The "superthin" disk galaxy IC 5249 has recently been studied by van der Kruit et al. (2001). In this paper, we re-analyse the ATCA HI observations reported in Abe et al. (1999), use the photometry of three Ken Freeman students, Claude Carignan (1983), Richard Wainscoat (1986) and Yong-Ik Byun (1992, 1998), and present a measurement of the stellar kinematics.
The HI observations show -contrary to the conclusions of Abe et al.- a rotation curve that is flat over a large part of the disk at Vmax = 105 ± 5 km s-1. From the available photometry we adopt hR = 7 ± 1 kpc, hz = 0.65 ± 0.05 kpc, µo ~ 24.5 B-mag arcsec-2 and Rmax = 17 ± 1 kpc. Note that hz is larger than that in our Galaxy! It follows that IC 5249 appears on the sky as a very thin disk, because it combines a low surface brightness with a very long scalelength and a truncation radius after only about 2.5 scalelengths.
From our data we can derive various kinematical and dynamical properties, using the equations given above.
|R = 7 kpc||R = 17 kpc|
|Vrot (km/s)||90 ± 5||105 ± 5|
|dVrot / dR (km/s.kpc)||3 ± 4||0 ± 1|
|(km/s.kpc)||20 ± 6||3.5 ± 0.4|
|R (km/s)||35 ± 5||-|
|Asym. drift (km/s)||10 ± 3||-|
|R (km/s)||-||25 ± 5|
|(M pc-2)||~ 25||~ 6|
|z (km/s)||19 ± 4||11 ± 2|
|Q||~ 2||~ 2|
|(FWHM)HI (kpc)||0.60 ± 0.17||1.5 ± 0.5|
At R = 7 kpc the stellar velocity dispersions are similar to the solar neighborhood, while the surface density of the disk is about half that of the solar neighborhood. Star formation must have proceeded much slower in IC 5249 in order to give the low surface brightness. But, surprisingly, as much dynamical evolution has occured as in the Galactic disk. Also note that with R, h / Vmax = 0.33 ± 0.05, IC 5249 falls on the Bottema relation (8).
Acknowledgments. I am grateful to the Mount Stromlo Observatory and John Norris for hospitality during the preparation of this review. Michiel Kregel kindly allowed me to quote from our recent paper and provided Figs. 4 and 5. I thank Ken Freeman for stimulating discussions and collaborations over many years and wish him well.