Multiwavelength observations of an evolved galaxy group: an end‐point of galaxy merging?

The group of galaxies RXJ1340.6+4018 has approximately the same bolometric X‐ray luminosity as other bright galaxy groups and poor clusters such as the Virgo cluster. However, 70 per cent of the optical luminosity of the group comes from a dominant giant elliptical galaxy, compared with 5 per cent from M87 in Virgo.The second brightest galaxy in RXJ1340.6+4018 is a factor of 10 fainter (Δ m 12 =2.5 mag) than the dominant elliptical, and the galaxy luminosity function has a gap at about L *. We interpret the properties of the system as a result of galaxy merging within a galaxy group. We find that the central galaxy lies on the Fundamental Plane of ellipticals, has an undisturbed, non‐cD morphology, and has no spectral features indicative of recent star formation, suggesting that the last major merger occurred ≳4 Gyr ago. The deviation of the system from the cluster L X − T relation in the opposite sense to most groups may be caused by an early epoch of formation of the group or a strong cooling flow. The unusual elongation of the X‐ray isophotes and the similarity between the X‐ray and optical ellipticities at large radii (∼230 kpc) suggest that both the X‐ray gas and the outermost stars of the dominant galaxy are responding to an elongated dark matter distribution. RXJ1340.6+4018 may be part of a filamentary structure related to infall in the outskirts of the cluster A1774.