The SAMI Galaxy Survey: The role of disc fading and progenitor bias in
  kinematic transitions
  We use comparisons between the SAMI Galaxy Survey and equilibrium galaxy
models to infer the importance of disc fading in the transition of spirals into
lenticular (S0) galaxies. The local S0 population has both higher photometric
concentration and lower stellar spin than spiral galaxies of comparable mass
and we test whether this separation can be accounted for by passive aging
alone. We construct a suite of dynamically self--consistent galaxy models, with
a bulge, disc and halo using the GalactICS code. The dispersion-dominated bulge
is given a uniformly old stellar population, while the disc is given a current
star formation rate putting it on the main sequence, followed by sudden
instantaneous quenching. We then generate mock observables (r-band images,
stellar velocity and dispersion maps) as a function of time since quenching for
a range of bulge/total (B/T) mass ratios. The disc fading leads to a decline in
measured spin as the bulge contribution becomes more dominant, and also leads
to increased concentration. However, the quantitative changes observed after 5
Gyr of disc fading cannot account for all of the observed difference. We see
similar results if we instead subdivide our SAMI Galaxy Survey sample by star
formation (relative to the main sequence). We use EAGLE simulations to also
take into account progenitor bias, using size evolution to infer quenching
time. The EAGLE simulations suggest that the progenitors of current passive
galaxies typically have slightly higher spin than present day star-forming disc
galaxies of the same mass. As a result, progenitor bias moves the data further
from the disc fading model scenario, implying that intrinsic dynamical
evolution must be important in the transition from star-forming discs to
passive discs.
