Probing the Ultraviolet Luminosity Function of the Earliest Galaxies with the Renaissance Simulations

O’Shea, B.W., Wise, J.H., Xu, H., & Norman, M.L., 2015, ApJL, 807, L12.

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Summary

Abstract

O’Shea and collaborators use the Renaissance Simulations to predict the ultraviolet luminosity function of galaxies at redshifts z = 8–25. The simulations agree with observations for bright galaxies, but predict far fewer faint galaxies than a standard luminosity function would suggest. This happens because small halos form stars less efficiently, and halos below about 7 × 106 solar masses do not contain stars at all.

1. Introduction

  • The faintest early galaxies are difficult to observe, but their abundance affects how many ionizing photons were available during reionization.
  • A steep luminosity function would suggest many faint galaxies, while a flatter function would mean that these galaxies were less common.
  • The paper uses simulations to predict the faint end and lower limit of the early galaxy ultraviolet luminosity function.

2. The Renaissance Simulations

  • The authors simulate three environments called the Rare peak, Normal, and Void regions to study the effects of cosmic environment.
  • The simulations include Population III and metal-enriched star formation, chemistry, cooling, radiation, and supernova feedback.
  • Together, the three regions contain hundreds of resolved galaxies between redshifts z = 25 and 8.

3. Results

  • The simulated luminosity function agrees with observations for bright galaxies but flattens for faint galaxies and ends near MUV ≈ −2.
  • Lower-mass halos form fewer stars and therefore produce less ultraviolet light than more massive halos.
  • All halos above about 2 × 108 solar masses recently formed stars, while no halos below about 7 × 106 solar masses contained stars.

4. Summary and Discussion

  • The flattening suggests that the early universe contained fewer extremely faint galaxies than simple fits to bright observations predict.
  • Star formation in small halos is limited by inefficient cooling, Lyman-Werner radiation, and gas loss caused by stellar feedback.
  • The brightest simulated galaxies may be visible in deep observations, especially when gravitational lensing magnifies them.

Best Figures

Figure 1
Ultraviolet luminosity functions from the Renaissance Simulations
The simulated luminosity function agrees with observations at the bright end but becomes flat at lower luminosities. The marked detection limits also show which galaxies could be observed by deep Hubble and JWST surveys.
Figure 3
Fraction of dark matter halos containing stars as a function of halo mass
The fraction of halos containing stars decreases rapidly at lower halo masses and reaches zero below about 7 × 106 solar masses, helping explain the faint-end flattening of the luminosity function.