Context. A moderate enhancement of the star formation rates (SFR) in local interacting galaxies has been reported, but the physical mechanisms leading to this increase are not clear.
Aims: We study the atomic gas content and the central stellar mass concentration for a sample of almost 1500 nearby galaxies to further investigate the nature of starbursts and the influence of galaxy-galaxy interactions on star formation.
Methods: We used a sample of catalogued interacting and non-interacting galaxies in the S4G survey - along with archival H I gas masses, stellar masses (M*), and SFRs from IRAS far-infrared fluxes - and calculate depletion times (τ) and gas fractions. We traced the central stellar mass concentration from the inner slope of the stellar component of the rotation curves, dRv*(0). Starbursts are defined as galaxies with a factor > 4 enhanced SFR relative to a control sample of non-interacting galaxies which are ±0.2 dex in stellar mass and ±1 in T-type.
Results: Starbursts are mainly early-type (T ≲ 5), massive spiral galaxies (M* ≳ 1010 M☉) that are not necessarily interacting. For a given stellar mass bin, starbursts are characterised by lower gas depletion times, similar gas fractions, and larger central stellar mass concentrations than non-starburst galaxies. The global distributions of gas fraction and gas depletion time of interacting galaxies are not statistically different from those of their non-interacting counterparts. However, in the case of currently merging galaxies, the median gas depletion time is a factor of 0.4 ± 0.2 that of control sample galaxies, and their SFRs are a factor of 1.9 ± 0.5 enhanced, even though the median gas fraction is similar.
Conclusions: Starbursts present long-lasting star formation in circumnuclear regions, which causes an enhancement of the central stellar density at z ≈ 0 in both interacting and non-interacting systems. Starbursts have low gas depletion timescales, yet similar gas fractions as normal main-sequence galaxies. Galaxy mergers cause a moderate enhancement of the star formation efficiency.