# HST Imaging of the Brightest z similar to 8-9 Galaxies from UltraVISTA: The Extreme Bright End of the UV Luminosity Function

Stefanon, M, Labbe, I, Bouwens, RJ, Brammer, GB, Oesch, P, Franx, M, Fynbo, JPU, Milvang-Jensen, B, Muzzin, A, Illingworth, GD, Le Fevre, O, Caputi, KI, Holwerda, BW, McCracken, HJ, Smit, R and Magee, Dan (2017) HST Imaging of the Brightest z similar to 8-9 Galaxies from UltraVISTA: The Extreme Bright End of the UV Luminosity Function. Astrophysical Journal, 851 (1). ISSN 0004-637X

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1706.04613v2.pdf - Accepted Version

We report on the discovery of three especially bright candidate ${z}_{\mathrm{phot}}\gtrsim 8$ galaxies. Five sources were targeted for follow-up with the Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3), selected from a larger sample of 16 bright ($24.8\lesssim H\lesssim 25.5$ mag) candidate $z\gtrsim 8$ Lyman break galaxies (LBGs) identified over 1.6 degrees2 of the COSMOS/UltraVISTA field. These were selected as Y and J dropouts by leveraging the deep (Y-to-${K}_{{\rm{S}}}\sim 25.3\mbox{--}24.8$ mag, $5\sigma$) NIR data from the UltraVISTA DR3 release, deep ground-based optical imaging from the CFHTLS and Suprime-Cam programs, and Spitzer/IRAC mosaics combining observations from the SMUVS and SPLASH programs. Through the refined spectral energy distributions, which now also include new HyperSuprimeCam g-, r-, i-, z-, and Y-band data, we confirm that 3/5 galaxies have robust ${z}_{\mathrm{phot}}\sim 8.0\mbox{--}8.7$, consistent with the initial selection. The remaining 2/5 galaxies have a nominal ${z}_{\mathrm{phot}}\sim 2$. However, with HST data alone, these objects have increased probability of being at $z\sim 9$. We measure mean UV continuum slopes $\beta =-1.74\pm 0.35$ for the three $z\sim 8\mbox{--}9$ galaxies, marginally bluer than similarly luminous $z\sim 4\mbox{--}6$ in CANDELS but consistent with previous measurements of similarly luminous galaxies at $z\sim 7$. The circularized effective radius for our brightest source is 0.9 ± 0.3 kpc, similar to previous measurements for a bright $z\sim 11$ galaxy and bright $z\sim 7$ galaxies. Finally, enlarging our sample to include the six brightest $z\sim 8$ LBGs identified over UltraVISTA (i.e., including three other sources from Labbé et al.) we estimate for the first time the volume density of galaxies at the extreme bright end (${M}_{\mathrm{UV}}\sim -22$ mag) of the $z\sim 8$ UV luminosity function. Despite this exceptional result, the still large statistical uncertainties do not allow us to discriminate between a Schechter and a double-power-law form.