The variation of the gas content of galaxy groups and pairs compared to
  isolated galaxies
  We measure how the atomic gas (HI) fraction ($f_{HI}={\rm
\frac{M_{HI}}{M_{*}}}$) of groups and pairs taken as single units vary with
average stellar mass ($\langle {\rm M_*} \rangle$) and average star-formation
rate ($\langle {\rm SFR} \rangle$), compared to isolated galaxies. The HI 21 cm
emission observation are from (i) archival ALFALFA survey data covering three
fields from the GAMA survey (provides environmental and galaxy properties), and
(ii) DINGO pilot survey data of one of those fields. The mean $f_{HI}$ for
different units (groups/pairs/isolated galaxies) are measured in regions of the
log($\langle {\rm M_*} \rangle$) -- log($\langle {\rm SFR} \rangle$) plane,
relative to the z $\sim 0$ star-forming main sequence (SFMS) of individual
galaxies, by stacking $f_{HI}$ spectra of individual units. For ALFALFA,
$f_{HI}$ spectra of units are measured by extracting HI spectra over the full
groups/pair areas and dividing by the total stellar mass of member galaxies.
For DINGO, $f_{HI}$ spectra of units are measured by co-adding HI spectra of
individual member galaxies, followed by division by their total stellar mass.
For all units the mean $f_{HI}$ decreases as we move to higher $\langle {\rm
M_*} \rangle$ along the SFMS, and as we move from above the SFMS to below it at
any $\langle {\rm M_*} \rangle$. From the DINGO-based study, mean $f_{HI}$ in
groups appears to be lower compared to isolated galaxies for all $\langle {\rm
M_*} \rangle$ along the SFMS. From the ALFALFA-based study we find
substantially higher mean $f_{HI}$ in groups compared to isolated galaxies
(values for pairs being intermediate) for ${\langle{\rm
M_*}\rangle}\lesssim10^{9.5}~{\rm M_{\odot}}$, indicating the presence of
substantial amounts of HI not associated with cataloged member galaxies in low
mass groups.
