Bar slowdown and the distribution of dark matter in barred galaxies

'Conspiracy' between the dark and the baryonic matter prohibits an unambiguous decomposition of disc galaxy rotation curves into the corresponding components. Several methods have been proposed to counter this difficulty, but their results are widely discrepant. In this paper, I revisit one of these methods, which relies on the relation between the halo density and the decrease of the bar pattern speed. The latter is routinely characterized by the ratio R of the corotation radius R-CR to the bar length L-b, R = R-CR/L-b. I use a set of N-body+SPH simulations, including subgrid physics, whose initial conditions cover a range of gas fractions and halo shapes. The models, by construction, have roughly the same azimuthally averaged circular velocity curve and halo density and they are all submaximal, i.e. according to previous works, they are expected to have all roughly the same R value, well outside the fast bar range (1.2 +/- 0.2). Contrary to these expectations, however, these simulations end up having widely different R values, either within the fast bar range or well outside it. This shows that the R value cannot constrain the halo density, nor determine whether galactic discs are maximal or submaximal. I argue that this is true even for early-type discs (S0s and Sas).