paper

How do minor mergers promote inside-out growth of ellipticals, transforming the size, density profile and dark matter fraction?

arXiv:1206.5004 · doi:10.1093/mnras/sts501

Abstract

There is observational evidence for inside-out growth of elliptical galaxies since , which is not driven by in-situ star formation. Many systems at high redshift have small sizes and surface brightness profiles with low Sersic indices n. The most likely descendants have, on average, grown by a factor of two in mass and a factor of four in size, indicating with . They also have surface brightness profiles with . This evolution can be qualitatively explained on the basis of two assumptions: compact ellipticals predominantly grow by collisionless minor or intermediate 'dry' mergers, and they are embedded in massive dark matter halos. We draw these conclusions from idealized collisionless mergers spheroidal galaxies - with and without dark matter - with mass ratios of 1:1, 1:5, and 1:10. The sizes evolve as with for mass-ratios of 1:1. For minor mergers of galaxies embedded in dark matter halos, the sizes grow significantly faster and the profile shapes change more rapidly. Mergers with moderate mass-ratios of 1:5 give and a final Sersic index of after doubling the stellar mass. This is accompanied by a significant increase of the dark matter fraction within the stellar half-mass radius, driven by the strong size increase probing larger, dark matter dominated regions. Only a few intermediate mass-ratio mergers of galaxies embedded in massive dark matter halos can result in the observed concurrent inside-out growth and the rapid evolution in profile shapes. Apart from negative stellar metallicity gradients such a 'minor' merger scenario also predicts significantly lower dark matter fractions for compact quiescent galaxies and their rare present day analogues (abbreviated).

accepted for publication

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