Bar pattern speeds in CALIFA galaxies III. Solving the puzzle of
  ultrafast bars
  More than 10% of the barred galaxies with a direct measurement of the bar
pattern speed host an ultrafast bar. These bars extend beyond the corotation
radius and challenge our understanding of the orbital structure of barred
galaxies. Most of them are found in spiral galaxies, rather than in lenticular
ones. We analysed the properties of the ultrafast bars detected in the CALIFA
Survey to investigate whether they are an artefact resulting from an
overestimation of the bar radius and/or an underestimation of the corotation
radius or a new class of bars, whose orbital structure has not yet been
understood. We revised the available measurements of the bar radius based on
ellipse fitting and Fourier analysis and of the bar pattern speed from the
Tremaine-Weinberg method. In addition, we measured the bar radius from the
analysis of the maps tracing the transverse-to-radial force ratio, which we
obtained from the deprojected i-band images of the galaxies retrieved from the
SDSS Survey. We found that nearly all the sample galaxies are spirals with an
inner ring or pseudo-ring circling the bar and/or strong spiral arms, which
hamper the measurement of the bar radius from the ellipse fitting and Fourier
analysis. According to these methods, the bar ends overlap the ring or the
spiral arms making the adopted bar radius unreliable. On the contrary, the bar
radius from the ratio maps are shorter than the corotation radius. This is in
agreement with the theoretical predictions and findings of numerical
simulations about the extension and stability of the stellar orbits supporting
the bars. We conclude that ultrafast bars are no longer observed when the
correct measurement of the bar radius is adopted. Deriving the bar radius in
galaxies with rings and strong spiral arms is not straightforward and a solid
measurement method based on both photometric and kinematic data is still
missing.
