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Abstract

I use volume- and mass-limited subsamples and recently published data from the Spitzer Survey of Stellar Structure in Galaxies (S⁴G) to investigate how the size of bars depends on galaxy properties. The known correlation between bar semimajor axis a and galaxy stellar mass (or luminosity) is actually bimodal: for log(M_⋆/M_⊙)≲10.1⁠, bar size is almost independent of stellar mass (⁠a_∝M_0.1^⋆⁠), while it is a strong function for higher masses (⁠a_∝M_0.6^⋆⁠). Bar size is a slightly stronger function of galaxy half-light radius R_e and (especially) exponential disc scale length h (a ∝ h^0.8). Correlations between stellar mass and galaxy size can explain the bar-size–M_⋆ correlation – but only for galaxies with log(M_⋆/M_⊙)≲10.1⁠; at higher masses, there is an extra dependence of bar size on M⋆ itself. Despite theoretical arguments that the presence of gas can affect bar growth, there is no evidence for any residual dependence of bar size on (present-day) gas mass fraction. The traditional dependence of bar size on Hubble type (longer bars in early-type discs) can be explained as a side effect of stellar mass–Hubble-type correlations. Finally, I show that galaxy size (R_e or h) can be modelled as a function of stellar mass and both bar presence and bar size: barred galaxies tend to be more extended than unbarred galaxies of the same mass, with larger bars correlated with larger sizes.