  A significant fraction of spiral galaxies are red, gas-poor, and have low
star formation rates (SFRs). We study these unusual objects using the
IllustrisTNG100 simulation. Among 1912 well-resolved disk galaxies selected
from the last simulation output, we identify 377 red objects and describe their
properties and origins using a few representative examples. The simulated red
spirals turn out to be typically very gas-poor, have very low SFRs, are more
metal-rich, and have larger stellar masses than the remaining disks. Only about
13% of red spirals suffered strong mass loss and thus could have resulted from
environmental quenching by ram pressure and tidal stripping of the gas, or
similar processes. The majority of red disks were probably quenched by feedback
from the active galactic nucleus (AGN). This conclusion is supported by the
higher black hole masses and lower accretion rates of red disks, as well as the
larger total AGN feedback energies injected into the surrounding gas in the
kinetic feedback mode implemented in the IllustrisTNG simulations. The
timescales of the gas loss correlate with the black hole growth for the
AGN-quenched galaxies and with the dark-matter loss for the environmentally
quenched ones. The red spirals are more likely to possess bars, and their bars
are stronger than in the remaining disks, which is probably the effect of gas
loss rather than the reason for quenching.
