Abstract
We extract from the Sloan Digital Sky Survey a sample of 347 systems involving early type galaxies separated by less than 30 kpc, in projection, and 500 km/s in radial velocity. These close pairs are likely progenitors of dry mergers. The (optical) spectra is used to determine how the interaction affects the star formation history and nuclear activity of the galaxies. The emission lines (or lack thereof) are used to classify the sample into AGN, star forming or quiescent. Increased AGN activity and reduced star formation in early-type pairs that already appear to be interacting indicate that the merging process changes the nature of nebular activity, a finding that is also supported by an increase in AGN luminosity with decreasing pair separation. Recent star formation is studied on the absorption line spectra, both through principal component analysis as well as via a comparison of the spectra with composite stellar population models. We find that the level of recent star formation in close pairs is raised relative to a control sample of early-type galaxies. This excess of residual star formation is found throughout the sample of close pairs and does not correlate with pair separation or with visual signs of interaction. Our findings are consistent with a scenario whereby the first stage of the encounter (involving the outer parts of the halos) trigger residual star formation, followed by a more efficient inflow towards the centre -- switching to an AGN phase -- after which the systems are quiescent.
Highlights
It is well known that early type galaxies are dominated by old stellar populations
In order to compare with a ’control’ sample, we show in each panel the histograms for the sample of elliptical galaxies (Rogers et al 2007), segregated with respect to NUV−r colour
Principal Component Analysis (PCA) indicates that there is little difference between the average populations of the two classes, which suggests that the visual level of disruption does not dictate the amount of recent star formation
Summary
It is well known that early type galaxies are dominated by old stellar populations (see e.g. Kodama & Arimoto 1997; Stanford et al 1998) It has been shown through studies of NUV photometry from GALEX (Yi et al 2005; Kaviraj et al 2007) as well as statistical disections of the optical spectra (Ferreras et al 2006; Rogers et al 2007; Nolan et al 2007) that a large fraction of early types have undergone small amounts of recent star formation. The young stellar mass content will be roughly independent of galaxy mass, which implies recent star formation will be more readily detectable in lower mass early-types, as observed. This scenario will result in enhanced recent star for-. The latter uses a number of age and metallicity sensitive spectral features measured via a new estimator of equivalent width that minimises the contamination from neighbouring lines (Rogers et al 2008)
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