We present new Hubble Space Telescope/Wide Field Camera 3 imaging of 25 extremely luminous (−23.2 ≤ M UV −21.2) Lyman-break galaxies (LBGs) at z 7. The sample was initially selected from 1.65 deg 2 of ground-based imaging in the UltraV-ISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman-α emitters, 'Himiko' and 'CR7'. A deconfusion analysis of the deep Spitzer photometry available suggests that these galaxies exhibit strong rest-frame optical nebular emission lines (EW 0 (Hβ + [OIII]) > 600Å). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common ( f multi > 0.4) in bright z 7 galaxies, and ubiquitous at the very bright end (M UV < −22.5). The galaxies have half-light radii in the range r 1/2 ∼ 0.5-3 kpc. The size measurements provide the first determination of the size-luminosity relation at z 7 that extends to M UV ∼ −23. We find the relation to be steep with r 1/2 ∝ L 1/2 . Excluding clumpy, multi-component galaxies however, we find a shallower relation that implies an increased star-formation rate surface density in bright LBGs. Using the new, independent, HST /WFC3 data we confirm that the rest-frame UV luminosity function at z 7 favours a power-law decline at the bright-end, compared to an exponential Schechter function drop-off. Finally, these results have important implications for the Euclid mission, which we predict will detect > 1000 similarly bright galaxies at z 7. Our new HST imaging suggests that the vast majority of these galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf star contamination.

We utilize deep near-infrared survey data from the UltraVISTA fourth data release (DR4) and the VIDEO survey, in combination with overlapping optical and Spitzer data, to search for bright star-forming galaxies at z 7.5. Using a full photometric redshift fitting analysis applied to the ∼ 6 deg 2 of imaging searched, we find 27 Lyman-break galaxies (LBGs), including 20 new sources, with best-fitting photometric redshifts in the range 7.4 < z < 9.1. From this sample we derive the rest-frame UV luminosity function (LF) at z = 8 and z = 9 out to extremely bright UV magnitudes (M UV −23) for the first time. We find an excess in the number density of bright galaxies in comparison to the typically assumed Schechter functional form derived from fainter samples. Combined with previous studies at lower redshift, our results show that there is little evolution in the number density of very bright (M UV ∼ −23) LBGs between z 5 and z 9. The tentative detection of an LBG with best-fit photometric redshift of z = 10.9 ± 1.0 in our data is consistent with the derived evolution. We show that a double power-law fit with a brightening characteristic magnitude (∆M * /∆z −0.5) and a steadily steepening bright-end slope (∆β/∆z −0.5) provides a good description of the z > 5 data over a wide range in absolute UV magnitude (−23 < M UV < −17). We postulate that the observed evolution can be explained by a lack of mass quenching at very high redshifts in combination with increasing dust obscuration within the first ∼ 1 Gyr of galaxy evolution.

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