They appear to be using narrow-band IR filters to detect faint galaxies at specific distances. The Z range of 2.1-2.3 corresponds to recession velocities a little over 0.8c (light speed) or a distance (using Hubble's Law that correlates recession speed with distance) of about 10.5 billion light years. Galaxies more or less distant than 10.5 billion light years will be filtered out. The small circles are probably the galaxies detected at this specific distance and the 3 large circles are where these galaxies cluster.<br><br>The coloring may be a galaxy density map but this can't be just the galaxies at 10.5 billion light years since the colors indicate a density range much larger than what's indicated by the small circles.<br><br>I'd guess the coloring represents the density of galaxies observed at all ranges but there's no way to tell for sure without additional info.