Michael Hilz, Thorsten Naab, Jeremiah P. Ostriker
There is observational evidence for inside-out growth of giant elliptical galaxies since $z \gtrsim 2-3$. Many of the $\sim 10^{11} M_{\odot}$ systems at high redshift have small sizes $\sim 1kpc$ 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. 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 '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. For minor mergers of galaxies embedded in dark matter halos, the sizes grow significantly faster and the profile shapes change more rapidly than for major mergers. After only two 1:5 mergers the Sersic index has increased to $n > 8$, reaching a final value of $n = 9.5$ after doubling the stellar mass. This is accompanied by a significant increase ($\gtrsim 80$ per cent) of the dark matter fraction within the half-mass radius, driven by the strong size increase probing larger, dark matter dominated regions. We conclude that only a few minor mergers ($\sim 3-5$ with 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. Apart from negative stellar metallicity gradients and, eventually, positive age 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 (abbreviated).
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http://arxiv.org/abs/1206.5004
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