From Red-Nuggets to Giant Ellipticals: Structural Evolution of Massive Galaxies

Despite tremendous recent observational progress at both high and low redshifts, there are still many open questions regarding the structural evolution of elliptical galaxies. A detailed examination of the photometric structure of the nearest and brightest elliptical galaxies can give useful clues to the dominant evolutionary pathway of these systems. Based on a detailed structural decomposition of a well-defined sample of 94 bright elliptical galaxies using the two-dimensional fitting code GALFIT, we find evidence that a large fraction of these local ellipticals contains three photometric subcomponents: a compact inner component with effective radius Re <1 kpc, an intermediate-scale middle component with Re ≈ 2.5 kpc, and an extended outer envelope with Re ≈ 10 kpc. We examine the impact of these results on the scaling relations of elliptical galaxies, and discuss their implications for different galaxy formation scenarios. Especially, we find that a hypothetical structure formed from combining the inner and middle components of local ellipticals follows a strikingly tight stellar mass-size relation, one that resembles the distribution of ETGs at z ≈ 1. Outside of the central kpc, the median stellar mass surface density profiles of this composite structure agree closest with those of massive galaxies that have similar cumulative number density at 1.5 < z < 2.0 within the uncertainty. We propose that the central substructures in nearby ellipticals are the evolutionary descendants of the "red nuggets" formed under highly dissipative ("wet") conditions at high redshifts, as envisioned in the initial stages of the two-phase formation scenario recently advocated for massive galaxies. Subsequent accretion, plausibly through non-dissipative ("dry") minor mergers, builds the outer regions of the galaxy identified as the outer envelope in our decomposition.