We report the detection of a 4 h _ { 70 } ^ { -1 } Mpc long large-scale filament leading into the massive galaxy cluster MACS J0717.5+3745 .
The extent of this object well beyond the cluster ’ s nominal virial radius ( \sim 2.3 Mpc ) rules out prior interaction between its constituent galaxies and the cluster and makes it a prime candidate for a genuine filament as opposed to a merger remnant or a double cluster .
The structure was discovered as a pronounced overdensity of galaxies selected to have V - R colors close to the cluster red sequence .
Extensive spectroscopic follow-up of over 300 of these galaxies in a region covering the filament and the cluster confirms that the entire structure is located at the cluster redshift of z = 0.545 .
Featuring galaxy surface densities of typically 15 Mpc ^ { -2 } down to luminosities of 0.10 L ^ { \ast } _ { V } , the most diffuse parts of the filament are comparable in density to the clumps of red galaxies found around A851 in the only similar study carried out to date ( Kodama et al . ) .
Our direct detection of an extended large-scale filament funneling matter onto a massive , distant cluster provides a superb target for in-depth studies of the evolution of galaxies in environments of greatly varying density , and supports the predictions from theoretical models and numerical simulations of structure formation in a hierarchical picture .