How Invisible Stellar Halos Bias Our Understanding of Ultrafaint Galaxies

We explore how a realistic surface brightness (SB) detection limit of μ_V ≈ 32.5 mag arcsec⁻² for stars at the edges of ultrafaint galaxies affects our ability to infer their underlying properties. We use a sample of 19 galaxies with stellar masses ≈400–40,000 M_⊙simulated with FIRE-2 physics and baryonic mass resolution of 30 M_⊙. The SB cut leads to smaller sizes, lower stellar masses, and lower stellar velocity dispersions than the values inferred without the cut. However, by imposing this realistic limit, our inferred galaxy properties lie closer to observed populations in the mass-size plane, better match observed velocity dispersions as a function of stellar mass, and better reproduce derived circular velocities as a function of half-light radius. For the most massive galaxies in our sample, the SB cut leads to higher mean [Fe/H] values, but the increase is not enough to match the observed MZR. Finally, we demonstrate that the common J. Wolf et al. dynamical mass estimator is less accurate when the SB cut is applied. For our lowest-mass galaxies, in particular, excluding the low-surface brightness outskirts causes us to overestimate their central dark-matter densities and virial masses. This suggests that attempts to use mass estimates of ultrafaint galaxies to constrain dark-matter physics or to place constraints on the low-mass threshold of galaxy formation must take into account surface brightness limits or risk significant biases.

C.W. acknowledges support from the Cal Poly Pomona Provost’s Teacher-Scholar Program and would like to give special thanks to Matt Povich for his unwavering support, mentorship, and belief throughout her faculty career and this paper. C.W. would also like to thank Eric Andersson and Martin Rey for providing their data, and Willem Geyskens for conversations that partially inspired this work. The authors also thank the anonymous referee for their thoughtful comments on the manuscript that significantly strengthened the paper. M.K.R.W. acknowledges support from NSF MPS Ascending Faculty Catalyst Award AST-2444751. F.J.M. is funded by the National Science Foundation (NSF) Award AST-2316748. J.S.B. is supported by NSF grant AST-2408247 and NASA grant 80NSSC22K0827. M.B.K. acknowledges support from NSF grants AST-1910346, AST-2108962, and AST-2408247; NASA grant 80NSSC22K0827; HST-GO-16686, HST-AR-17028, HST-AR-17043, JWST-GO-03788, and JWST-AR-06278 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555; and from the Samuel T. and Fern Yanagisawa Regents Professorship in Astronomy at UT Austin. S.R.L. acknowledges support from NSF grant AST-2109234 and HST grant AR-16624 from STScI. All authors would like to thank all of the amazing people who make our work possible: the cleaning and the clerical staff, the food service workers, the technical support personnel, and so many more.