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

There is observational evidence for inside-out growth of giant elliptical galaxies since z greater than or similar to 2-3, which is - in contrast to disc galaxies - not driven by in situ star formation. Many of the similar to 10(11) M-circle dot systems at high redshift have small sizes similar to 1 kpc and surface brightness profiles with low-Sersic 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 r proportional to M-alpha with alpha greater than or similar to 2. They also have surface brightness profiles with n greater than or similar to 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 r proportional to M-alpha with alpha < 2 for mass-ratios of 1:1 (and 1:5 without dark matter haloes) and, while doubling the stellar mass, the Sersic index increases from n similar to 4 to n similar to 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 alpha similar to 2.3 and after only two merger generations (similar to 40 per cent added stellar mass) the Sersic index has increased to n > 8 (n similar to 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 similar to 40 to greater than or similar 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 (similar to 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 Sersic 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 similar to 2 compact quiescent galaxies and their rare present-day analogues.