A correlation predicting galaxies without dark matter

Abstract

The standard theory of galaxy formation predicts that all galaxies should contain dark matter, yet a handful of recently discovered galaxies appear to lack it, challenging our understanding of galaxy formation. We investigate whether such dark-matter deficient objects can be identified from their baryonic properties alone, analogously to the radial-acceleration relation, which tightly links baryon and dark matter distributions in spiral galaxies. Using a sample of ultra-diffuse and dwarf spheroidal galaxies -- systems whose baryonic properties resemble those of the confirmed dark-matter-deficient galaxies -- we systematically search for a formula to predict baryonic fractions from stellar mass, effective radius, distance to the host, and the host's baryonic mass. We find that baryonic fraction correlates most strongly with the gravitational acceleration expected from baryons alone, $a_\mathrm{bar}$, or equivalently, with mean surface brightness, following an approximately $a_\mathrm{bar}^{-1}$ dependence. This scaling resembles the radial-acceleration relation but differs in functional form and applies to a different galaxy population. Strikingly, the dark-matter-deficient galaxies occupy the extreme end of the correlation. This suggests that they result from standard formation processes operating at unusual intensities rather than from exotic mechanisms. Importantly, the correlation predicts that all ultra-diffuse galaxies brighter than approximately 25 mag arcsec$^{-2}$ in the $g$-band should have very low dark matter content, offering a straightforward observational criterion for identifying these rare objects.