role in the secular evolution of their hosts. Indeed, they redistribute material
within the galaxies, while rotating around the centre at a given angular frequency,
the bar pattern speed.
When formed in an isolated galaxy, a bar is expected to be born as fast rotating
with a bar rotation rate R (a parameter used to describe the bar pattern speed)
equal to 1.0 ≤ R ≤ 1.4. During its evolution, the bar can be slowed through the
exchange of angular momentum with the other components and/or when an efficient
dynamical friction is exerted by the dark matter (DM) halo. In this case, R is
shifted in the slow regime (R > 1.4), while the bar radius and strength are increasing.
On the other hand, ultrafast (UF) bars, with R < 1.0, are physically unstable.
Measuring the bar rotation rate becomes desirable both to investigate the secular
evolution of barred galaxies and to test whether the measured DM distribution matches
that predicted by cosmological simulations in the cold DM framework.
The only model-independent way to recover the bar pattern speed (and derive R) is
the Tremaine-Weinberg (TW) method, nowadays largely applied thanks to the advent
of integral-field spectroscopy: most of the analysed bars are compatible with the
fast regime, while a non-negligible fraction belongs to the unstable UF regime.
As a consequence, the question arises whether these results are biased by an
improper application of the method or instead they come from a not completely
theoretically understanding of the nature of slow/UF bars.
We explore the open questions on bar pattern speed with the TW method by
1. testing the reliability of the TW measurements which led to UF bars
2. pushing further the quest of slow bars applying the TW method to a sample of
dwarf galaxies, the best candidates to host slowly-rotating bars, since they are
commonly thought to host a massive and centrally-concentrated DM halo.
We measure the bar radius from the analysis of the maps tracing the transverse-to-radial
force ratio, showing that UF bars are no longer observed when the correct measurement
of the bar radius is adopted to derive R.
We apply the TW method to dedicated MUSE observations of a sample of 5 dwarf barred

galaxies from the Virgo cluster, showing that the analysed bars are slowly-rotating. This
suggests they could have been slowed down by a dense and massive DM halo.

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