Unveiling the nature of bright z ≃ 7 galaxies with the Hubble Space Telescope

We present new Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) imaging of 25 extremely luminous (-23.2 <= MUV less than or similar to -21.2) Lyman-break galaxies (LBGs) at z similar or equal to 7. The sample was initially selected from 1.65 deg(2) of ground-based imaging in the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman alpha 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 (EW0(H beta + [O III]) > 600 angstrom). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common (f(multi) > 0.4) in bright z similar or equal to 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) similar to 0.53 kpc. The size measurements provide the first determination of the size-luminosity relation at z similar or equal to 7 that extends to M-UV similar to -23. We find the relation to be steep with r(1/2) alpha L-1/2. Excluding clumpy, multicomponent 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 similar or equal to 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 similar or equal to 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.