Kinematics and dynamics of the "superthin" edge-on disk galaxy IC 5249

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: jjimenez@astro.rug.nl, kregel@astro.rug.nl
² Research School of Astronomy and Astrophysics, Mount Stromlo and Siding Spring Observatories, Australian National University, Private Bag, Weston Creek, 2611 Canberra, Australia e-mail: kcf@mso.anu.edu.au

Corresponding author: P. C. van der Kruit, vdkruit@astro.rug.nl

Received: 8 June 2001
Accepted: 6 September 2001

Abstract

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^-1, but contrary to the conclusion of Abe et al. ([CITE]) 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^-1. 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 kpc, a vertical scaleheight of kpc and a disk truncation radius of 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 ( kpc) as respectively 35 km s^-1, 30 km s^-1 and 20 km s^-1. 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.