JWST CAPERS: Spectroscopic evaluation of very high redshift galaxy candidates

JWST/NIRCam imaging has enabled efficient selection of a large number of galaxy candidates at very high redshifts. Accurate and robust selection is crucial for statistical analyses, such as measurement of the ultraviolet luminosity function (UV LF). A variety of selection methods and criteria have been proposed in the literature. We evaluate the selection methodology of galaxy candidates at $z\ge 8.5$ using 183 candidates with secure spectroscopic redshifts obtained from the CAPERS JWST/NIRSpec PRISM program. The success of high-$z$ confirmation is strongly correlated with the photometric signal-to-noise ratio (SNR) in the detection filter, the strength of the Lyman-$\alpha$ break, and the $\mathrm{\Delta }{\chi }^2$ between the best-fitting low-redshift and high-redshift spectral energy distribution model. However, the most efficient selection method is the use of a UV LF prior on the inferred $p(z)$ of a source, raising the success rate from $\simeq 35\mathrm{\%}$ to $\simeq 77\mathrm{\%}$ while achieving greater completeness than conservative SNR and $\mathrm{\Delta }{\chi }^2$ cuts. Out of the 183 candidates we spectroscopically confirm 48 sources at $z\ge 8.5$ via detections of emission lines and/or the Lyman-$\alpha$ break (particularly important for those with no emission line detection). We report the highest-redshift source in CAPERS as UDS-111893 at $z_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}}=12.98$, with no sources confirmed at $z>13$. This work provides spectroscopic quantification of JWST/NIRCam photometric selection at $z\ge 8.5$, and demonstrates the effectiveness of a UV LF prior for high-redshift photometric selection.