The Spiderweb Galaxy: a Forming Massive Cluster Galaxy

We present a deep image of the radio galaxy MRC 1138 262 taken with the Hubble Space Telescope (HST) at a redshift of . The galaxy is known to have properties of a cD galaxy progenitor and be surrounded z p 2.2 by a 3 Mpc–sized structure, identified with a protocluster. The morphology shown on the new deep HST ACS image is reminiscent of a spider’s web. More than 10 individual clumpy features are observed, apparently starforming satellite galaxies in the process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago. There is an extended emission component, implying that star formation was occurring over a kpc region 50 # 40 at a rate of more than 100 yr . A striking feature of the newly named “Spiderweb galaxy” is the presence M, of several faint linear galaxies within the merging structure. The dense environments and fast galaxy motions at the centers of protoclusters may stimulate the formation of these structures, which dominate the faint resolved galaxy populations in the Hubble Ultra Deep Field. The new image provides a unique testbed for simulations of forming dominant cluster galaxies. Subject headings: galaxies: active — galaxies: clusters: general — galaxies: elliptical and lenticular, cD — galaxies: high-redshift