Discerning Galaxies from Brown Dwarfs in JWST/MIRI Coronagraphy Using Color–Color Diagrams

Abstract JWST’s Mid-Infrared Instrument (MIRI) features observing modes covering a wavelength range of ∼5–28 μ m. Particularly, its coronagraphic modes promise to expand exoplanet and brown dwarf direct imaging by providing unparalleled sensitivity to substellar objects, and its wide-field imager allows galactic and extragalactic surveys to complement those conducted at shorter wavelengths. MIRI is sensitive to both substellar objects and faint, unresolved galaxies, necessitating a method for discerning the two in MIRI data. This investigation aims to determine if color comparison is a viable method to determine whether an unresolved point source is more likely to be a substellar object or a galaxy. We model populations of galaxies and substellar objects and compare them in color–color space. We find that some of the coldest substellar objects overlap with galaxy populations, but varying substellar atmospheric properties and galaxy types produce parameter spaces where objects are better differentiated.