2020A&A...638L..14C

Context. We report new HI 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 HI 21 cm emission line, in low-z star formation galaxies taken from the Valparaiso ALMA/APEX Line Emission Survey (VALES). Together with previous Atacama Large Millimeter/submillimeter Array (ALMA) CO(J=1-0) observations, the HI data provides crucial information to measure the gas mass and dynamics.
Methods. As a pilot HI galaxy survey, we targeted four local star-forming galaxies at z∼0.05. In particular, one of them has already been detected in HI 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 0.7mJy/beam at a 1.7km/s 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 HI emission line of extra-galactic sources. The HI 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 HI emission line profiles are similar. The dynamical mass estimated from the HI 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 HI 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.