The dependence of bar frequency on galaxy mass, colour, and gas content - and angular resolution - in the local universe

I use distance- and mass-limited subsamples of the Spitzer Survey of Stellar Structure in Galaxies (S⁴G) to investigate how the presence of bars in spiral galaxies depends on mass, colour, and gas content and whether large, Sloan Digital Sky Survey (SDSS)-based investigations of bar frequencies agree with local data. Bar frequency reaches a maximum of f_bar ≈ 0.70 at M_⋆ ∼ 10^9.7M_⊙, declining to both lower and higher masses. It is roughly constant over a wide range of colours (g - r ≈ 0.1-0.8) and atomic gas fractions (log (M_{H I}/ M_{\star }) ≈ -2.5 to 1). Bars are thus as common in blue, gas-rich galaxies are they are in red, gas-poor galaxies. This is in sharp contrast to many SDSS-based studies of z ∼ 0.01-0.1 galaxies, which report f_bar increasing strongly to higher masses (from M_⋆ ∼ 10¹⁰ to 10¹¹M_⊙), redder colours, and lower gas fractions. The contradiction can be explained if SDSS-based studies preferentially miss bars in, and underestimate the bar fraction for, lower mass (bluer, gas-rich) galaxies due to poor spatial resolution and the correlation between bar size and stellar mass. Simulations of SDSS-style observations using the S⁴G galaxies as a parent sample, and assuming that bars below a threshold angular size of twice the point spread function full width at half-maximum cannot be identified, successfully reproduce typical SDSS f_bar trends for stellar mass and gas mass ratio. Similar considerations may affect high-redshift studies, especially if bars grow in length over cosmic time; simulations suggest that high-redshift bar fractions may thus be systematically underestimated.