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Kinematics and dynamics of the ``superthin'' edge-on disk galaxy IC 5249
We present spectroscopic observations of the stellar motions in the diskof the superthin edge-on spiral galaxy IC 5249 and re-analyse synthesisobservations of the HI. We find that the HI rotation curve risesinitially to about 90-100 km s-1, but contrary to theconclusion of Abe et al. (\cite{Abe99}) flattens well before the edge ofthe optical disk. Over most part of the optical disk we have been ableto establish that the (tangential) stellar velocity dispersion is 25-30km s-1. We argue that the central light concentration in thedisk is not a bulge in the classical Population II sense, but mostlikely represents structure in the disk component. From earlier surfacephotometry we adopt a value for the radial scalelength of the disk of 7+/- 1 kpc, a vertical scaleheight of 0.65 +/- 0.05 kpc and a disktruncation radius of 17 +/- 1 kpc. The HI disk has a measurablethickness but from our analysis we conclude that this is due to a smallinclination away from perfectly edge-on. The very thin appearance of IC5249 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 onlyabout 2.5 scalelengths. In terms of the ratio of the radial scalelengthand the vertical scaleheight of the disk, IC 5249 is not very flat; infact it is slightly fatter than the disk of our Galaxy. From variousarguments we derive the stellar velocity dispersions at a position oneradial scalelength out in the disk (R ~ 7 kpc) as respectively sigmaR ~ 35 km s-1, sigma theta ~ 30 kms-1 and sigma z ~ 20 km s-1. This iscomparable to the values for the disk of our Galaxy in the solarneighborhood. Near the edge of the disk the ratio of radial to verticalvelocity dispersion is probably higher. Presumably the angular momentumdistribution of the gas that formed the disk in IC 5249 was such that,compared to the Galaxy, a much more extended distribution resulted inspite of the lower overall rotation and mass. The low surface densitythat arose from that resulted in a thicker HI layer in which starformation proceeded at a much slower rate, but disk heating proceeded ata similar pace.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
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