This composite image contains three distinct features: the bright star-filled central region of galaxy NGC 6946 in optical light (blue), the dense hydrogen tracing out the galaxy’s sweeping spiral arms and galactic halo (orange), and the extremely diffuse and extended field of hydrogen engulfing NGC 6946 and its companions (red). The new GBT data show the faintly glowing hydrogen bridging the gulf between the larger galaxy and its smaller companions. This faint structure is precisely what astronomers expect to appear as hydrogen flows from the intergalactic medium into galaxies or from a past encounter between galaxies.
D.J. Pisano (WVU)/B. Saxton (NRAO/AUI/NSF)/Palomar Observatory – Space Telescope Science Institute 2nd Digital Sky Survey (Caltech)/Westerbork Synthesis Radio Telescope