (mass ratio close to unity) between the two massive subclusters, with the merger largely along the line of sight (∼80° between the merger axis and the plane of the sky from the simplified model) and probably at an early stage (less than ∼0.4–0.7 Gyr since the merger began). G036N hosts a small (∼27 kpc), moderate cool core (cooling time $t_{cool}\sim 2.6$–4.7 Gyr), while G036S has at most a very weak cool core ($t_{cool}\sim 5.7$–10.3 Gyr) in the central ∼40 kpc region. The difference in core cooling times is unlikely to be caused by the ongoing merger disrupting a pre-existing cool core in G036S. G036N also hosts an unresolved radio source in the center, which may be heating the gas if the radio source is extended. The total mass of the whole cluster determined from XMM-Newton is $\sim (5.9-8.0)\times {10}^{14}{M}_{\odot }$, and is $\sim (6.7-9.9)\times {10}^{14}{M}_{\odot }$ from Chandra. The Planck derived mass, $\sim (5.1-6.0)\times {10}^{14}{M}_{\odot }$, is higher than the X-ray measured mass of either subcluster, but is lower than the X-ray measured mass of the whole cluster, due to the fact that Planck does not resolve PLCK G036.7+14.9 into subclusters and interprets it as a single cluster. This mass discrepancy could induce significant bias to the mass function if such previously unresolved systems are common in the Planck cluster sample. High resolution X-ray observations are necessary to identify the fraction of such systems and correct such a bias for the purpose of precision cosmological studies.