2014ApJ...797L..28S

We present the correlation between the extrapolated central disk surface brightness (µ) and extrapolated central surface mass density (Σ) for galaxies in the DiskMass sample. This µ-Σ relation has a small scatter of 30% at the high surface brightness (HSB) end. At the low surface brightness (LSB) end, galaxies fall above the µ-Σ relation, which we attribute to their higher dark matter content. After correcting for the dark matter as well as for the contribution of gas and the effects of radial gradients in the disk, the LSB end falls back on the linear µ-Σ relation. The resulting scatter around the corrected µ-Σ relation is 25% at the HSB end and about 50% at the LSB end. The intrinsic scatter in the µ-Σ relation is estimated to be 10%-20%. Thus, if µ_K, 0_ is known, the stellar surface mass density is known to within 10%-20% (random error). Assuming disks have an exponential vertical distribution of mass, the average γ_*^K is 0.24 M_☉/L_☉, with an intrinsic scatter around the mean of at most 0.05 M_☉/L_☉. This value for γ_*^K is 20% smaller than we found in Martinsson et al., mainly due to the correction for dark matter applied here. This small scatter means that among the galaxies in our sample, variations in scale height, vertical density profile shape, and/or the ratio of vertical over radial velocity dispersion must be small.