The Effects of X-Ray and UV Background Radiation on the Low-Mass Slope of the Galaxy Mass Function

TL;DR

This paper investigates how X-ray and UV background radiation influence the low-mass end slope of the galaxy mass function, showing that external heating mechanisms flatten the slope and suppress star formation in low-mass halos.

Contribution

It introduces a physical model and simulation results demonstrating the impact of UV and X-ray backgrounds on low-mass galaxy formation and the resulting mass function slope.

Findings

UV and X-ray backgrounds flatten the low-mass slope from -1.5 to -0.75.

Critical halo mass for star formation increases with background radiation.

Simulations show the mean gas temperature at star formation epoch determines the slope.

Abstract

Even though the dark-matter power spectrum in the absence of biasing predicts a number density of halos n(M) ~ M^-2 (i.e. a Schechter alpha value of -2) at the low-mass end (M < 10^10 M_solar), hydrodynamic simulations have typically produced values for stellar systems in good agreement with the observed value alpha ~ -1. We explain this with a simple physical argument and show that an efficient external gas-heating mechanism (such as the UV background included in all hydro codes) will produce a critical halo mass below which halos cannot retain their gas and form stars. We test this conclusion with GADGET-2-based simulations using various UV backgrounds, and for the first time we also investigate the effect of an X-ray background. We show that at the present epoch alpha is depends primarily on the mean gas temperature at the star-formation epoch for low-mass systems (z <~ 3): with no background we find alpha ~ -1.5, with UV only alpha ~ -1.0, and with UV and X-rays alpha ~ -0.75. We find the critical final halo mass for star formation to be ~4x10^8 M_solar with a UV background and ~7x10^8 M_solar with UV and X-rays.