  We introduce SPARC (Spitzer Photometry & Accurate Rotation Curves): a sample
of 175 nearby galaxies with new surface photometry at 3.6 um and high-quality
rotation curves from previous HI/Halpha studies. SPARC spans a broad range of
morphologies (S0 to Irr), luminosities (~5 dex), and surface brightnesses (~4
dex). 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 (Vbar/Vobs) for
different characteristic radii and values of the stellar mass-to-light ratio
(M/L) at [3.6]. Assuming M/L=0.5 Msun/Lsun (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) Vbar/Vobs varies with luminosity
and surface brightness: high-mass, high-surface-brightness galaxies are nearly
maximal, while low-mass, low-surface-brightness galaxies are submaximal. These
basic properties are lost for low values of M/L=0.2 Msun/Lsun as suggested by
the DiskMass survey. The mean maximum-disk limit in bright galaxies is M/L=0.7
Msun/Lsun at [3.6]. The SPARC data are publicly available and represent an
ideal test-bed for models of galaxy formation.
