The Evolution of the Galaxy Rest-Frame Ultraviolet Luminosity Function Over the First Two Billion Years (1410.5439v2)

Abstract: We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity function at z=4-8. We use deep Hubble Space Telescope imaging over the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Year 1 Hubble Frontier Field deep parallel observations. These surveys provides an effective volume of 0.6-1.2 x 10^6 Mpc^3 over this epoch, allowing us to perform a robust search for faint (M_UV=-18) and bright (M_UV < -21) galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 galaxies at 3.5<z\<8.5, with \>1000 galaxies at z~6-8. We measure the luminosity function using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end our results agree with previous studies, yet we find a higher abundance of UV-bright galaxies at z>6, with M* ~ -21 at z>5, different than that inferred based on previous trends at lower redshift. At z=8, a single power-law provides an equally good fit to the UV luminosity function, while at z=6 and 7, an exponential cutoff at the bright-end is moderately preferred. We compare to semi-analytical models, and find that the lack of evolution in M* is consistent with models where the impact of dust attenuation on the bright-end of the luminosity function decreases at higher redshift. We measure the evolution of the cosmic star-formation rate density, correcting for dust attenuation, and find that it declines as (1+z)^-4.3 +/- 0.5 at z>4, consistent with observations at z>9. Our observations are consistent with a reionization history that starts at z>10, completes at z>6, and reaches a midpoint (x_HII = 0.5) at 6.7<z<9.4. Finally, our observations predict that the abundance of bright z=9 galaxies is likely higher than previous constraints, though consistent with recent estimates of bright z~10 galaxies. [abridged]

• The paper demonstrates that the UV characteristic magnitude (M*_UV) remains nearly constant at -21 for z≥5, challenging previous dimming trends.
• It employs a large HST galaxy sample with advanced photometric redshift techniques and MCMC-driven Schechter and SWML methods to derive robust luminosity functions.
• The study finds a cosmic SFR density decline proportional to (1+z)^-4.3, offering key insights into galaxy-driven reionization processes.

Analysis of the Evolution of the Galaxy Rest-Frame Ultraviolet Luminosity Function Over the First Two Billion Years

This paper presents a comprehensive analysis of the rest-frame ultraviolet (UV) luminosity functions for galaxies at redshifts $z = 4$ to $8$, using data from the Hubble Space Telescope (HST), notably from the CANDELS/GOODS fields and other deep-field observations. The paper employs a sample of 7446 candidate galaxies to examine the evolution of the luminosity function over a significant span of cosmic time, approximately the first two billion years after the Big Bang.

Key Findings and Methodology

1. Sample Selection and Data Sources:
   • The analysis is based on a large dataset from the HST, including the CANDELS/GOODS fields, the Hubble Ultra Deep Field, and Hubble Frontier Field observations. This substantial volume allows the identification of both faint (M$_\mathrm{UV} = -18$) and bright (M$_\mathrm{UV} < -21$) high-redshift galaxies, thus providing a robust basis for investigating the rest-frame UV luminosity functions.
2. Photometric Redshift Technique:
   • The galaxies were selected using a photometric redshift technique, which was cross-validated against existing spectroscopic redshifts. This approach ensures a high-confidence sample with a low estimated contamination rate, suggesting the derived galaxy samples are reliable.
3. Luminosity Function Measurements:
   • The paper utilizes both a parametric Schechter function and a non-parametric stepwise maximum likelihood (SWML) approach to derive the luminosity functions. The use of Markov Chain Monte Carlo (MCMC) methods facilitates the estimation of robust uncertainties in the derived parameters.
   • Surprisingly, the characteristic magnitude ($M^*_\mathrm{UV}$) shows minimal evolution, remaining consistent with $-21$ at $z \geq 5$. This result contrasts with previous work suggesting a dimming at higher redshifts.
4. Implications on Dust and Feedback:
   • The observed lack of evolution in $M^*_\mathrm{UV}$ aligns with models indicating reduced dust attenuation effects at higher redshifts. However, the possibility of a decreasing impact of feedback mechanisms also cannot be ruled out.
5. Star-Formation Rate (SFR) Density:
   • The cosmic SFR density is calculated by integrating the observed luminosity functions down to a limiting magnitude, then correcting for dust attenuation. The SFR density exhibits a decline proportional to $(1 + z)^{-4.3 \pm 0.5}$ at $z > 4$, which is consistent with observations extending to $z \geq 9$.
6. Implications for Reionization:
   • The observed galaxy luminosity functions support a reionization history suggesting the universe became fully ionized by $z \sim 6$, with a midpoint at redshifts between 6.7 and 9.4. The analysis indicates that galaxies, even those below the current detection threshold, play a critical role in the reionization process.

Future Directions

The paper underscores the importance of future wide-field surveys and improved datasets, such as those anticipated from the James Webb Space Telescope (JWST), to refine these measurements, particularly at the faint end and at higher redshifts. The evolution of the galaxy population, as characterized by the UV luminosity function, provides crucial insight into the underlying physical processes dictating star formation and feedback over cosmic time. Addressing the role of dust and contrasting theoretical models with empirical findings will remain a critical avenue of inquiry. Additionally, the results pose significant implications for understanding the sources and timeline of cosmic reionization, thus contributing to broader cosmological models and narratives.

In conclusion, by providing a nuanced understanding of the UV luminosity function and its evolution, this research offers essential insights into the early universe's structure and continua, illuminating the pathways of galaxy formation during the universe's formative epochs.