MIGHTEE-HI / LADUMA: Investigating the link between baryons and dynamics with 130 resolved HI-selected galaxies

The baryonic Tully-Fisher relation (bTFR) and the radial acceleration relation (RAR) link the observed dynamics in galaxies to that expected from their baryonic mass distributions. The relations' small intrinsic scatters place strong constraints on galaxy formation models, dark matter properties and theories of modified dynamics, yet detailed measurements beyond the very local Universe remain limited. We use 130 purely HI-selected galaxies with resolved HI kinematics and baryonic mass profiles to measure the bTFR and RAR up to z about0.09. We measure a tight RAR with an acceleration scale a 0=(1.50 plus or minus0.05) times10-10, m,s-2 and an intrinsic scatter of 0.096 plus or minus0.006 dex, consistent with local results. We fit the bTFR in the `inverse' direction, conditioning on M bar to mitigate HI flux-related selection effects, measuring a logarithmic slope of 0.27 plus or minus0.01 (corresponding to a forward slope of 3.72 plus or minus0.16), with vertical intrinsic scatter sigma about0.05 dex. Fitting the general delta-family of MOND interpolating functions to the RAR, we infer delta=4.10+1.4-0.68, consistent with the value required by Solar System gravitational constraints and a null Wide Binary Test. We find no significant redshift evolution in the RAR acceleration scale for our pure HI-selected sample. However, the bTFR zero-point shows an apparent evolutionary trend that is strongly dependent on the fit direction: the traditional forward fit yields an 8.7 sigma preference for z evolution, while for our fiducial inverse fit, this reduces to 3.4 sigma, within about2 sigma of the RAR evolution constraint. This suggests selection effects bias the forward fit; a careful consideration of such effects will be required in future endeavours to robustly measure the redshift evolution of dynamical scaling relations.