Tidally Triggered Star Formation in Close Pairs of Galaxies

Abstract

We analyze optical spectra of a sample of 502 galaxies in close pairs and N-tuples, separated by ≤50 h-1 kpc. We extracted the sample objectively from the CfA2 redshift survey, without regard to the surroundings of the tight systems; we remeasure the spectra with longer exposures, to explore the spectral characteristics of the galaxies. We use the new spectra to probe the relationship between star formation and the dynamics of the systems of galaxies. The equivalent widths of Hα [EW(Hα)] and other emission lines anticorrelate strongly with pair spatial separation (ΔD) and velocity separation; the anticorrelations do not result from any large-scale environmental effects that we detect. We use the measured EW(Hα) and the starburst models of Leitherer et al. to estimate the time since the most recent burst of star formation began for galaxies in our sample. In the absence of a large contribution from an old stellar population to the continuum around Hα that correlates with the orbit parameters, the observed ΔD-EW(Hα) correlation signifies that starbursts with larger separations on the sky are, on average, older. We also find a population of galaxies with small to moderate amounts of Balmer absorption. These galaxies support our conclusion that the sample includes many aging bursts of star formation; they have a narrower distribution of velocity separations, consistent with a population of orbiting galaxies near apogalacticon. By matching the dynamical timescale to the burst timescale, we show that the data support a simple picture in which a close pass initiates a starburst; EW(Hα) decreases with time as the pair separation increases, accounting for the anticorrelation. Recent N-body/smoothed particle hydrodynamics simulations of interacting pairs suggest a physical basis for the correlation—for galaxies with shallow central potentials, they predict gas infall before the final merger. This picture leads to a method for measuring the duration and the initial mass function of interaction-induced starbursts: our data are compatible with the starburst models and orbit models in many respects, as long as the starburst lasts longer than ~108 yr and the delay between the close pass and the initiation of the starburst is less than a few times 107 yr. If there is no large contribution from an old stellar population to the continuum around Hα, the Miller-Scalo and cutoff (M ≤ 30 M☉) Salpeter initial mass functions (IMFs) fit the data much better than a standard Salpeter IMF.