SAMI-H i: The H i view of the Hα Tully-Fisher relation and data release

We present SAMI-H i, a survey of the atomic hydrogen content of 296 galaxies with integral field spectroscopy available from the SAMI Galaxy Survey. The sample spans nearly 4 dex in stellar mass (M* = 107.4-1011.1 M⊙), redshift z < 0.06, and includes new Arecibo observations of 153 galaxies, for which we release catalogues and H i spectra. We use these data to compare the rotational velocities obtained from optical and radio observations and to show how systematic differences affect the slope and scatter of the stellar-mass and baryonic Tully-Fisher relations. Specifically, we show that Hα rotational velocities measured in the inner parts of galaxies (1.3 effective radii in this work) systematically underestimate H i global measurements, with H i/ Hα velocity ratios that increase at low stellar masses, where rotation curves are typically still rising and Hα measurements do not reach their plateau. As a result, the Hα; stellar mass Tully-Fisher relation is steeper (when M⊙ is the independent variable) and has larger scatter than its H i counterpart. Interestingly, we confirm the presence of a small fraction of low-mass outliers of the Hα; relation that are not present when H i velocity widths are used and are not explained by 'aperture effects'. These appear to be highly disturbed systems for which Hα widths do not provide a reliable estimate of the rotational velocity. Our analysis reaffirms the importance of taking into account differences in velocity definitions as well as tracers used when interpreting offsets from the Tully-Fisher relation, at both low and high redshifts and when comparing with simulations.