Kinematic modeling of disk galaxies III. The warped Spindle NGC 2685

This is the third of a series of papers in which the structure and kinematics of disk galaxies is studied. By employing direct tilted-ring fits to the data cube as introduced in Paper I, we analyzed the Spindle galaxy NGC 2685, previously regarded as a two-ringed polar ring galaxy. Deep HI and optical (i'-band) observations are presented. The HI observations strongly suggest that the gaseous structure of NGC 2685 does not consist of two separate mutually inclined regions, but forms a coherent, extremely warped disk, the appearance of two rings being due to projection effects. By comparing the HI total-intensity maps with the optical image, we demonstrate that at large radii a faint stellar disk is well aligned with the outer HI disk. The shape of the dust lanes obscuring the NE part of the inner stellar body indicates that, also at smaller radii, NGC 2685 possesses a disk containing gas, dust, and stars in which the various constituents are aligned. At smaller radii, this disk is kinematically decoupled from the central stellar body; hence, in the region of the bright, central stellar body, NGC 2685 appears to consist of two disks that share a common center, but have different orientation: a bright stellar lenticular body apparently devoid of dust and gas, and a heavily warped low-surface brightness disk containing stars, gas, and dust. The low-surface-brightness disk changes its orientation gradually and at large radii assumes the orientation of the central stellar S0 disk. Since, according to our analysis, the intrinsic orientation of the low-surface-brightness disk changes through 70 degrees, the gaseous disk is coherent, and is at no radius oriented perpendicularly with respect to the central stellar body, NGC 2685 is not likely to be a classical polar-ring galaxy.