Candidate z~8-9 Galaxies from WFC3 Imaging
Lorenzoni, Silvio; Bunker, Andrew; Wilkins, Stephen; Stanway, Elizabeth; Jarvis, Matt; Caruana, Joseph
Abstract
We present a search for galaxies at 7.5<z<10 using the latest HST WFC3 near-infrared data, based on the Lyman-break technique. We search for galaxies which have large (Y-J) colours (Y-drops) on account of the Lyman-alpha forest absorption, and with (J-H) colours inconsistent with being low-redshift contaminants. We identify 22 candidates at redshift z~8-9 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 a both phi* 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 galaxies below our detection limit provide a significant UV contribution. The faint-end slope, alpha, is not well constrained. However, adopting a similiar faint-end slope to that determined at z=3-6 (alpha=-1.7) and a Salpeter IMF, then the ionizing photon budget still falls short if f_esc<0.5, even integrating down to M_UV=-8. A steeper faint end slope or a low-metallicity population (or a top-heavy IMF) might still provide sufficient photons for star-forming galaxies to reionize the Universe, but confirmation of this might have to await the James Webb Space Telescope.