Abstract

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.