2007ApJ...665L...9R

We have identified perhaps the largest major galaxy merger ever seen. While analyzing Spitzer IRAC images of CL 0958+4702, an X-ray-selected cluster at z=0.39, we discovered an unusual plume of stars extending ≳110 h^–1 kpc outward from the bright central galaxy (BCG). Three galaxies 1-1.5 mag fainter than the BCG lie within 17 kpc (projected) of the BCG and are probably participating in the merger. The plume is detected in all four IRAC channels and at optical wavelengths in images from the WIYN telescope; the surface brightness is remarkably high (µ_r~24.8 mag/arcsec2 at 50 kpc). The optical and infrared colors are consistent with those of other BCGs, suggesting that the plume is composed of old stars and negligible recent star formation (hence a ``dry merger''). The luminosity in the plume is at least equivalent to a 4L^* galaxy. A diffuse halo extending 110 h^–1 kpc from the BCG in one IRAC image suggests the total amount of diffuse light is L_r∼1.3x10¹¹ h^–2 L_☉. A Chandra observation shows an X-ray image and spectrum typical of moderate-mass clusters. We use MMT/Hectospec to measure 905 redshifts in a 1 deg² region around the cluster. The velocities of two of the BCG companions indicate a merger timescale for the companion galaxies of ∼110 Myr and ∼0.5-1 Gyr for the plume. We conclude that the BCG and intracluster light of CL 0958 is formed by major mergers at moderate redshifts. After the major merger is complete, CL 0958 will likely become a fossil cluster.