1997A&A...321..754V

We have studied the neutral hydrogen component of the ringed and unbarred Scd galaxy NGC 6015 with the Westerbork telescope at a resolution of 13.6"x15.4" (αxδ) and a velocity resolution of 33km/s. These data have been supplemented with CCD-BVRI surface photometry and optical spectroscopy. The study of the stellar and HI gas component of NGC 6015 shows that its structure changes from small to large radii. In the inner 60" there is a blue two armed spiral structure, previously measured by Elmegreen and Elmegreen. In the following 30" the pattern changes to a flocculent one. At 90" two pseudoarms start, the outer parts winding in a blue pseudoring 153.1"x56.1" in size. About 41% of the HI emission is concentrated in this ring. The extent of the HI gas is about twice that of the optical emission, and an analysis of the HI distribution and velocity field indicates that NGC 6015 is warped at radii larger than 170". In the inner parts the velocity field can be modelled well by normal circular rotation in a disk of the same extent as the optical one. Assuming, as is known to be the case for barred galaxies, that the outer pseudoring is at the outer Lindblad resonance, we derive a value for the pattern speed of 26.5km/s/kpc. The expected locations for the other resonances then follow from the combined optical/HI rotation curve, and in particular we find that the end of the inner spiral structure is located at the ultraharmonic (4:1) resonance, in agreement with the result found by Patsis et al. for Sb and Sc type galaxies.