The effects of X-ray and UV background radiation on the low-mass slope of the galaxy mass function

• Published in: Monthly notices of the Royal Astronomical Society Vol. 413; no. 4; pp. 2421 - 2428
• Main Authors: Hambrick, D. C., Ostriker, J. P., Johansson, P. H., Naab, T.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.06.2011; Wiley-Blackwell; Oxford University Press
• Subjects: Astronomy; Dark matter; Earth, ocean, space; Exact sciences and technology; galaxies: dwarf; galaxies: formation; methods: numerical; Star & galaxy formation; Stars & galaxies; Ultraviolet astronomy; X-ray astronomy; galaxies: dwarf; methods: numerical; galaxies: formation; Power spectra; Digital simulation; Numerical method; Critical mass; Ultraviolet radiation; Mass function; Star formation; Stellar mass; Dark matter; Galaxy formation; Dwarf galaxies; Hydrodynamic model; Background radiation; Stellar systems
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2011.18312.x

Even though the dark-matter power spectrum in the absence of biasing predicts a number density of haloes n(M) ∝M −2 (i.e. a Schechter α value of −2) at the low-mass end (M < 1010 M⊙), hydrodynamic simulations have typically produced values for stellar systems in good agreement with the observed value α≃−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 haloes 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 α depends primarily on the mean gas temperature at the star-formation epoch for low-mass systems (z≲ 3): with no background we find α≃−1.5, with UV only α≃−1.0 and with UV and X-rays α≃−0.75. We find the critical final halo mass for star formation to be ∼4 × 108 M⊙ with a UV background and ∼7 × 108 M⊙ with UV and X-rays.