Scaling Relations for Galaxies Prior to Reionization

Chen, P., Wise, J.H., Norman, M.L., Xu, H., & O’Shea, B.W., 2014, ApJ, 795, 144.

Click to Read Full Article

View Article

Summary

Abstract

Chen and collaborators use the Rarepeak simulation to study relationships between the dark matter, gas, stars, and metals in more than 3,300 galaxies at redshifts z ≥ 15. They find an important change near the atomic cooling limit: smaller halos form stars slowly and are strongly affected by stellar feedback, while larger halos retain more gas and form stars more efficiently. The results provide scaling relations that can be used in models of early galaxy formation, chemical enrichment, and reionization.

1. Introduction

  • The first galaxies were small and faint, but they became the building blocks of later galaxies and contributed to cosmic reionization.
  • Radiation and supernovae from early stars could remove gas from small halos and temporarily stop further star formation.
  • The paper studies how the gas, stellar mass, star-formation rate, and metallicity of early galaxies depend on their halo properties.

2. Simulation Setup

  • The authors analyze the Rarepeak cosmological simulation, which follows Population III and metal-enriched stars along with their radiation and supernova feedback.
  • The simulated region contains 3,338 resolved halos at z = 15, with halo masses extending above 109 solar masses.
  • Data from redshifts 18.4 to 15 are used to measure relationships between halo mass, gas mass, stellar mass, accretion, star formation, and metallicity.

3. Results

  • Small halos often lose gas through radiation and supernova feedback, while larger halos retain and accrete gas more efficiently.
  • Star formation becomes much more efficient near a halo mass of 108 solar masses, where atomic hydrogen cooling becomes effective.
  • Metallicity does not simply increase with halo mass: early supernovae enrich small galaxies, later gas inflows dilute them, and sustained star formation enriches the larger galaxies again.

4. Discussion and Conclusions

  • The atomic cooling limit separates small, feedback-sensitive galaxies from larger galaxies with more stable and efficient star formation.
  • The first supernovae can enrich galaxies well above their original Population III metallicity before new metal-poor gas dilutes them.
  • These scaling relations can improve models of early galaxy formation, reionization, and the chemical evolution of present-day dwarf galaxies.

Best Figures

Figure 5
Relationship between stellar mass and dark matter halo mass in early galaxies
Stellar mass increases with halo mass, with star formation becoming more efficient as halos approach and pass the atomic cooling limit.
Figure 7
Gas and stellar metallicity shown as functions of halo mass
Metallicity first decreases as fresh, metal-poor gas dilutes early supernova enrichment, then rises in larger halos as sustained star formation produces more metals.