The specific angular momentum of disc galaxies and its connection with
  galaxy morphology, bar structure and disc gravitational instability
  The specific angular momenta ($j\equiv J/M$) of stars ($j_{\star}$), gas
($j_{\mathrm{gas}}$), baryons as a whole ($j_{\mathrm{b}}$) and dark matter
haloes ($j_{\mathrm{h}}$) contain clues of vital importance about how galaxies
form and evolve. Using one of the largest samples of disc galaxies (S0-BCD)
with high-quality rotation curves and near-infrared surface photometry, we
perform a detailed comparative analysis of $j$ that stretches across a variety
of galaxy properties. Our analysis imposes tight constraints on the "retained"
fractions of specific angular momentum ($j_{\star}/j_{\mathrm{h}}$,
$j_{\mathrm{HI}}/j_{\mathrm{h}}$ and $j_{\mathrm{b}}/j_{\mathrm{h}}$), as well
as on their systematic trends with mass fraction and galaxy morphology, thus on
how well specific angular momentum is conserved in the process of disc galaxy
formation and evolution. Besides, our analysis demonstrates how challenging it
is to characterize barred galaxies from a gravitational instability point of
view. This is true not only for the popular Efstathiou, Lake & Negroponte
(1982) bar instability criterion, which fails to separate barred from
non-barred galaxies in about 55% of the cases, but also for the mass-weighted
Toomre (1964) parameter of atomic gas, $\langle Q_{\mathrm{HI}}\rangle$, which
succeeds in separating barred from non-barred galaxies, but only in a
statistical sense.
