2000ApJ...538..569R

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 Å≤λ≤5315 Å) 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{d5}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 Å 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(Hδ_abs) and the strength of the 4000 Å break (D₄₀₀₀), with lopsidedness and find a 3-4 σ correlation between the two. We also find a 3.2 σ correlation between EW(Hβ_emission) and lopsidedness. Using the evolutionary population synthesis code of Bruzual & Charlot 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(Hδ_abs) and the strength of the 4000 Å break (D₄₀₀₀) between the most and least symmetric thirds of our sample, we infer that ∼1x10⁹ 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 10¹⁰ M_☉). This corresponds to a factor of 8 increase in the star formation rate over the past 5x10⁸ years. For the nuclear spectra, all of the above correlations except D₄₀₀₀ versus <A{d5}₁> are weaker than for the disk, indicating that in lopsided galaxies, the star formation boost is not dominated by the nucleus.