Structure-dynamics relations for late-type spiral and dwarf irregular galaxies revisited

Abstract

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 µ 0 <µ 0,[3.6] >= 19.63 ± 0.11 mag arcsec −2 allows for a clear-cut distinction between low and high surface brightness galaxies. At a given luminosity, LSB galaxies are more extended than HSB galaxies and the rotation curves have smaller inner circular velocity gradients dv(R d )/dr at one disk scale length R d . Irrespective of luminosity, the geometry of rotation curves is characterized by the relation dv(R d )/dr ≈ v max /R max , with v max being the maximum circular velocity reached at R max . For the rotation curve decompositions disk mass-tolight ratios are restricted to have constant, but semi-free best-fit values 0.2, 0.5, or 0.8 M /L at [3.6]; they exhibit an asymmetric bimodal distribution with the dominant peak located at the median value of 0.2 (minimum disks) and with the subdominant peak at 0.8 (maximum disks). Assuming dark matter halos of Burkert and of pseudo-isothermal (PITS) type, the former provide better fits for about two thirds of all galaxies. While the halo core densities ρ 0 are about equal, the core radii r 0 of PITS halos are systematically lower by a factor of about 0.6 as compared with those of the Burkert type. Focussing on the Burkert halo, 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 average radial decrease of the baryonic mass fraction within galaxies is quantified. The Burkert halo parameters obey ρ 0 ∝ r −1.5±0.1 0 with considerable scatter, but allowing v max as a third variable we find ρ 0 ∝ r −1.84±0.07 0 v 2.00±0.11 max with small scatter. The halo central surface density ρ 0 r 0 , with a sample median <ρ 0 r 0 > ≈ 121 M pc −2 (σ = 112), weakly correlates with µ 0 and with compactness C and strongly correlates with the observed radial acceleration g obs = v 2 obs (r)/r at different galactocentric radii. Consequently, because R max ∝ r 0 , we have a tight central halo column density versus maximum circular velocity relation v 2 max ∝ ρ 0 r 2 0 . Halo cores barely extend over the luminous disk, but their sizes do not correlate with the optical radii. We introduce an alternative to a prominent conventional universal rotation curve; it is based on the non-singular total matter density profile ρ total (r) = (v 2 max /4πGr 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 . Fitting the synthetic URC to a selection of galaxies, the co-added doubly-normalized rotation curves exhibit a high degree of similarity. A couple of analytic URC decompositions into a baryonic disk and a dark matter component is accomplished.