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

We present SAMI-HI, a surv e y of the atomic hydrogen content of 296 galaxies with integral field spectroscopy available from the SAMI Galaxy Surv e y. The sample spans nearly 4 de x in stellar mass ( M * = 10( 7 . 4) - 10( 11 . )1 M-?), redshift z < 0.06, and includes ne w Arecibo observ ations of 153 galaxies, for which we release catalogues and HI spectra. We use these data to compare the rotational velocities obtained from optical and radio observations and to sho w ho w systematic dif ferences af fect the slope and scatter of the stellar-mass and baryonic Tully-Fisher relations. Specifically, we show that H alpha rotational velocities measured in the inner parts of galaxies (1.3 ef fecti ve radii in this work) systematically underestimate HI global measurements, with HI/H alpha velocity ratios that increase at low stellar masses, where rotation curves are typically still rising and H alpha measurements do not reach their plateau. As a result, the H alpha stellar mass Tully-Fisher relation is steeper (when M * is the independent variable) and has larger scatter than its HI counterpart. Interestingly, we confirm the presence of a small fraction of low-mass outliers of the H alpha relation that are not present when HI velocity widths are used and are not explained by "aperture effects'. These appear to be highly disturbed systems for which H alpha 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.