The impact of feedback on disc galaxy scaling relations

Abstract

We use a disc galaxy evolution model to investigate the impact of mass outflows (a.k.a. feedback) on disc galaxy scaling relations, mass fractions and spin parameters. Our model follows the accretion, cooling, star formation and ejection of baryonic mass inside growing dark matter haloes, with cosmologically motivated angular momentum distributions, and dark matter halo structure. Models without feedback produce discs that are too small, too gas-poor and which rotate too fast. Feedback reduces the galaxy formation efficiency ∈ GF (defined as the fraction of the universally available baryons that end up as stars and cold gas in a given galaxy), resulting in larger discs with higher gas fractions and lower rotation velocities. Models with feedback can reproduce the zero-points of the scaling relations among rotation velocity, stellar mass and disc size, but only in the absence of adiabatic contraction. Our feedback mechanism is maximally efficient in expelling mass, but our successful models require 25 per cent of the supernova (SN) energy or 100 per cent of the SN momentum to drive an outflow. It remains to be seen whether such high efficiencies are realistic or not. Our energy- and momentum-driven wind models result in different slopes of various scaling relations. Energy-driven winds result in steeper slopes to the galaxy-mass―halo-mass and stellar-mass-halo-mass relations, a shallower slope to the galaxy-size-stellar-mass relation at z = 0 and a steeper slope to the cold gas metallicity-stellar-mass relation at z ≃ 2. Observations favour the energy-driven wind at stellar masses below M star ≤ 10 10.5 M ⊙ , but the momentum-driven wind model at high masses. The ratio between the specific angular momentum of the baryons to that of the halo (j gal /m gal ) is not unity in our models with inflow and outflow. Yet this is the standard assumption in models of disc formation. Above a halo mass of M vir ≃ 10 12 M ⊙ , cooling becomes increasingly inefficient, which results in (j gal /m gal ) decreasing with increasing halo mass. Below a halo mass of M vir ≃ 10 12 M ⊙ , feedback becomes increasingly efficient. Feedback preferentially ejects low-angular-momentum material because star formation is more efficient at smaller galactic radii and at higher redshifts. This results in (j gal /m gal ) increasing with decreasing halo mass. This effect helps to resolve the discrepancy between the high spin parameters observed for dwarf galaxies with the low spin parameters predicted from A cold dark matter.