A fossil galaxy cluster: an X-ray and optical study of RX J1416.4+2315

We present a detailed X-ray and optical study of a distant fossil system RX J1416.4+2315(z ~ 0.137), combining Chandra and XMM-Newton observations, optical photometry and spectroscopy. X-ray emitting hot gas imaged by both the Chandra and XMM-Newton shows a globally relaxed spatial distribution, supporting the idea that fossil groups are old galaxy systems with no recent mergers. However, the diffuse X-ray emission shows signs of asymmetries in the core of the system. With a mean gas temperature of ~4.0 keV and total gravitational mass of 3.1 × 10¹⁴M_solar, within the virial radius, this is better described as a fossil galaxy cluster rather than a fossil group. The temperature profile shows no sign of a significant cooler core despite a cooling time dropping to 5 Gyr within the resolved core. We find a mass concentration parameter c₂₀₀ ~ 11 which is relatively high for a cluster of this mass, indicative of an early formation epoch.

Using the spectroscopically identified cluster members, we present the galaxy luminosity function for this fossil system. We measure the velocity dispersion of the galaxies to be ~700 km s^-1 based on 18 confirmed members. The dynamical mass is nearly twice the total gravitational mass derived from the X-ray analysis. The measured R-band mass-to-light ratio, within the virial radius, is ~440 M_solar/L_solar which is not unusual for clusters of galaxies. The central giant elliptical galaxy has discy isophotes and spectral features typical of elliptical galaxies.