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

Abstract

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. (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 −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 7 ± 1 kpc, a vertical scaleheight of 0.65 ± 0.05 kpc and a disk truncation radius of 17 ± 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 ∼ 7 kpc) as respectively σR ∼ 35 km s −1 , σ θ ∼ 30 km s −1 and σz ∼ 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.

The dark matter halo shape of edge-on disk galaxies

“…One possible explanation for the discrepancy between our measured surface density and that of van der Kruit et al (2001), is the lower sensitivity of their observations. In addition to the 12 h 6.0C ATCA observation used by all parties, we acquired an additional 30 h of Hi synthesis observations spread over the ATCA EW352, 750D and 6.0A array configurations.…”

“…The measured Hi radial surface density has a shallow gradient with dips at around 11-12 kpc in agreement with the features in the deprojected surface density obtained by van der Kruit et al (2001). However these authors also measured a central Hi depression, while we find a small central peak when measuring from the left-hand side and a plateaued central surface density when measuring from the right-hand side.…”

“…The measured rotation curve is reasonably symmetrical, in agreement with that obtained by van der Kruit et al (2001) which was derived by modelling the XV distribution using the fixed velocity dispersion XV modelling method of van der Kruit (1981, see also Sect. 2.5 in Paper II).…”

“…2.5 in Paper II). Like that of van der Kruit et al (2001), our measurement shows a steep inner rise -as opposed to the earlier less accurate measurements by Abe et al (1999) which appear to have been measured from the peak flux ridge of the XV map.…”

“…In addition to the 12 h 6.0C ATCA observation used by all parties, we acquired an additional 30 h of Hi synthesis observations spread over the ATCA EW352, 750D and 6.0A array configurations. This resulted in a peak signal-to-noise of 46.1 and a synthesised beam FWHM θ of 8 in the XV map, which appears to be a significantly higher sensitivity and spatial resolution than that acquired by van der Kruit et al (2001).…”