Size Evolution of Spheroids in a Hierarchical Universe

Details Size Evolution of Spheroids in a Hierarchical Universe Size Evolution of Spheroids in a Hierarchical Universe

2011-05-30

arxiv.org/abs/1105.6043v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

24 pages, 20 Figures. Submitted to MNRAS

Scientific paper

We present basic predictions of a detailed semi-analytic hierarchical galaxy formation model that grows bulges via mergers and disk instabilities. Overall, we find that while spheroids below Ms ~ 10^11 Msun grow their sizes via a mixture of disk instability and mergers, galaxies above it mainly evolve via mergers. The model produces a flattening of the size-mass relation below Ms at variance with the data. Including gas dissipation in major mergers, efficiently shrinks galaxies with final mass < 3*10^10 Msun reproducing the data. The scatter in sizes at fixed stellar mass is predicted to be only marginally larger than the observed one at the 10%-40% level. Galaxies that have mainly grown their bulge via disk instabilities are predicted to have systematically lower B/T and half-mass radiuses at fixed stellar mass, in good agreement with the measured structural properties of pseudobulges. Spheroids are more compact at higher redshifts at fixed stellar mass, and at fixed redshift and stellar mass larger galaxies tend to be more starforming. For B/T>0.7 a nearly mass-independent decrease in sizes is found. However, for galaxies below the characteristic mass Ms, the size evolution depends on the structure. The z=2 progenitors of ~ 10^11 Msun galaxies with B/T>0.7 at z=0, are mostly disk-dominated galaxies with a median B/T ~ 0.3, with only ~ 20% remaining bulge-dominated. A flat size-age relation at fixed stellar mass is also predicted at z=0. 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 reproduce the tilt of the fundamental plane, the correlations with stellar mass and central black holes, and their different redshift evolution. The tilt does not depend on galaxy age but it decreases with increasing redshift.

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