The Influence of Metallicity on Star Formation in Protogalaxies

TL;DR

This study investigates how small amounts of metal enrichment in early protogalaxies influence gas cooling, collapse, and star formation, using high-resolution SPH simulations to identify a potential critical metallicity for star formation modes.

Contribution

The paper presents high-resolution simulations that explore the impact of low metallicity on gas cooling and fragmentation, identifying a possible threshold metallicity for forming a Salpeter-like initial mass function.

Findings

Metal line cooling has minimal effect at Z < 0.001 Z_sun for densities below 1 cm-3.

High-resolution simulations improve understanding of metal enrichment effects.

Potential critical metallicity for efficient fragmentation is identified.

Abstract

In cold dark matter cosmological models, the first stars to form are believed to do so within small protogalaxies. We wish to understand how the evolution of these early protogalaxies changes once the gas forming them has been enriched with small quantities of heavy elements, which are produced and dispersed into the intergalactic medium by the first supernovae. Our initial conditions represent protogalaxies forming within a fossil H II region, a previously ionized region that has not yet had time to cool and recombine. We study the influence of low levels of metal enrichment on the cooling and collapse of ionized gas in small protogalactic halos using three-dimensional, smoothed particle hydrodynamics (SPH) simulations that incorporate the effects of the appropriate chemical and thermal processes. Our previous simulations demonstrated that for metallicities Z < 0.001 Z_sun, metal line cooling alters the density and temperature evolution of the gas by less than 1% compared to the metal-free case at densities below 1 cm-3) and temperatures above 2000 K. Here, we present the results of high-resolution simulations using particle splitting to improve resolution in regions of interest. These simulations allow us to address the question of whether there is a critical metallicity above which fine structure cooling from metals allows efficient fragmentation to occur, producing an initial mass function (IMF) resembling the local Salpeter IMF, rather than only high-mass stars.