Lopsided galaxies, weak interactions, and boosting the star formation rate

To investigate the link between weak tidal interactions in disk galaxies and the boosting of their recent star formation, we obtain images and spatially integrated spectra (3615 Angstrom less than or equal to lambda less than or equal to 5315 Angstrom) for 40 late-type spiral galaxies (Sab-Sbc) with varying degrees of lopsidedness (a dynamical indicator of weak interactions). We quantify lopsidedness as the amplitude [(A) over tilde(1)] of the m = 1 Fourier component of the azimuthal surface brightness distribution averaged over a range of radii. The median spectrum of the most lopsided galaxies shows strong evidence for a more prominent young stellar population (i.e., strong Balmer absorption, strong nebular emission, a weak 4000 Angstrom break, and a blue continuum) when compared to the median spectrum of the most symmetric galaxies. We compare the young stellar content, quantified by EW(H delta(abs)) and the strength of the 4000 Angstrom break (D-4000), with lopsidedness and find a 3-4 sigma correlation between the two. We also find a 3.2 sigma correlation between EW(H beta(emission)) and lopsidedness. Using the evolutionary population synthesis code of Bruzual & Chariot we model the spectra as an underlying population and a superimposed boost population with the aim of constraining the fractional boost in the SFR averaged over the past 0.5 Gyr (the characteristic lifetime of lopsidedness). From the difference in both EW(H delta(abs)) and the strength of the 4000 Angstrom break (D-4000) between the most and least symmetric thirds of our sample, we infer that similar to 1 x 10(9) M. of stars are formed over the duration of a lopsided event in addition to the underlying star formation history (assuming a final galactic stellar mass of 10(10) M.). This corresponds to a factor of 8 increase in the star formation rate over the past 5 x 10(8) years. For the nuclear spectra, all of the above correlations except D-4000 versus [(A) over tilde(1)] are weaker than for the disk, indicating that in lopsided galaxies, the star formation boost is not dominated by the nucleus.