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 use the catalogue on interacting and merging galaxies in the S ^ { 4 } G survey of along with archival H i gas masses , stellar masses ( M _ { \ast } ) , and SFRs ( from IRAS far-IR fluxes ) , and calculate depletion times ( \tau ) and gas fractions .
We trace the central stellar mass concentration from the inner slope of the stellar component of the rotation curves , d _ { R } v _ { \ast } ( 0 ) . Results .
Starbursts – defined as galaxies with a factor > 4 enhanced SFR relative to a control sample ( \pm 0.2 dex in stellar mass , \pm 1 in T -type , non-interacting ) – are mainly early-type ( T \lesssim 5 ) massive spiral galaxies ( M _ { \ast } \gtrsim 10 ^ { 10 } M _ { \odot } ) , 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 are not statistically different for interacting and non-interacting galaxies .
However , in the case of currently merging galaxies , the median gas depletion time is a factor of 0.4 \pm 0.2 that of control sample galaxies , and their star formation rates are a factor of 1.9 \pm 0.5 enhanced , even though the median gas fraction is similar . Conclusions .
Starbursts present long-lasting star formation in the circumnuclear regions that causes an enhancement of the central stellar density at z \approx 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 .