2025ApJ...978...63F

Highly inclined (edge-on) disk galaxies offer the unique perspective to constrain their intrinsic flattening, c/a, where c and a are, respectively, the vertical and long radial axes of the disk measured at suitable stellar densities. The ratio c/a is a necessary quantity in the assessment of galaxy inclinations, three-dimensional structural reconstructions, and total masses, as well as a constraint to galaxy formation models. The 3.6 μm maps of 133 edge-on spiral galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S⁴G) and its early-type galaxy extension are used to revisit the assessment of c/a free from dust extinction and away from the influence of a stellar bulge. We present a simple definition of c/a and explore trends with other galactic physical parameters: total stellar mass, concentration index, total H I mass, mass of the central mass concentration, circular velocity, model-dependent scales, and Hubble type. Other than a dependence on early/late Hubble types and a related trend with light concentration, no other parameters were found to correlate with the intrinsic flattening of spiral galaxies. The latter is mostly constant with 〈c/a〉 = 0.124 ± 0.001 (stat) ± 0.033 (intrinsic/systematic) and greater for earlier types.