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

Abstract

Context.We report new H Iobservations of fourz ∼ 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 I21 cm emission line, in low-zstar 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 Idata provides crucial information to measure the gas mass and dynamics.Methods.As a pilot H Igalaxy survey, we targeted four local star-forming galaxies atz ∼ 0.05. In particular, one of them has already been detected in H Iby 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−1at a 1.7 km s−1velocity 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 Iemission line of extra-galactic sources. The H Iemission 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-zgalaxies. We find that the CO(J = 1−0) and H Iemission line profiles are similar. The dynamical mass estimated from the H Idata 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 Iis 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.