THE SMALL SCATTER OF THE BARYONIC TULLY-FISHER RELATION

In a Lambda cold dark matter (Lambda CDM) cosmology, the baryonic Tully-Fisher relation (BTFR) is expected to show significant intrinsic scatter resulting from the mass-concentration relation of dark matter halos and the baryonic-to-halo mass ratio. We study the BTFR using a sample of 118 disk galaxies (spirals and irregulars) with data of the highest quality: extended H I rotation curves (tracing the outer velocity) and Spitzer photometry at 3.6 mu m (tracing the stellar mass). Assuming that the stellar mass-to-light ratio (gamma(*)) is nearly constant at 3.6 mu m, we find that the scatter, slope, and normalization of the BTFR systematically vary with the adopted gamma(*). The observed scatter is minimized for gamma(*) greater than or similar to 0.5M(circle dot) L-circle dot, corresponding to nearly maximal disks in high-surface-brightness galaxies and BTFR slopes close to similar to 4. For any reasonable value of gamma(*), the intrinsic scatter is similar to 0.1 dex, below general Lambda CDM expectations. The residuals show no correlations with galaxy structural parameters (radius or surface brightness), contrary to the predictions from some semi-analytic models of galaxy formation. These are fundamental issues for Lambda CDM cosmology.