A tight angular-momentum plane for disc galaxies
  The relations between the specific angular momenta ($j$) and masses ($M$) of
galaxies are often used as a benchmark in analytic models and hydrodynamical
simulations as they are considered to be amongst the most fundamental scaling
relations. Using accurate measurements of the stellar ($j_\ast$), gas ($j_{\rm
gas}$), and baryonic ($j_{\rm bar}$) specific angular momenta for a large
sample of disc galaxies, we report the discovery of tight correlations between
$j$, $M$, and the cold gas fraction of the interstellar medium ($f_{\rm gas}$).
At fixed $f_{\rm gas}$, galaxies follow parallel power laws in 2D $(j,M)$
spaces, with gas-rich galaxies having a larger $j_\ast$ and $j_{\rm bar}$ (but
a lower $j_{\rm gas}$) than gas-poor ones. The slopes of the relations have a
value around 0.7. These new relations are amongst the tightest known scaling
laws for galaxies. In particular, the baryonic relation ($j_{\rm bar}-M_{\rm
bar}-f_{\rm gas}$), arguably the most fundamental of the three, is followed not
only by typical discs but also by galaxies with extreme properties, such as
size and gas content, and by galaxies previously claimed to be outliers of the
standard 2D $j-M$ relations. The stellar relation ($j_{\ast}-M_{\ast}-f_{\rm
gas}$) may be connected to the known $j_\ast-M_\ast-$bulge fraction relation;
however, we argue that the $j_{\rm bar}-M_{\rm bar}-f_{\rm gas}$ relation can
originate from the radial variation in the star formation efficiency in
galaxies, although it is not explained by current disc instability models.
