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Fig. 4.

image

Measured RAR, which compares the total gravitational acceleration gobs with the expected baryonic acceleration gbar of galaxies. At high accelerations we show the M16 RAR measurements from galaxy rotation curves (all data as the blue 2D histogram, the mean as red squares). Using weak gravitational lensing we were able to extend this measurement to lower accelerations, using both the spectroscopic GAMA and the photometric KiDS-bright isolated lens samples (blue and black points with 1σ error bars). Comparing our lensing observations to two MG models: MOND (the M16 fitting function; grey solid line) and EG (assuming a point mass; red dashed line) we find that GAMA results are in agreement with the two models, while those from KiDS-bright are systematically higher. At very low accelerations (corresponding to , light blue shaded region) the uncertainty in the photometric KiDS redshifts affects the isolated lens selection, resulting in systematically higher values of gobs due to the possible contribution of satellites. The results from the spectroscopic GAMA survey, however, are still reliable within this region. The impact of stellar mass uncertainty (ΔM = 0.2 dex) on the measurement is shown as the grey band. We show the MOND prediction including the EFE (with e = 0.003, see Eq. (13)) as the grey dashed line. In addition, we show the effect on the RAR of KiDS-bright galaxies if gbar contained an additional isothermal hot gas contribution within a radius, with a nominal gas mass equal to the stellar mass (orange crosses with 1σ error bars). We emphasise that this is only a rough order of magnitude estimate of the possible effect of gaseous haloes, which are extremely difficult to observe.

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