Detection of young ($\leq$20 Myr) stellar populations in apparently
  quenched low-mass galaxies using red spectral line indices
  We report on the detection of a small contribution (around and below 1% in
mass) from young stellar components with ages $\leq$20 Myr in low-mass galaxies
purposely selected from the MaNGA survey to be already-quenched systems. Among
the sample of 28 galaxies, eight of them show signatures of having suffered a
very recent burst of star formation. The detection has been done through the
analysis of line-strength indices in the red spectral range [5700,8800] \AA.
The increasing contribution of red supergiants to this red regime is
responsible for a deviation of the index measurements with respect to their
position within the model grids in the standard spectral range [3600,5700] \AA.
We demonstrate that a combination of red indices, as well as a qualitative
assessment of the mean luminosity-weighted underlying stellar population, is
required in order to distinguish between a true superyoung population and other
possible causes of this deviation, such as abundance ratio variations. Our
result implies that many presumably quenched low-mass galaxies actually contain
gas that is triggering some level of star formation. They have, therefore,
either accreted external gas, internally recycled enough gas from stellar
evolution to trigger new star formation, or they kept a gas reservoir after the
harassment or stripping process that quenched them in first place. Internal
processes are favoured since we find no particular trends between our
non-quenched galaxies and their environment, although more work is needed to
fully discard an external influence.
