The Origin of the Morphology–Density Relation

Abstract

The history of a galaxy is encoded in its morphology: the angular momentum and dissipative properties of gas leads to the formation of a disk, within which stars form; mergers of galaxies and tidal interactions randomize stellar orbits, forming a bulge; and the presence or lack of a gas disk drives the level of disk star formation and (apparently) spiral features. The history of a galaxy is also encoded in its environment: sub-halos and their galaxies rarely merge, except at the bottom of the potential well (the centre of the main halo) in which they live; and gas disks and the hot gas which feeds them can only be stripped in a dense surrounding medium, found only in massive haloes (e.g. [Chung et al., ApJ 138, 1741 (2009), McCarthy et al., MNRAS 383, 593 (2008)]). To distinguish the mechanisms driving galaxy evolution, we examine the halo mass dependence of morphology separately for central and satellite galaxies at z ∼ 0. We place constraints on evolution utilizing a variety of physical prescriptions applied to semi-analytic models and compared to a SDSS-RC3 matched sample at z ∼ 0. We find ellipticals primarily at the centre of M_halo ≳ 10¹³ M_⊙ haloes, compatible with expectations of bulge growth in mergers; S0s and passive disk galaxies are created via two independent channels: as central galaxies in M_halo ≳ 10¹² M_⊙ haloes, with star formation suppressed possibly as a result of AGN feedback, and as satellite galaxies in M_halo ≳ 10¹³ M_⊙ haloes in which stripping of the hot gas associated to a galaxy is probably responsible for the suppression.