Gas fractions and depletion times in galaxies with different degrees of interaction

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 S(4)G survey - along with archival HI gas masses, stellar masses (M-*), and SFRs from IRAS far-infrared fluxes - and calculate depletion times (tau) and gas fractions. We traced the central stellar mass concentration from the inner slope of the stellar component of the rotation curves, d(R)v(*)(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 less than or similar to 5), massive spiral galaxies (M-*greater than or similar to 10(10) M-circle dot) 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 approximate to 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.