Star-forming galaxies at z approximate to 8-9 from Hubble Space Telescope/WFC3: implications for reionization
Bunker, Andrew J.
Wilkins, Stephen M.
Stanway, Elizabeth R.
We present a search for galaxies at 7.6 < z < 9.8 using the latest Hubble Space Telescope/Wide Field Camera 3 (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 alpha forest absorption, and with (J - H) colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift z approximate to 8-9 (15 are robust and a further nine more marginal but consistent with being high redshift) over an area of approximate to 50 arcmin(2). Previous searches for Y-drops with WFC3 have focused only on the Hubble Ultra Deep Field, and our larger survey (involving two other nearby deep fields and a wider area survey) has trebled the number of robust Y-drop candidates. For the first time, we have sufficient z approximate to 8-9 galaxies to fit 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 approximate to 8-9, consistent with an evolution mainly in M*. The candidate z approximate to 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 similar faint-end slope to that determined at z = 3-6 (alpha = -1.7) and a Salpeter initial mass function (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.
Published inMonthly Notices of the Royal Astronomical Society
RelationsSchool of Physics, Astronomy and Mathematics
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