Distribution of star formation in galactic bars as seen with Hα and stacked GALEX UV imaging

Abstract Context: Stellar bars are known to gradually funnel gas to the central parts of disk galaxies. It remains a matter of debate why the distribution of ionized gas along bars and in the circumnuclear regions varies among galaxies. Aims: Our goal is to investigate the spatial distribution of star formation (SF) within bars of nearby low-inclination disk galaxies (i < 65°) from the S⁴G survey. We aim to link the loci of SF to global properties of the hosts (morphological type, stellar mass, gas fraction, and bar-induced gravitational torques), providing constraints for the conditions that regulate SF in bars. Methods: We use archival GALEX far- and near-UV imaging for 772 barred galaxies, and for a control sample of 423 non-barred galaxies. We also assemble a compilation of continuum-subtracted Hα images for 433 barred galaxies, 70 of which we produced from ancillary photometry and MUSE and CALIFA integral field unit data cubes. We employ two complementary approaches: (i) the analysis of bar (2D) and disk (1D) stacks built from co-added UV images (oriented and scaled with respect to the stellar bars and the extent of disks) of hundreds of galaxies that are binned based on their Hubble stage (T) and bar family; and (ii) the visual classification of the morphology of ionized regions (traced from Hα and UV data) in individual galaxies into three main SF classes: (A) only circumnuclear SF; (B) SF at the bar ends, but not along the bar; and (C) SF along the bar. Barred galaxies with active and passive inner rings are likewise classified. Results: Massive, gas-poor, lenticular galaxies typically belong to SF class A; this is probably related to bar-induced quenching of SF in the disk. The distribution of SF class B peaks for early- and intermediate-type spirals; this most likely results from the interplay of gas flow, shocks, and enhanced shear in massive centrally concentrated galaxies with large bar amplitudes (the latter is supported by the lack of a dip in the radial distribution of SF in non-barred galaxies). Late-type gas-rich galaxies with high gravitational torques are mainly assigned to SF class C; we argue that this is a consequence of low shear among the faintest galaxies. In bar stacks of spiral galaxies the UV emission traces the stellar bars and dominates on their leading side, as witnessed in simulations. Among early-type spirals the central UV emission is ∼0.5 mag brighter in strongly barred galaxies, relative to their weakly barred counterparts; this is probably related to the efficiency of strong bars sweeping the disk gas and triggering central starbursts. On the contrary, in later types the UV emission is stronger at all radii in strongly barred galaxies than in weakly barred and non-barred ones. We also show that the distributions of SF in inner-ringed galaxies are broadly the same in barred and non-barred galaxies, including a UV and Hα deficit in the middle part of the bar; this hints at the effect of resonance rings trapping gas that is no longer funneled inwards. Conclusions: Distinct distributions of SF within bars are reported in galaxies of different morphological types. Star-forming bars are most common among late-type gas-rich galaxies. Bars are important agents in the regulation of SF in disks