The atomic gas of star-forming galaxies at z$\sim$0.05 as revealed by
  the Five-hundred-meter Aperture Spherical Radio Telescope
  We report new HI observations of four z$\sim$0.05 star-forming galaxies
undertaken during the commissioning phase of the Five-hundred-meter Aperture
Spherical Radio Telescope (FAST). FAST is the largest single-dish telescope
with a 500 meter aperture and a 19-Beam receiver. Exploiting the unprecedented
sensitivity provided by FAST, we aim to study the atomic gas, via the HI 21cm
emission line, in low-$z$ star-forming galaxies taken from the Valpara\'iso
ALMA/APEX Line Emission Survey (VALES) project. Together with previous ALMA
CO($J=1-0$) observations, the HI data provides crucial information to measure
the gas mass and dynamics. As a pilot HI survey, we targeted four local
star-forming galaxies at $z\sim0.05$. 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
minutes ON-target integration time. We demonstrate the great capabilities of
the FAST 19-beam receiver for pushing the detectability of the HI emission line
of extra-galactic sources. The HI emission line detected by FAST shows good
consistency with the previous ALFALFA results. Our observations are put in
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.
