Mass models for 175 disk galaxies with SPARC

SPARC spans a broad range of morphologies (S0 to Irr), luminosities (~5dex), and surface brightnesses (~4dex). We derive [3.6] surface photometry and study structural relations of stellar and gas disks.

We find that both the stellar mass-HI mass relation and the stellar radius-HI radius relation have significant intrinsic scatter, while the HI mass-radius relation is extremely tight. We build detailed mass models and quantify the ratio of baryonic to observed velocity (V_bar_/V_obs_) for different characteristic radii and values of the stellar mass-to-light ratio ({Upsilon}_*_) at [3.6]. Assuming {Upsilon}_*_{simeq}0.5M_{Sun}_/L_{Sun}_ (as suggested by stellar population models), we find that (i) the gas fraction linearly correlates with total luminosity; (ii) the transition from star-dominated to gas-dominated galaxies roughly corresponds to the transition from spiral galaxies to dwarf irregulars, in line with density wave theory; and (iii) V_bar_/V_obs_ varies with luminosity and surface brightness: high-mass, high-surface-brightness galaxies are nearly maximal, while low-mass, low-surface-brightness galaxies are submaximal.