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Astron. Astrophys. 328, 517-525 (1997)

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The maximum rotation of a galactic disc

Roelof Bottema

Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 20 February 1997 / Accepted 12 June 1997

Abstract

The observed stellar velocity dispersions of galactic discs show that the maximum rotation of a disc is on average 63% of the observed maximum rotation. This criterion can, however, not be applied to small or low surface brightness (LSB) galaxies because such systems show, in general, a continuously rising rotation curve until the outermost measured radial position. That is why a general relation has been derived, giving the maximum rotation for a disc depending on the luminosity, surface brightness, and colour of the disc. As a physical basis of this relation serves an adopted fixed mass-to-light ratio as a function of colour. That functionality is consistent with results from population synthesis models and its absolute value is determined from the observed stellar velocity dispersions. The derived maximum disc rotation is compared with a number of observed maximum rotations, clearly demonstrating the need for appreciable amounts of dark matter in the disc region and even more so for LSB galaxies. Matters have been illustrated for two examples; the galaxy NGC 6503 and LSB galaxy NGC 1560.

Key words: galaxies: general – galaxies: kinematics and dynamics – galaxies: luminosity function, mass function – galaxies: spiral

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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