Beasts in the Bubbles: Remarkably UV-bright Galaxies at z=9-10

Keck/MOSFIRE | 2N | PI - C. Casey

We propose Keck MOSFIRE J-band spectroscopy for four UV-bright (H=24.5–25.4 mag) novel photometric candidates at z=9-10 from the new COSMOS2020 catalogs, with deeper, homogeneous near-IR and ultra-deep optical coverage that allowed for the identification of such rare, extraordinary candidates. All candidates have (a) clear Lyman-break features from Y to H bands, (b) IRAC detection, (c) tight distributions in PDF(z) at z=9-10, (d) inconsistency with common high-z galaxy contaminants (low-mass stars and dusty lower-z galaxies), and (e) are estimated to sit at z>9 with four independent fits to their photometry. The volume density of these four candidates is also consistent with the bright-end of the latest UV luminosity function (UVLF) studies at z=9-10. With the proposed observations, we will achieve the following three major goals: 1) redshift confirmation and thus the power-law nature of the UVLF at z=9-10; 2) testing if UV-bright galaxies at z=9-10 reside in ionized bubbles; and 3) if they host AGN measured via the Nv1243 line. Importantly, our targets are > 10-20 times brighter than the only other source confirmed with line emission at these redshifts. The spectroscopic confirmation of our targets will provide essential constraints on the bright/massive-end of luminosity/mass function at z>9 and the cold dark matter formation of early, massive galaxies.