RX J1548.9+0851, a fossil cluster?

TL;DR

This study investigates the fossil galaxy candidate RX J1548.9+0851 using spectroscopy and imaging to analyze its galaxy population, dynamics, and morphology, aiming to clarify whether such systems are a distinct class resulting from unique formation histories.

Contribution

The paper provides a detailed spectroscopic and morphological analysis of RX J1548.9+0851, expanding the sample of well-studied fossil systems and comparing its properties with existing fossil data.

Findings

Identified 54 group members within 1 Mpc, confirming the system's massiveness.

Found a velocity dispersion of 568 km/s and a mass of ~2.5 x 10^14 solar masses.

Discovered two central bright ellipticals with a significant magnitude gap.

Abstract

Fossil galaxy groups are spatially extended X-ray sources with X-ray luminosities above L_X,bol > 10^42 h_50^-2 ergs s^-1 and a central elliptical galaxy dominating the optical, the second-brightest galaxy being at least 2 magnitudes fainter in the R band. Whether these systems are a distinct class of objects resulting from exceptional formation and evolution histories is still unclear, mainly due to the small number of objects studied so far, mostly lacking spectroscopy of group members for group membership confirmation and a detailed kinematical analysis. To complement the scarce sample of spectroscopically studied fossils down to their faint galaxy populations, the fossil candidate RX J1548.9+0851 (z=0.072) is studied in this work. Our results are compared with existing data from fossils in the literature. We use ESO VLT VIMOS multi-object spectroscopy to determine redshifts of the faint galaxy population and study the luminosity-weighted dynamics and luminosity function of the system. The full-spectrum fitting package ULySS is used to determine ages and metallicities of group members. VIMOS imaging data are used to study the morphology of the central elliptical. We identify 40 group members spectroscopically within the central ~300 kpc of the system and find 31 additional redshifts from the literature, resulting in a total number of 54 spectroscopically confirmed group members within 1 Mpc. RX J1548.9+0851 is made up of two bright ellipticals in the central region with a magnitude gap of m_1,2 = 1.34 in the SDSS r' band leaving the definition of RX J1548.9+0851 being a fossil to the assumption of the virial radius. We find a luminosity-weighted velocity dispersion of 568 km s^-1 and a mass of ~2.5 x 10^14 M_sun for the system confirming previous studies that revealed fossils to be massive. (abridged)