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Abstract: We have identified perhaps the largest major galaxy merger ever seen.
Whileanalysing Spitzer IRAC images of CL0958+4702, an X-ray selected cluster atz=0.39, we discovered an unusual plume of stars extending $\gtrsim$110 kpcoutward from the bright central galaxy BCG.
Three galaxies 1-1.5 mag fainterthan the BCG lie within 17 kpc projected of the BCG and are probablyparticipating in the merger.
The plume is detected in all four IRAC channelsand at optical wavelengths in images from the WIYN telescope; the surfacebrightness is remarkably high $\mu r\approx$24.8 mag arcsec$^{-2}$ 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 starformation hence a -dry merger-.
The luminosity in the plume is at leastequivalent to a 4L^* galaxy.
A diffuse halo extending 110 kpc from the BCG inone IRAC image suggests the total amount of diffuse light is L r\sim1.3x10^{11}h^{-2} L sun.
A Chandra observation shows an X-ray image andspectrum typical of moderate-mass clusters.
We use MMT-Hectospec to measure 905redshifts in a 1 deg^2 region around the cluster.
The velocities of two of theBCG companions indicate a merger timescale for the companion galaxies of$\sim$110 Myr and $\sim$0.5-1 Gyr for the plume.
We conclude that the BCG andintracluster light of CL0958 is formed by major mergers at moderate redshifts.After the major merger is complete, CL0958 will likely become a fossil cluster.

Autor: Kenneth Rines CfA, Rose Finn Siena, Alexey Vikhlinin CfA


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