Photometric Confirmation of the Brightest Known Galaxy Candidate at z > 9

We request two orbits of WFC3 imaging with the F098M filter to confirm the Lyman break and constrain the redshift of a remarkably bright H=24.5 galaxy in the CANDELS COSMOS field. Photometric redshift analyses including previous HST (V, I, J and H-band) and ground-based imaging overwhelmingly prefer a redshift of z 9.5. A galaxy this luminous (in the searched volume) so early in the universe sets tight constraints on the onset of star formation and AGN growth, thus even this single source would be a game changer (regardless of its ionizing source).

However, the inclusion of Spitzer/IRAC photometry complicates the picture. Two different deblending methods both successfully subtract bright neighboring galaxies, measuring significant flux at the position of this exceptional source at both 3.6 and 4.5 um. However, the [3.6]-[4.5] color is slightly different between these two methods, with one method measuring a slightly red color, consistent with z 9.5, and the other measuring a slightly blue color, preferring z 2. This systematic uncertainty in the deblending process prevents a concrete determination of the nature of this source.

An efficient two orbits of F098M imaging will break this redshift degeneracy. At this depth, if the galaxy is truly at z 2 we will detect it at 5-sigma significance, while a non-detection will rule out all low-redshift solutions, regardless of the IRAC photometry used, at >99.5% confidence. These observations are urgent, as once the nature of this source is known, deep spectroscopy can be pursued to measure the precise redshift, allowing detailed investigations with JWST Cycle 1 spectroscopy.