Ad hoc RC model for the barred galaxy NGC 4548 and the effect of the dark matter halo on the force profile. Left panel: solid curves indicate the RC model based on the stellar component of the rotation curve, inferred from the 3.6 μm photometry. The amplitude of the URC halo component (isothermal sphere) has been corrected so that the total rotation curve matches the H i maximum velocity at the optical radius (R_opt = 3.2 h_R). The dotted line displays the URC model (exponential disk + isothermal sphere), while the dashed lines correspond to the models used in B2004. Central panel: Q_T profiles, with and without halo correction. Right panel: halo-corrected Q_T profiles resulting from a measurement error in input parameters. A 20% overestimate on the input parameters is assumed (in the mass-to-light ratio, the distance to the galaxy, and the observed velocity amplitude). We also assess the impact of reducing by one mag the I-band total luminosity (affecting the core radius in the URC models). The effect of matching the observed maximum velocity at 2.2h_R instead of at 3.2 h_R is tested as well.