Journal Article

Star-forming galaxies at <i>z</i>≈ 8–9 from <i>Hubble Space Telescope</i>/WFC3: implications for reionization

Silvio Lorenzoni, Andrew J. Bunker, Stephen M. Wilkins, Elizabeth R. Stanway, Matt J. Jarvis and Joseph Caruana

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 414, issue 2, pages 1455-1466
Published in print June 2011 | ISSN: 0035-8711
Published online June 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18479.x
Star-forming galaxies at z≈ 8–9 from Hubble Space Telescope/WFC3: implications for reionization

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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 (YJ) colours (the ‘Y-drops’) on account of the Lyman α forest absorption, and with (JH) colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift z≈ 8–9 (15 are robust and a further nine more marginal but consistent with being high redshift) over an area of ≈50 arcmin2. 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≈ 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 galaxies below our detection limit provide a significant UV contribution. The faint-end slope, α, is not well constrained. However, adopting a similar faint-end slope to that determined at z= 3–6 (α=−1.7) and a Salpeter initial mass function (IMF), then the ionizing photon budget still falls short if fesc < 0.5, even integrating down to MUV=−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.

Keywords: galaxies: evolution; galaxies: formation; galaxies: high-redshift; galaxies: starburst; ultraviolet: galaxies

Journal Article.  10026 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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