Star-Forming Galaxies at z ≈ 8 − 9 from HST/WFC3: Implications for Reionization

We present a search for galaxies at 7.6 < z < 9.8 using the latest HST WFC3 near-infrared data, based on the Lyman-break technique. We search for galaxies which have large (Y−J) colours (the “Y-drops”) on account of the Lyman-α forest absorption, and with (J −H) colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift z ≈ 8 − 9 (15 are robust and a further 9 more marginal but consistent with being high redshift) over an area of ≈ 50 square arcminutes. Previous searches for Y-drops with WFC3 have focussed only on the Hubble Ultra Deep Field (HUDF), and our larger survey (involving two other nearby deep fields and a wider area survey) has trebelled the number of robust Y-drop candidates. For the first time, we have sufficient z ≈ 8 − 9 galaxies to fit both φ ∗ and M ∗ of the UV Schechter luminosity function. There is evidence for evolution in this luminosity function from z = 6−7 to z = 8−9, in the sense that there are fewer UV-bright galaxies at z ≈ 8−9, consistent with an evolution mainly in M∗. The candidate z ≈ 8−9 galaxies we detect have insufficient ionizing flux to reionize the Universe, and it is probable that galaxie