Templates for Fitting Photometry of Ultra-High-Redshift Galaxies

Recent data from the James Webb Space Telescope allow a first glimpse of galaxies at $z \gtrsim 11$. The most successful tool for identifying ultra-high-redshift candidates and inferring their properties is photometric template fitting. However, current methods rely on templates derived from much lower-redshift conditions, including stellar populations older than the age of the Universe at $z > 12$, a stellar initial mass function which is physically disallowed at $z > 6$, and weaker emission lines than currently observed at $z > 7.5$. Here, two sets of synthetic templates, optimized for the expected astrophysics of galaxies at $8 < z < 12$ and $z > 12$, are developed and used to fit three galaxies at $z > 12$ from the SMACS0723 field. Using these improved templates, quantitative estimates are produced of the bias in inferred properties from JWST observations at $z>8$ due to these effects. The best-fit redshifts are similar to those found with previous template sets, but the inferred stellar masses drop by as much as 1--1.6 dex, so that stellar masses are no longer seemingly inconsistent with $\Lambda$CDM. The two new template sets are released in formats compatible with EAZY and LePhare.