The atomic gas of star-forming galaxies at z ～ 0.05 as revealed by the Five-hundred-meter Aperture Spherical Radio Telescope

Context. We report new H？I observations of four z？～？0.05 VALES galaxies undertaken during the commissioning phase of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST).Aims. FAST is the largest single-dish telescope in the world, with a 500 m aperture and a 19-Beam receiver. Exploiting the unprecedented sensitivity provided by FAST, we aim to study the atomic gas content, via the H？I 21 cm emission line, in low-z star formation galaxies taken from the Valparaíso ALMA/APEX Line Emission Survey (VALES). Together with previous Atacama Large Millimeter/submillimeter Array (ALMA) CO(J？=？1？0) observations, the H？I data provides crucial information to measure the gas mass and dynamics.Methods. As a pilot H？I galaxy survey, we targeted four local star-forming galaxies at z？～？0.05. In particular, one of them has already been detected in H？I by the Arecibo Legacy Fast ALFA survey (ALFALFA), allowing a careful comparison. We use an ON-OFF observing approach that allowed us to reach an rms of 0.7 mJy beam^{？1 at a 1.7 km s？1 velocity resolution within only 20 min ON-target integration time.Results. In this Letter, we demonstrate the extraordinary capability of the FAST 19-beam receiver to push the detectability of the H？I emission line of extra-galactic sources. The H？I emission line detected by FAST shows good consistency with the previous Arecibo telescope ALFALFA results. Our observations are put into context with previous multi-wavelength data to reveal the physical properties of these low-z galaxies. We find that the CO(J？=？1？0) and H？I emission line profiles are similar. The dynamical mass estimated from the H？I data is an order of magnitude higher than the baryon mass and the dynamical mass derived from the CO observations, implying that the mass probed by dynamics of H？I is dominated by the dark matter halo. In one case, a target shows an excess of CO(J？=？1？0) in the line centre, which can be explained by an enhanced CO(J？=？1？0) emission induced by a nuclear starburst showing high-velocity dispersion}