We present spectroscopic observations of the stellar motions in the disk of the superthin edge-on spiral galaxy IC 5249 and re-analyse synthesis observations of the HI . We find that the HI rotation curve rises initially to about 90-100 km/s , but contrary to the conclusion of Abe et al .
( 1999 ) flattens well before the edge of the optical disk .
Over most part of the optical disk we have been able to establish that the ( tangential ) stellar velocity dispersion is 25-30 km/s .
We argue that the central light concentration in the disk is not a bulge in the classical Population II sense , but most likely represents structure in the disk component .
From earlier surface photometry we adopt a value for the radial scalelength of the disk of 7 \pm 1 kpc , a vertical scaleheight of 0.65 \pm 0.05 kpc and a disk truncation radius of 17 \pm 1 kpc .
The HI disk has a measurable thickness but from our analysis we conclude that this is due to a small inclination away from perfectly edge-on . The very thin appearance of IC 5249 on the sky is the result of a combination of a low ( face-on ) surface brightness , a long scalelength and a a sharp truncation at only about 2.5 scalelengths .
In terms of the ratio of the radial scalelength and the vertical scaleheight of the disk , IC 5249 is not very flat ; in fact it is slightly fatter than the disk of our Galaxy .
From various arguments we derive the stellar velocity dispersions at a position one radial scalelength out in the disk ( R \sim 7 kpc ) as respectively \sigma _ { R } \sim 35 km/s , \sigma _ { \theta } \sim 30 km/s and \sigma _ { z } \sim 20 km/s .
This is comparable to the values for the disk of our Galaxy in the solar neighborhood .
Near the edge of the disk the ratio of radial to vertical velocity dispersion is probably higher . Presumably the angular momentum distribution of the gas that formed the disk in IC 5249 was such that , compared to the Galaxy , a much more extended distribution resulted in spite of the lower overall rotation and mass .
The low surface density that arose from that resulted in a thicker HI layer in which star formation proceeded at a much slower rate , but disk heating proceeded at a similar pace .