A Break in Spiral Galaxy Scaling Relations at the Upper Limit of Galaxy Mass

Super spirals are the most massive star-forming disk galaxies in the universe. We measured rotation curves for 23 massive spirals with the Southern African Large Telescope (SALT) and found a wide range of fast rotation speeds (240–570 km s−1), indicating enclosed dynamical masses of (0.6−4) × 1012M⊙. Super spirals with mass in stars $\mathrm{log}{M}_{\mathrm{stars}}/{M}_{\odot }\gt 11.5$ break from the baryonic Tully–Fisher relation (BTFR) established for lower-mass galaxies. The BTFR power-law index breaks from 3.75 ± 0.11 to 0.25 ± 0.41 above a rotation speed of ∼340 km s−1. Super spirals also have very high specific angular momenta that break from the Fall relation. These results indicate that super spirals are undermassive for their dark matter halos, limited to a mass in stars of $\mathrm{log}{M}_{\mathrm{stars}}/{M}_{\odot }\lt 11.8$. Most giant elliptical galaxies also obey this fundamental limit, which corresponds to a critical dark halo mass of $\mathrm{log}{M}_{\mathrm{halo}}/{M}_{\odot }\simeq 12.7$. Once a halo reaches this mass, its gas can no longer cool and collapse in a dynamical time. Super spirals survive today in halos as massive as $\mathrm{log}{M}_{\mathrm{halo}}/{M}_{\odot }\simeq 13.6$, continuing to form stars from the cold baryons they captured before their halos reached critical mass. The observed high-mass break in the BTFR is inconsistent with the Modified Newtonian Dynamics theory.