How do galaxies acquire their mass?

¹ Laboratoire d’Astrophysique de Marseille, UMR 6110 CNRS - Univ. de Provence, 38 rue Frédréric Joliot-Curie, 13388 Marseille Cedex 13, France
e-mail: Andrea.Cattaneo@oamp.fr
² Centre de Recherche Astrophysique de Lyon, UMR 5574 CNRS - Univ. Claude Bernard - ENS Lyon, 9 av. Charles André, 69561 Saint Genis Laval Cedex, France
³ Astrophysikalisches Institut Potsdam, an der Sternwarte 16, 14482 Potsdam, Germany
⁴ Institut d’Astrophysique de Paris, UMR 7095 CNRS - UPMC, 98bis boulevard Arago, 75014 Paris, France
⁵ Astrophysics and BIPAC, Department of Physics, University of Oxford, Keble Rd, Oxford OX1 3RH, UK
⁶ Departamento de Fisica Teorica, Modulo C-XI, Universidad Autonoma de Madrid, 28049 Cantoblanco, Madrid, Spain

Received: 17 September 2010
Accepted: 9 June 2011

Abstract

We study the growth of galaxy masses, via gas accretion and galaxy mergers. We introduce a toy model that describes (in a single equation) how much baryonic mass is accreted and retained into galaxies as a function of halo mass and redshift. In our model, the evolution of the baryons differs from that of the dark matter because 1) gravitational shock heating and AGN jets suppress gas accretion mainly above a critical halo mass of M_shock ~ 10¹² M_⊙; 2) the intergalactic medium after reionisation is too hot for accretion onto haloes with circular velocities v_circ ≲ 40 km   s^-1; 3) stellar feedback drives gas out of haloes, mainly those with v_circ ≲ 120    km   s^-1. We run our model on the merger trees of the haloes and sub-haloes of a high-resolution dark matter cosmological simulation. The galaxy mass is taken as the maximum between the mass given by the toy model and the sum of the masses of its progenitors (reduced by tidal stripping). Designed to reproduce the present-day stellar mass function of galaxies, our model matches fairly well the evolution of the cosmic stellar density. It leads to the same z = 0 relation between central galaxy stellar and halo mass as the one found by abundance matching and also as that previously measured at high mass on SDSS centrals. Our model also predicts a bimodal distribution (centrals and satellites) of stellar masses for given halo mass, in very good agreement with SDSS observations. The relative importance of mergers depends strongly on stellar mass (more than on halo mass). Massive galaxies with m_stars > m_crit ~ Ω_b/Ω_mM_shock ~ 10¹¹ M_⊙ acquire most of their final mass through mergers (mostly major and gas-poor), as expected from our model’s shutdown of gas accretion at high halo masses. However, although our mass resolution should see the effects of mergers down to m_stars ≃ 10^10.6 h^-1 M_⊙, we find that mergers are rare for m_stars ≲ 10¹¹ h^-1 M_⊙. This is a consequence of the curvature of the stellar vs. halo mass relation set by the physical processes of our toy model and found with abundance matching. So gas accretion must be the dominant growth mechanism for intermediate and low mass galaxies, including dwarf ellipticals in clusters. The contribution of galaxy mergers terminating in haloes with mass M_halo < M_shock (thus presumably gas-rich) to the mass buildup of galaxies is small at all masses, but accounts for the bulk of the growth of ellipticals of intermediate mass (~10^10.5 h^-1 M_⊙), which we predict must be the typical mass of ULIRGs.