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

Abstract

There is observational evidence for inside-out growth of giant elliptical galaxies since z≳ 2-3, which is - in contrast to disc galaxies - not driven by in situ star formation. Many of the ~1011M⊙ systems at high redshift have small sizes ~1 kpc and surface brightness profiles with low-Śersic indices n. The most likely descendants at z = 0 have, on average, grown by a factor of 2 in mass and a factor of 4 in size, indicating rα Mα with α ≳2. They also have surface brightness profiles with n ≳ 5. This evolution can be qualitatively explained on the basis of two assumptions: compact ellipticals predominantly grow by collisionless minor (mass-ratio 1:10) or intermediate (mass-ratio 1:5) 'dry' mergers, and they are embedded in massive dark matter haloes which support the stripping of merging satellite stars at large radii. 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 rα Mα withα <2 for mass-ratios of 1:1 (and 1:5 without dark matter haloes) and, while doubling the stellar mass, the Śersic index increases from n ~ 4 to n ~ 5. For minor mergers of galaxies embedded in dark matter haloes, the sizes grow significantly faster and the profile shapes change more rapidly. Surprisingly, already mergers with moderate mass-ratios of 1:5, well motivated by recent cosmological simulations, give α ~ 2.3 and after only two merger generations (~40 per cent added stellar mass) the Śersic index has increased to n > 8 (n ~ 5.5 without dark matter), reaching a final value of n = 9.5 after doubling the stellar mass. This is accompanied by a significant increase of the dark matter fraction (from ~40 to ≳70 per cent) within the stellar half-mass radius, driven by the strong size increase probing larger, dark matter-dominated regions. For equal-mass mergers the effect is much weaker. We conclude that only a few intermediate mass-ratio mergers (~3-5 with initial mass-ratios of 1:5) of galaxies embedded in massive dark matter haloes can result in the observed concurrent inside-out growth and the rapid evolution in profile shapes. This process might explain the existence of present-day giant ellipticals with sizes, r > 4 kpc, high Śersic indices, n > 5, and a significant amount of dark matter within the half-light radius. Apart from negative stellar metallicity gradients such a 'minor' merger scenario also predicts significantly lower dark matter fractions for z ~ 2 compact quiescent galaxies and their rare present-day analogues. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.