New Constraints on the Faint-end of the UV Luminosity Function at z~7-8 using the Gravitational Lensing of the Hubble Frontier Fields Cluster A2744

TL;DR

This study uses gravitational lensing from the Hubble Frontier Fields to measure the faint-end of the UV luminosity function at redshifts 7-8, revealing a steep slope down to very faint luminosities and advancing understanding of early galaxy populations.

Contribution

First to demonstrate that the faint-end slope of the UV luminosity function remains steep at very faint luminosities at z~7-8 using lensing data from HFF clusters.

Findings

Extended the UV LF to M_UV ~ -15.5 at z~7

Found a steep faint-end slope of α ≈ -1.88

Confirmed the steep slope persists at very faint luminosities

Abstract

Exploiting the power of gravitational lensing, the Hubble Frontier Fields (HFF) program aims at observing six massive galaxy clusters to explore the distant Universe far beyond the depth limits of blank field surveys. Using the complete Hubble Space Telescope observations of the first HFF cluster Abell 2744, we report the detection of 50 galaxy candidates at $z \sim 7$ and eight candidates at $z \sim 8$ in a total survey area of 0.96 arcmin$^{2}$ in the source plane. Three of these galaxies are multiply-imaged by the lensing cluster. Using an updated model of the mass distribution in the cluster we were able to calculate the magnification factor and the effective survey volume for each galaxy in order to compute the ultraviolet galaxy luminosity function at both redshifts 7 and 8. Our new measurements extend the $z \sim 7$ UV LF down to an absolute magnitude of $M_{UV} \sim -15.5$. We find a characteristic magnitude of $M^{\star}_{UV}=-20.63^{+0.69}_{-0.56}$ mag and a faint-end slope $\alpha = -1.88^{+0.17}_{-0.20}$ close to previous determinations in blank fields. We show here for the first time that this slope remains steep down to very faint luminosities of 0.01$L^{\star}$. Although prone to large uncertainties, our results at $z \sim 8$ also seem to confirm a steep faint-end slope below 0.1$L^{\star}$. The HFF program is therefore providing an extremely efficient way to study the faintest galaxy populations at $z > 7$ that would otherwise be inaccessible with current instrumentation. The full sample of six galaxy clusters will provide yet better constraints on the build-up of galaxies at early epochs and their contribution to cosmic reionization.