z~2-9 Galaxies Magnified by the Hubble Frontier Field Clusters II: Luminosity Functions and Constraints on a Faint-End Turnover

TL;DR

This study uses gravitational lensing from Hubble Frontier Field clusters to measure the faint-end of the UV luminosity functions of galaxies at redshifts 2 to 9, providing new constraints on galaxy evolution and star formation history.

Contribution

It presents the first consistent faint-end slope measurements from lensed and blank-field samples across z=2-9, extending to lower luminosities and constraining the shape of the UV luminosity function.

Findings

Faint-end slope alpha flattens from -2.28 at z=9 to -1.53 at z=2.

No evidence for a turnover brighter than -13 mag at z~3, -14.3 mag at z~6, or -15.5 mag at other redshifts.

Unobscured star formation dominates at z>~4, obscured star formation at lower redshifts.

Abstract

We present new determinations of the rest-UV luminosity functions (LFs) at z=2-9 to extremely low luminosities (>-14 mag) from a sample of >2500 lensed galaxies found behind the HFF clusters. For the first time, we present faint-end slope results from lensed samples that are fully consistent with blank-field results over the redshift range z=2-9, while reaching to much lower luminosities than possible from the blank-field studies. Combining the deep lensed sample with the large blank-field samples allows us to set the tight constraints on the faint-end slope alpha of the z=2-9 UV LFs and its evolution. We find a smooth flattening in alpha from -2.28+/-0.10 (z=9) to -1.53+/-0.03 (z=2) with cosmic time (d(alpha)/dz=-0.11+/-0.01), fully consistent with dark matter halo buildup. We utilize these new results to present new measurements of the evolution in the UV luminosity density rho(UV) brightward of -13 mag from z~9 to z~2. Accounting for the SFR densities to faint luminosities implied by our LF results, we find that unobscured star formation dominates the SFR density at z>~4, with obscured star formation dominant thereafter. Having shown we can quantify the faint-end slope alpha of the LF accurately with our lensed HFF samples, we also quantify the apparent curvature in the shape of the UV LF through a curvature parameter delta. The constraints on the curvature delta strongly rule out the presence of a turn-over brightward of -13.1 mag at z~3, -14.3 mag at z~6, and -15.5 mag at all other redshifts between z~9 to z~2.