A lack of evolution in the very bright-end of the galaxy luminosity function from z = 8-10

Abstract

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 $z\gtrsim 7.5$. Using a full photometric redshift fitting analysis applied to the $\sim 6{\mathrm{deg}}^2$ 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 ($M_{\mathrm{UV}}\simeq -23$) 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 ($M_{\mathrm{UV}}\sim -23$) LBGs between $z\simeq 5$ and $z\simeq 9$. The tentative detection of an LBG with best-fit photometric redshift of $z=10.9\pm 1.0$ in our data is consistent with the derived evolution. We show that a double power-law fit with a brightening characteristic magnitude ($\Delta M^{\ast }/\Delta z\simeq -0.5$) and a steadily steepening bright-end slope ($\Delta \beta /\Delta z\simeq -0.5$) provides a good description of the $z>5$ data over a wide range in absolute UV magnitude ($-23<M_{\mathrm{UV}}<-17$). 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 $\sim 1\mathrm{Gyr}$ of galaxy evolution.

Updated to match MNRAS accepted version (accepted 27/01/2020) after minor revisions. 18 pages, 10 figures and 7 tables in the main text, plus additional figures in the appendix

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• The UV Luminosity Function of Protocluster Galaxies at $z\sim 4$: the Bright-end Excess and the Enhanced Star Formation Rate Density
• SuperBoRG: Search for The Brightest of Reionizing Galaxies and Quasars in HST Parallel Imaging Data
• The rapid transition from star-formation to AGN dominated rest-frame UV light at z ~ 4
• Augmenting machine learning photometric redshifts with Gaussian mixture models
• Inception of a first quasar at cosmic dawn
• Quasi-equilibrium models of high-redshift disc galaxy evolution
• Effects of self-consistent rest-ultraviolet colours in semi-empirical galaxy formation models
• MIGHTEE-HI: Discovery of an HI-rich galaxy group at z = 0.044 with MeerKAT