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) × 10¹² M _⊙. Super spirals with mass in stars {log}{M}_stars}/{M}_⊙ > 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 {log}{M}_stars}/{M}_⊙ < 11.8. Most giant elliptical galaxies also obey this fundamental limit, which corresponds to a critical dark halo mass of {log}{M}_halo}/{M}_⊙ ≃ 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 {log}{M}_halo}/{M}_⊙ ≃ 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.