# The dark nemesis of galaxy formation: why hot haloes trigger black hole growth and bring star formation to an end

Bower, R, Schaye, J, Frenk, CS, Theuns, T, Schaller, M, Crain, RA and McAlpine, S (2017) The dark nemesis of galaxy formation: why hot haloes trigger black hole growth and bring star formation to an end. Monthly Notices of the Royal Astronomical Society, 465 (1). pp. 32-44. ISSN 0035-8711

## Abstract

Galaxies fall into two clearly distinct types: blue-sequence' galaxies that are rapidly forming young stars, and red-sequence' galaxies in which star formation has almost completely ceased. Most galaxies more massive than $3\times10^{10} M_\odot$ follow the red-sequence while less massive central galaxies lie on the blue sequence. We show that these sequences are created by a competition between star formation-driven outflows and gas accretion on to the supermassive black hole at the galaxy's center. We develop a simple analytic model for this interaction. In galaxies less massive than $3\times10^{10} M_\odot$, young stars and supernovae drive a high entropy outflow that is more buoyant that any diffuse corona. The outflow balances the rate of gas inflow, preventing high gas densities building up in the central regions. More massive galaxies, however, are surrounded by a hot corona. We argue that above a halo mass of $\sim 10^{12} M_\odot$, the supernova-driven outflow is no longer buoyant and star formation is unable to prevent the build up of gas in the central regions. This triggers a strongly non-linear response from the black hole. Its accretion rate rises rapidly, heating the galaxy's corona, disrupting the incoming supply of cool gas and starving the galaxy of the fuel for star formation. The host galaxy makes a transition to the red sequence, and further growth predominantly occurs through galaxy mergers. We show that the analytic model provides a good description of galaxy evolution in the EAGLE hydrodynamic simulations, and demonstrate that, so long as star formation-driven outflows are present, the transition mass scale is almost independent of subgrid parameter choice. The transition mass disappears entirely, however, if star formation driven outflows are absent.

Item Type: Article This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. astro-ph.GA; astro-ph.GA Q Science > QB AstronomyQ Science > QC Physics Astrophysics Research Institute Oxford University Press Author 06 Dec 2016 10:52 06 Sep 2017 13:55 10.1093/mnras/stw2735 http://researchonline.ljmu.ac.uk/id/eprint/5027