Quenching Timescales in the IllustrisTNG Simulation
  The timescales for galaxy quenching offer clues to its underlying physical
drivers. We investigate central galaxy quenching timescales in the IllustrisTNG
100-1 simulation, their evolution over time, and the pre-quenching properties
of galaxies that predict their quenching timescales. Defining quenching
duration $\tau_q$ as the time between crossing sSFR thresholds, we find that
$\sim$40% of galaxies quench rapidly with $\tau_q<$1 Gyr, but a substantial
tail of galaxies can take up to 10 Gyr to quench. Furthermore, 29% of galaxies
that left the star forming main sequence (SFMS) more than 2 Gyr ago never fully
quench by $z=0$. While the median $\tau_q$ is fairly constant with epoch, the
rate of galaxies leaving the SFMS increases steadily over cosmic time, with the
rate of slow quenchers being dominant around $z\sim2$ to 0.7. Compared to fast
quenchers ($\tau_q<$1 Gyr), slow-quenching galaxies ($\tau_q>$1 Gyr) were more
massive, had more massive black holes, had larger stellar radii and accreted
gas with higher specific angular momentum (AM) prior to quenching. These
properties evolve little by $z=0$, except for the accreting gas AM for fast
quenchers, which reaches the same high AM as the gas in slow quenchers. By
$z=0$, slow quenchers also have residual star formation in extended gas rings.
Using the expected relationship between stellar age gradient and $\tau_q$ for
inside-out quenching we find agreement with MaNGA IFU observations. Our results
suggest the accreting gas AM and potential well depth determine the quenching
timescale.
