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Aims.We have investigated the accuracy and reliability of six methods used to
determine the length of stellar bars in galaxies or N-body simulations.
Methods. All these methods use ellipse fitting and Fourier decomposition of the
surface brightness. We have applied them to N-body simulations that include
stars, gas, star formation, and feedback. Stellar particles were
photometrically calibrated to make B and K-band mock images. Dust absorption
is also included. We discuss the advantages and drawbacks of each method,
the effects of projection and resolution, as well as the uncertainties
introduced by the presence of dust.
Results. The use of N-body simulations allows us to compare the location of Ultra
Harmonic Resonance (UHR or 4/1) and corotation (CR) with measured bar
lengths. We show that the minimum of ellipticity located just outside the
bulk of the bar is correlated with the corotation, whereas the location of
the UHR can be approximated using the phase of the fitted ellipses or the
phase of the m=2 Fourier development of the surface brightness. We give
evidence that the classification of slow/fast bars, based on the ratio
R=RCR​/Rbar​ could increase from 1 (fast
bar) to 1.4 (slow bar) just by a change of method. We thus conclude that
one has to select the right bar-length estimator depending on the
application, since these various estimators do not define the same physical
area
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