Lopsided Galaxies, Weak Interactions, and Boosting the Star Formation Rate

To investigate the link between weak tidal interactions in disk galaxies andthe boosting of their recent star formation, we obtain images and spatiallyintegrated spectra (3615A < lambda < 5315A) for 40 late-type spiral galaxies(Sab-Sbc) with varying degrees of lopsidedness (a dynamical indicator of weakinteractions). We quantify lopsidedness as the amplitude of the m=1Fourier component of the azimuthal surface brightness distribution, averagedover a range of radii. We compare the young stellar content, quantified byEW(H\delta_abs) and the strength of the 4000 Angstrom break (D_4000), withlopsidedness and find a 3-4 sigma correlation between the two. We also find a3.2 sigma correlation between EW(H\beta_emission) and lopsidedness. Using theevolutionary population synthesis code of Bruzual & Charlot we model thespectra as an ``underlying population'' and a superimposed ``boost population''with the aim of constraining the fractional boost in the SFR averaged over thepast 0.5 Gyr (the characteristic lifetime of lopsidedness). From the differencein both EW(H\delta_abs) and D_4000 between the most and least symmetric thirdsof our sample, we infer that ~ 1x10^9 M_solar of stars are formed over theduration of a lopsided event in addition to the ``underlying'' SFH (assuming afinal galactic stellar mass of 10^10 M_solar). This corresponds to a factor of8 increase in the SFR over the past 5x10^8 years. For the nuclear spectra, allof the above correlations except D_4000 vs. are weaker than for the disk,indicating that in lopsided galaxies, the SF boost is not dominated by thenucleus.Comment: 35 pages, including 10 figures, to appear in the Astrophysical Journal, abridged abstrac