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 ≤ 50h −1 kpc. We extracted the sample objectively from the CfA2 redshift survey, without regard to the surroundings of the tight systems; we re-measure 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 anti-correlate strongly with pair spatial separation (∆D) and velocity separation; the anti-correlations 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 ∆D -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 suport 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 anti-correlation. Recent n-body/SPH 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 ∼10 8 years and the delay between the close pass and the initiation of the starburst is less than a few×10 7 years. 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 fit the data much better than a standard Salpeter IMF.

Galaxy pairs in the Sloan Digital Sky Survey – VIII. The observational properties of post-merger galaxies

“…There has been considerable observational work over the last decades to confirm these theoretical predictions, which bear out many of the general, qualitative predictions of the simulations over a range of redshifts (e.g. Kennicutt et al 1987;Barton, Geller & Kenyon 2000;Kewley, Geller & Barton 2006;Woods, Geller & Barton 2006;Smith et al 2007;Jogee et al 2009;Rupke, Kewley & Chien 2010;Koss et al 2010Koss et al , 2012Silverman et al 2011;Wong et al 2011;Ramos-Almeida et al 2011, 2012Xu et al 2012;Cotini et al 2013;Lanz et al 2013). However, a full and detailed picture of the merger transformation requires large samples of galaxies that span a range in mass, mass ratio, structural properties, environments and projected separation, since all of these properties are likely to impact the specific outcome of a merger.…”

“…Indeed, this has been the typical separation that most studies of close pairs have selected, with enhanced star formation rates (SFRs) usually detected within projected separations ∼ 30 h −1 70 kpc (e.g. Barton et al 2000;Alonso et al 2007;Domingue et al 2009;Wong et al 2011). However, it has recently been shown that the effects of galaxy interactions can be detected in pairs with separations in excess of 50 h −1 70 kpc (e.g.…”

Spectral classification and composites of galaxies in LAMOST DR4

“…These targets are candidates of galaxy pairs. A galaxy pair is typically defined from the projected distance rp and recessional velocity difference |∆V | of two neighbouring galaxies, and it is useful to probe the process of galaxy interactions or galaxy mergers (Barton et al 2000, Nikolic et al 2004. A large number of galaxy pairs are identified from SDSS main galaxy sample (Ellison et al 2008(Ellison et al , 2011.…”

An investigation of star formation and dust attenuation in major mergers using ultraviolet and infrared data

“…Their work is less affected by dust extinction than are previous studies based on UV and optical bands (e.g. Barton et al 2000;Woods et al 2006;Ellison et al 2008), and it has better resolution than previous infrared (IR) works (e.g. Kennicutt et al 1987;Telesco et al 1988;Xu & Sulentic 1991).…”