Star formation and nuclear activity in close pairs of early-type galaxies

Details Star formation and nuclear activity in close pairs of early-type galaxies Star formation and nuclear activity in close pairs of early-type galaxies

2009-05-20

arxiv.org/abs/0905.3386v2

Mon.Not.Roy.Astron.Soc. 399 (2009) 2172-2182

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

12 pages, 12 figures, 1 table. Accepted for publication in MNRAS

Scientific paper

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.

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