Corotation resonance and the velocity field of NGC 4321

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Session 76 -- Spiral and Bar Structure in Galaxies
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[76.08] Corotation resonance and the velocity field of NGC 4321

B.Canzian (U.S.Naval Obs., Flagstaff Station), R.J.Allen (STScI)

The corotation resonance of the two-armed spiral pattern in the disk of NGC~4321 has been located. To find the resonance, we studied the kinematics of $\rm H\alpha$-emitting gas as a tracer of gas motion in the disk of the galaxy. The $\rm H\alpha$ emission line data cube was obtained using the TAURUS-II imaging Fabry-Perot interferometer on the WHT at La~Palma in 1988. The kinematic data were fitted with functions that describe circular rotation plus a perturbation due to a spiral density wave from linear theory. Our kinematic analysis shows that the corotation resonance is within the range 101--$128\rm\,arcsec$ radius. Analysis of the data is made difficult by the declining signal-to-noise beyond this radius and especially by the presence of substructure in the velocity field caused by spurs, bridges, and other strong features not part of the two-armed spiral pattern. The largest velocity perturbations caused by these extra features reach 60--$80\rm\,km\,s^{-1}$ in the plane of the disk.

The kinematically-derived location of the corotation resonance agrees with prior assertions by Elmegreen, Elmegreen, \& Seiden (1989) and Elmegreen, Elmegreen, \& Montenegro (1992) that the resonance is at $118\,\rm arcsec$ radius. Those works were based on identifying resonances with azimuthally symmetric surface brightness variations. Our kinematic verification of the resonance location in NGC~4321 suggests that the symmetrized surface brightness method for locating resonances has merit.

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