There is observational evidence for inside-out growth of giant elliptical galaxies since z \gtrsim 2 - 3 , which is - in contrast to disk galaxies - not driven by in-situ star formation .
Many of the \sim 10 ^ { 11 } M _ { \odot } systems at high redshift have small sizes \sim 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 two in mass and a factor of four in size , indicating r \propto M ^ { \alpha } with \alpha \gtrsim 2 .
They also have surface brightness profiles with n \gtrsim 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 halos 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 \propto M ^ { \alpha } with \alpha < 2 for mass-ratios of 1:1 ( and 1:5 without dark matter halos ) and , while doubling the stellar mass , the Sersic index increases from n \sim 4 to n \sim 5 .
For minor mergers of galaxies embedded in dark matter halos , 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 \sim 2.3 and after only two merger generations ( \sim 40 per cent added stellar mass ) the Sersic index has increased to n > 8 ( n \sim 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 \sim 40 per cent to \gtrsim 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 ( \sim 3 - 5 with initial mass-ratios of 1:5 ) of galaxies embedded in massive dark matter halos 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 \sim 2 compact quiescent galaxies and their rare present day analogues .