Assessing Ultra-Cool Dwarf Contamination in Photometrically Selected High-Redshift Galaxy Samples

Abstract

Ultra-cool dwarf stars (UCDs) are a common source of contamination in high-redshift galaxy searches because both populations have red colors and these early-forming galaxies can have sizes that are difficult to resolve even with space telescopes. We develop \textbf{F}oreground \textbf{C}ontamination \textbf{E}valuator of \textbf{N}earby dwarf stars in high-\textbf{Z} photometrically selected \textbf{O}bjects (FC-ENZO), a code that predicts magnitude-dependent UCD contamination for a given survey setup and galaxy selection method. FC-ENZO combines UCD number-density models, synthetic spectral libraries, and photometric selection criteria to estimate contaminant populations. We find that the predicted contamination is sensitive to the adopted UCD spectral model, stellar metallicity, and Galactic distribution models, with the Galactic distribution model producing variations of up to several factors. The Elf Owl models predict higher contamination than the Bobcat models, primarily because of their broader coverage of T and Y dwarf spectra. The dominant contaminants are late L to early Y-type UCDs, which are most commonly misclassified as galaxies at $z \sim 8$. For surveys with similar areas, broad-band observations from different space telescopes produce comparable numbers of contaminants at a given redshift, but contaminants are concentrated near the survey limiting magnitude, resulting in higher bright-end contamination fractions for shallower surveys. Adding medium-band filters can reduce the UCD contamination rate at $z\sim8$ by a factor of a few. At $z\gtrsim30$, F356W dropout selections can suffer substantial contamination from Y dwarfs. FC-ENZO provides a practical tool for survey design and for identifying fields suitable for spectroscopic follow-up.