The role of mergers and halo spin in shaping galaxy morphology

• Published in: Monthly notices of the Royal Astronomical Society Vol. 467; no. 3; pp. 3083 - 3098
• Main Authors: Rodriguez-Gomez, Vicente, Sales, Laura V., Genel, Shy, Pillepich, Annalisa, Zjupa, Jolanta, Nelson, Dylan, Griffen, Brendan, Torrey, Paul, Snyder, Gregory F., Vogelsberger, Mark, Springel, Volker, Ma, Chung-Pei, Hernquist, Lars
• Format: Journal Article
• Language: English
• Published: Oxford University Press 21.05.2017
• Subjects: galaxies: formation; galaxies: structure; galaxies: kinematics and dynamics; methods: numerical; galaxies: interactions; galaxies: haloes
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx305

Abstract Mergers and the spin of the dark matter halo are factors traditionally believed to determine the morphology of galaxies within a Λ cold dark matter (ΛCDM) cosmology. We study this hypothesis by considering approximately 18 000 central galaxies at z = 0 with stellar masses M * = 109–1012 M⊙ selected from the Illustris cosmological hydrodynamic simulation. The fraction of accreted stars – which measures the importance of massive, recent and dry mergers – increases steeply with galaxy stellar mass, from less than 5 per cent in dwarfs to 80 per cent in the most massive objects, and the impact of mergers on galaxy morphology increases accordingly. For galaxies with M * ≳ 1011 M⊙, mergers have the expected effect: If gas-poor, they promote the formation of spheroidal galaxies, whereas gas-rich mergers favour the formation and survivability of massive discs. This trend, however, breaks at lower masses. For objects with M * ≲ 1011 M⊙, mergers do not seem to play any significant role in determining the morphology, with accreted stellar fractions and mean merger gas fractions that are indistinguishable between spheroidal and disc-dominated galaxies. On the other hand, halo spin correlates with morphology primarily in the least massive objects in the sample (M * ≲ 1010 M⊙), but only weakly for galaxies above that mass. Our results support a scenario where (1) mergers play a dominant role in shaping the morphology of massive galaxies, (2) halo spin is important for the morphology of dwarfs, and (3) the morphology of medium-sized galaxies – including the Milky Way – shows little dependence on galaxy assembly history or halo spin, at least when these two factors are considered individually.