HST-ACS image (F814W filter, logarithmic scale, arbitrary unit) with membership identification: galaxy group members are marked using blue ellipses (which define the apertures used to extract spectra), foreground and background objects are marked using magenta diamonds and red rectangles, respectively, a star is identified by a brown triangle, and objects with undefined spectroscopic redshifts are shown with green circles. Thicker red symbols correspond to two background galaxies, which we refer to as CGr30-237 (the most central) and CGr30-86 (the northernmost) and which show Mg ii in absorption at the redshift of the ionised structure. Logarithmic [O ii] contours, smoothed using a 0.2′′ FWHM Gaussian, are overlaid in black. Contour levels correspond to 1.5, 3.3, 7.3, 16.2, and 35.9 × 10^-18 erg s^-1 cm^-2 arcsec^-2. The physical scale at the redshift of the structure is indicated at the bottom right.