The Turndown of the Baryonic Tully-Fisher Relation and Changing Baryon Fraction at Low Galaxy Masses

The ratio of baryonic-to-dark matter in present-day galaxies constrains galaxy formation theories and can be determined empirically via the baryonic Tully-Fisher relation (BTFR), which compares a galaxy’s baryonic mass (M _bary) to its maximum rotation velocity (V _max). The BTFR is well determined at M _bary > 10⁸ M _⊙, but poorly constrained at lower masses due to small samples and the challenges of measuring rotation velocities in this regime. For 25 galaxies with high-quality data and M _bary ≲ 10⁸ M _⊙, we estimate M _bary from infrared and H i observations and V _max from the H i gas rotation. Many of the V _max values are lower limits because the velocities are still rising at the edge of the detected H i disks (R _max); consequently, most of our sample has lower velocities than expected from extrapolations of the BTFR at higher masses. To estimate V _max, we map each galaxy to a dark matter halo assuming density profiles with and without cores. In contrast to noncored profiles, we find the cored profile rotation curves are still rising at R _max values, similar to the data. When we compare the V _max values derived from the cored density profiles to our M _bary measurements, we find a turndown of the BTFR at low masses that is consistent with Λ cold dark matter predictions and implies baryon fractions of 1%-10% of the cosmic value.