z~7 Galaxies in the HUDF: First Epoch WFC3/IR Results

TL;DR

This paper reports the discovery of 16 z~7 galaxies in the HUDF using WFC3/IR, providing new insights into their properties, luminosity function, and implications for cosmic reionization.

Contribution

First epoch WFC3/IR data enabled the identification of faint z~7 galaxies and the determination of the luminosity function's faint end slope, advancing understanding of early galaxy populations.

Findings

Detected 16 robust z~7 galaxies with deep WFC3/IR imaging.

Measured a steep faint-end slope of the luminosity function, alpha=-1.77.

Faint galaxies could significantly contribute to reionization.

Abstract

We present a sample of 16 robust z~7 z-drop galaxies detected by the newly installed WFC3/IR camera on the Hubble Space Telescope. Our analysis is based on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep Field with 60 orbits of Y, J, and H observations. These remarkable data cover 4.7 arcmin^2 and are the deepest NIR images ever taken, reaching to ~29 mag AB (5 sigma). The 16 z~6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z-Y) vs. (Y-J) colors. They have magnitudes J = 26.0-29.0 (AB), an average apparent half-light radius of ~0.16 arcsec (<~1 kpc), and show very blue colors (some even beta<~-2.5), in particular at low luminosities. The WFC3/IR data confirms previous NICMOS detections indicating that the dropout selection at z~7 is very reliable. Our data allow a first determination of the faint end slope of the z~7 luminosity function, reaching down to M_UV ~ -18, a full magnitude fainter than previous measurements. When fixing phi*=1.4e-3/Mpc^3/mag to the value previously measured at z~6, we find a best-fit value of alpha=-1.77+-0.20, with a characteristic luminosity of M_*=-19.91+-0.09. This steep slope is similar to what is seen at z~2-6 and indicates that low luminosity galaxies could potentially provide adequate flux to reionize the universe. The remarkable depth and resolution of these new images provide insights into the coming power of JWST.