Size evolution of spheroids in a hierarchical Universe

Shankar, Francesco, Marulli, Federico, Bernardi, Mariangela, Mei, Simona, Meert, Alan and Vikram, Vinu (2013) Size evolution of spheroids in a hierarchical Universe. Monthly Notices of the Royal Astronomical Society, 428 (1), 109-128. (doi:10.1093/mnras/sts001).

Abstract

Unveiling the structural evolution of spheroids, and in particular the origin of the tight size–stellar mass relation, has become one of the hottest topics in cosmology in the last years and it is still largely debated. To this purpose, we present and discuss basic predictions of an updated version of the latest release of the Munich semi-analytic hierarchical galaxy formation model that grows bulges via mergers and disc instabilities. We find that while spheroids below a characteristic mass Ms ~ 10¹¹ M_sun grow their sizes via a mixture of disc instability and mergers, galaxies above it mainly evolve via dry mergers. Including gas dissipation in major mergers efficiently shrinks galaxies, especially those with final mass Ms ≤ 10¹¹ M_sun that are the most gas-rich, improving the match with different observables. We find that the predicted scatter in sizes at fixed stellar mass is still larger than the observed one by up to ~ 40 %. Spheroids are, on average, more compact at higher redshifts at fixed stellar mass, and at fixed redshift and stellar mass larger galaxies tend to be more star forming. More specifically, while for bulge-dominated galaxies the model envisages a nearly mass-independent decrease in sizes, the predicted size evolution for intermediate-mass galaxies is more complex. The z = 2 progenitors of massive galaxies with M_star ~ (1–2) × 10¹¹ M_sun and B/T > 0.7 at z = 0 are found to be mostly disc-dominated galaxies with a median B/T ~ 0.3, with only ~ 20 per cent remaining bulge-dominated. The model also predicts that central spheroids living in more massive haloes tend to have larger sizes at fixed stellar mass. Including host halo mass dependence in computing velocity dispersions allows the model to properly reproduce the correlations with stellar mass. We also discuss the Fundamental Plane, the correlations with galaxy age, the structural properties of pseudo-bulges and the correlations with central black holes.

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