  The dynamics of a dense binary mixture of soft dumbbells, each subject to an
active propulsion force and thermal fluctuations, shows a sudden arrest, first
to a translational then to a rotational glass, as one reduces temperature $T$
or the self-propulsion force $f$. Is the temperature-induced glass different
from the activity-induced glass? To address this question, we monitor the
dynamics along an iso-relaxation-time contour in the $(T-f)$ plane. We find
dramatic differences both in the fragility and in the nature of dynamical
heterogeneity which characterise the onset of glass formation - the
activity-induced glass exhibits large swirls or vortices, whose scale is set by
activity, and appears to diverge as one approaches the glass transition. This
large collective swirling movement should have implications for collective cell
migration in epithelial layers.
