  We report the detection of a 4/h70 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 (~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(*,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.
