The bright end of the galaxy luminosity function at $z\simeq 7$ from the VISTA VIDEO survey

Abstract

We have conducted a search for $z\simeq 7$ Lyman break galaxies over 8.2 square degrees of near-infrared imaging from the VISTA Deep Extragalactic Observations (VIDEO) survey in the XMM-Newton - Large Scale Structure (XMM-LSS) and the Extended Chandra Deep Field South (ECDF-S) fields. Candidate galaxies were selected from a full photometric redshift analysis down to a $Y+J$ depth of 25.3 ($5\sigma$), utilizing deep auxiliary optical and Spitzer/IRAC data to remove brown dwarf and red interloper galaxy contaminants. Our final sample consists of 28 candidate galaxies at $6.5\le z\le 7.5$ with $-23.5\le M_{\mathrm{UV}}\le -21.6$. We derive stellar masses of $9.1\le {\log }_{10}(M/M_{\odot })\le 10.9$ for the sample, suggesting that these candidates represent some of the most massive galaxies known at this epoch. We measure the rest-frame UV luminosity function (LF) at $z\simeq 7$, confirming previous findings of a gradual decline in number density at the bright-end ($M_{\mathrm{UV}}<-22$) that is well described by a double-power law (DPL). We show that quasar contamination in this magnitude range is expected to be minimal, in contrast to conclusions from recent pure-parallel Hubble studies. Our results are up to a factor of ten lower than previous determinations from optical-only ground-based studies at $M_{\mathrm{UV}}\lesssim -23$. We find that the inclusion of $YJH{K}_s$ photometry is vital for removing brown-dwarf contaminants, and $z\simeq 7$ samples based on red-optical data alone could be highly contaminated ($\gtrsim 50$ per cent). In comparison with other robust $z>5$ samples, our results further support little evolution in the very bright-end of the rest-frame UV LF from $z=5-10$, potentially signalling a lack of mass quenching and/or dust obscuration in the most massive galaxies in the first Gyr.

15 pages, 6 figures, 5 tables (plus additional figures/tables in Appendix). Accepted for publication in MNRAS

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