Structure-dynamics relations for late-type spiral and dwarf irregular galaxies revisited
Scaling relations among structural and kinematical features of 79 late-type spiral and dwarf irregular galaxies of the SPARC sample are revisited or newly established. The mean central surface brightness = 19.63 $\pm$ 0.11 mag arcsec$^{-2}$ allows for a clear-cut distinction between low and high surface brightness galaxies irrespective of luminosity. The geometry of rotation curves is characterized by the relation $dv(R_d)/dr \approx v_{max}/R_{max}$. For the rotation curve decompositions we apply dark matter halos of Burkert and of pseudo-isothermal type. The disk mass-to-light ratios exhibit an asymmetric bimodal distribution with the dominant peak located at 0.2. The baryonic mass fraction at intermediate radii is included to address both an adjusted baryonic Tully-Fisher relation and the significance of deviations from the mean radial acceleration relation. The mean radial decrease of the baryonic mass fraction within galaxies is quantified. The Burkert halo parameters obey $\rho_0 \propto r_0^{-1.5}$ with considerable scatter, but allowing $v_{max}$ as a third variable we find $\rho_0 \propto r_0^{-1.84}v_{max}^{2.00}$ with small scatter. The halo central surface density $\rho_0r_0$ strongly correlates with the observed radial acceleration at different galactocentric radii. As a consequence, there is a tight central halo column density versus maximum circular velocity relation $v_{max}^2 \propto \rho_0 r_0^2$. We introduce an alternative universal rotation curve that is based on the non-singular total matter density profile $\rho_{total}(r) \propto (v_{max}^2/r^2)[1-(1-r/r_c)\exp(-r/r_c)]^2$, with the scaling parameter $r_c$ correlating with the halo core size $r_0$ and with $R_{max}$. Fitting the synthetic URC to a selection of galaxies, the co-added doubly-normalized rotation curves exhibit a high degree of similarity.