What Shapes the Galaxy Mass Function? Exploring the Roles of Supernova-Driven Winds and AGN

TL;DR

This study uses a phenomenological model to explore how supernova and AGN feedback influence the galaxy stellar mass function, highlighting the importance of feedback strength and halo dependence in galaxy formation.

Contribution

It introduces a new feedback model with explicit gas recapture and compares different wind scaling laws, providing insights into galaxy formation processes.

Findings

Modest wind speed with high mass loading fits the flat SMF

AGN feedback improves agreement with observed SMF above 10^9 h^-1 M_sun

Expulsion models predict different galaxy assembly histories and star formation trends

Abstract

The observed stellar mass function (SMF) is very different to the halo mass function predicted by Lambda-CDM, and it is widely accepted that this is due to energy feedback from supernovae and black holes. However, the strength and form of this feedback is not understood. In this paper, we use the phenomenological model GALFORM to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on 'expulsion' models in which the wind mass loading, beta, is proportional to 1/\vdisk^n, with n=0,1,2 and contrast these models with the successful Bower et al.\ 2008 model (B8W7). A crucial development is that our code explicitly accounts for the recapture of expelled gas as the system's halo mass (and thus gravitational potential) increases. We find that a model with modest wind speed but high mass loading matches the flat portion of the SMF. When combined with AGN feedback, the model provides a good description of the observed SMF above 10^9 h^-1 Msol. However, in the expulsion models, the brightest galaxies are assembled more recently than in B8W7, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models also tend to have a cosmic star formation density that is dominated by lower mass galaxies at z=1-3, and dominated high mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper give us important physical insight to the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed universe.