F814W HST-ACS image of the filamentary structure found in CGr172 at z ∼ 0.696 in logarithmic scale (arbitrary unit). The circles correspond to galaxies in the COSMOS2015 catalog within the field with secure redshifts, either within the same group as the extended emission (black), in the foreground (blue), or in the background (red). The green star marks a foreground star and the emission around it is an artifact. The black plus symbol and cross show the position of the group barycenter and X-ray center, respectively. The IDs of galaxies within CGr172 are indicated. The blue contours represent the [O II] doublet in emission, at levels 0.25, 0.5, 1, and 2 × 10⁻¹⁷ erg s⁻¹ cm⁻² arcsec⁻², after a Gaussian spatial smoothing of two pixels FWHM. Contours are estimated by collapsing the MUSE cube over 20 Å at the observed wavelength of both lines, and by subtracting the continuum using 50 Å wide adjacent wavelength ranges on both sides of the lines.