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tokens: than the RM of the low- z DLA sample. Such a probability indicates no significant effect. Any additional RM contribution from the plasma of the intragroup medium (see Section 3.7 ) must be low, indicating either low electron density, a weak magnetic field, or field reversals along the line of sight. The most likely scenarios are either that there is a (1 + z ) 2 dilution in the RM of background quasars, or that the Galactic RM foreground varies substantially between samples. There is no evidence for a (1 + z ) 2 dilution in the RM of the DLAs themselves, which do not show a marked difference from the control samples. Previous attempts to observe the former scenario of an evolving background quasar, e.g. Hammond, Robishaw Gaensler ( 2012 ), find that any (1 + z ) 2 dilution in RM due to cosmological expansion is weak out to z ∼ 3.5 (see also Xu Han 2014 ). If our data do therefore show an evolution in the background quasars, it may be due to a low-redshift selection effect on, for example SDSS quasars relative to high-redshift SDSS quasars, or possibly a low-redshift evolution in the magnetoionic environment surrounding the quasars themselves. To constrain the latter scenario of a varying Galactic RM foreground between samples, we use the best available model from Oppermann et al. ( 2015 ). A plot of the distribution of Galactic RMs (GRMs) towards each source is shown in Fig. 9 . There is again no difference between specific samples. We calculate a Bayesian probability of 61.5 per cent that the GRM towards J1243+4043 is greater than the GRM towards the low- z DLA sample based on 21-cm absorption measurements. This is again consistent with no effect. The combination of no difference in RM, together with no difference in GRM, means that we are consequently unable to draw conclusions about the coherent magnetic fields in the DLA in J1243+4043 itself. The only difference between the data sets is the distributions of the high- versus low-redshift RMs, which display no subsequent difference in the GRMs. This is consistent with our earlier interpretation of either an uncontrolled selection effect or an evolution of the quasar magnetoionic environment at low redshifts. Figure 9. The distribution of GRMs for high- z DLAs (top panel) and low- z 21-cm absorbing QGPs (bottom panel). The ‘control’ refers to sight lines with no DLA ( top ) and no 21-cm absorption ( bottom ), respectively. In terms of J1243+4043, the magnetic field properties do not allow us to distinguish between different scenarios in order to place strong constraints on this source. Future broad-band spectropolarimetric observations of this source will enable improved tests that can attempt to determine where this sight line fits within the greater source population. 3.7 H <sc>i</sc> emission from the associated galaxy group: the environment of the foreground galaxy In the WSRT data, we serendipitously detect H i 21-cm emission associated with other members of the galaxy group. The group