From star-forming galaxies to AGN: the global HI content from a stacking
  experiment
  We study the atomic neutral hydrogen (HI) content of $\sim$1600 galaxies up
to $z \sim 0.1$ using stacking techniques. The observations were carried out
with the Westerbork Synthesis Radio Telescope (WSRT) in the area of the SDSS
South Galactic Cap (SSGC), where we selected a galaxy sample from the SDSS
spectroscopic catalog. Multi-wavelength information is provided by SDSS, NVSS,
GALEX, and WISE. We use the collected information to study HI trends with
color, star-forming, and active galactic nuclei (AGN) properties.
  Using NUV-r colors, galaxies are divided into blue cloud, green valley and
red sequence galaxies. As expected based on previous observations, we detect HI
in green valley objects with lower amounts of HI than blue galaxies, while
stacking only produces a 3-$\sigma$ upper limit for red galaxies with M$_{\rm
HI}$ $<$ (5 $\pm$ 1.5) $\times$ 10$^{8}$ M$_{\odot}$ and M$_{\rm HI}/\rm{L}_r$
$<$ 0.02 $\pm$ 0.006 $\rm M_{\odot} / \rm L_{\odot} $. We find that the HI
content is more dependent on NUV-r color, and less on ionization properties, in
the sense that regardless of the presence of an optical AGN (based on optical
ionization line diagnostics), green-valley galaxies always show HI, whereas red
galaxies only produce an upper limit. This suggests that feedback from optical
AGN is not the (main) reason for depleting large-scale gas reservoirs.
  Low-level radio continuum emission in our galaxies can stem either from star
formation, or from AGN. We use the WISE color-color plot to separate these
phenomena by dividing the sample into IR late-type and IR early-type galaxies.
We find that the radio emission in IR late-type galaxies stems from enhanced
star formation, and this group is detected in HI. However, IR early-type
galaxies lack any sign of HI gas and star formation activity, suggesting that
radio AGN are likely to be the source of radio emission in this population.
