UV Luminosity Functions at redshifts z~4 to z~10: 10000 Galaxies from HST Legacy Fields (1403.4295v4)

Abstract: The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map out the evolution of the UV LF from z~10 to z~4. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at z~4, z~5, z~6, z~7, z~8, and z~10, respectively from the ~1000 arcmin*2 area probed. The selection of z~4-8 galaxies over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are apparent at z>=7. Our new LF determinations at z~4 and z~5 span a 6-mag baseline (-22.5 to -16 AB mag). These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of >L galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to 3.4 sigma significance for a steeper faint-end slope to the UV LF at z>4, with alpha evolving from alpha=-1.64+/-0.04 at z~4 to alpha=-2.06+/-0.13 at z~7 (and alpha = -2.02+/-0.23 at z~8), consistent with that expected from the evolution of the halo mass function. With our improved constraints at the bright end, we find less evolution in the characteristic luminosity M* over the redshift range z~4 to z~7; the observed evolution in the LF is now largely represented by changes in phi*. No evidence for a non-Schechter-like form to the z~4-8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio of halos ((1+z)**-1.5) provides a good representation of the observed evolution.

• The paper presents a comprehensive analysis of 10,000 galaxies from HST legacy fields to examine UV luminosity function evolution between z~4 and z~10.
• The study reveals that while the characteristic magnitude (M*) remains nearly constant, the faint-end slope steepens significantly from α = -1.64 at z~4 to α = -2.06 at z~7.
• The methodology minimizes cosmic variance through diverse field sampling, providing robust insights into early galaxy formation, AGN feedback, and dust extinction processes.

Overview of "UV Luminosity Functions at Redshifts $z\sim4$ to $z\sim10$: 10000 Galaxies from HST Legacy Fields"

This paper presents a comprehensive analysis of the ultraviolet (UV) luminosity functions (LFs) spanning redshifts from $z\sim4$ to $z\sim10$, using a sample of 10,000 galaxies selected from Hubble Space Telescope (HST) legacy fields. The primary focus is on assessing how the population of galaxies has evolved over time, as evidenced by changes in their LFs, which describe the number density of galaxies as a function of their UV luminosity.

Methodology

The paper utilizes deep HST observations from several key programs (CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES) that provide data across the optical to near-infrared spectrum, allowing for robust selection of high-redshift galaxies. The researchers apply new color criteria that leverage this full wavelength coverage, minimizing incompleteness and eliminating redshift gaps. The paper identifies galaxy candidates through their Lyman-break features, a method effective for selecting galaxies over a range of high redshifts.

Simulations were extensively used to estimate the completeness and contamination of the samples, with particular attention to maintaining accurate measurements of galaxy sizes and flux across various background noise levels.

Key Findings

1. Evolution of the LF: The paper finds significant build-up in the number densities and luminosities of galaxies from $z\sim8$ to $z\sim4$. The LF shows clear evidence for the increase in the number density of bright galaxies over time, indicating significant galaxy assembly during this epoch.
2. Characteristic Magnitude and Faint-End Slope: The analysis shows no significant evolution in the characteristic magnitude $M^*$ over the redshift range considered, with $M^*$ remaining approximately constant. However, there is a pronounced evolution in the faint-end slope $\alpha$, from $\alpha=-1.64\pm0.04$ at $z\sim4$ to $\alpha=-2.06\pm0.13$ at $z\sim7$. This steepening is consistent with theoretical expectations from halo mass function evolution.
3. Field-to-Field Variations: The paper harnesses the diverse fields to control for cosmic variance, estimating that results are robust to $\sim$10% due to the use of independent sightlines.
4. Implications for Galaxy Formation Models: The observed flattening of the LF at the bright end might suggest mechanisms such as AGN feedback or dust extinction playing significant roles. The limited evolution in $M^*$ suggests the cut-off is not solely due to cosmic variation but likely involves these processes.
5. Comparison to Previous Work: The present results indicate more luminous galaxies at high redshift than previously reported, highlighting the impact of the improved completeness and contamination corrections. Comparisons to earlier studies show broad agreement at the faint end but divergence at the bright end, attributed to methodological advances and the inclusion of larger data samples.

Theoretical and Practical Implications

The results have strong implications for the models of early galaxy formation and evolution. The evolving faint-end slope $\alpha$ supports models predicting an increased efficiency of star formation in lower mass halos at higher redshifts. The near constancy of $M^*$ places constraints on models of AGN feedback and dust evolution in early galaxies.

Future observations, particularly as part of the James Webb Space Telescope (JWST) and other upcoming facilities, will further refine these LFs by pushing the boundaries on faint and distant galaxy detections, offering deeper insights into the cosmic dawn and reionization period.

In summary, this paper presents a robust analysis of high-redshift galaxy evolution, supported by rich observational data and comprehensive simulations, which form a critical contribution to our understanding of the early universe.