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In decomposing the HI rotation curves of disc galaxies, it is necessary tobreak a degeneracy between the gravitational fields of the disc and the darkhalo by estimating the disc surface density. This is done by combiningmeasurements of the vertical velocity dispersion of the disc with the discscale height. The vertical velocity dispersion of the discs is measured fromabsorption lines (near the V-band) of near-face-on spiral galaxies, with thelight coming from a mixed population of giants of all ages. However, the scaleheights for these galaxies are estimated statistically from near-IR surfacephotometry of edge-on galaxies. The scale height estimate is thereforedominated by a population of older (> 2 Gyr) red giants. In this paper, wedemonstrate the importance of measuring the velocity dispersion for the sameolder population of stars that is used to estimate the vertical scale height.We present an analysis of the vertical kinematics of K-giants in the solarvicinity. We find the vertical velocity distribution best fit by two componentswith dispersions of 9.6 +/- 0.5 km/s and 18.6 +/- 1.0 km/s, which we interpretas the dispersions of the young and old disc populations respectively.Combining the (single) measured velocity dispersion of the total young + olddisc population (13.0 +/- 0.1 km/s) with the scale height estimated for theolder population would underestimate the disc surface density by a factor of ~2. Such a disc would have a peak rotational velocity that is only 70% of thatfor the maximal disc, thus making it appear submaximal.

The Influence of a Kinematically Cold Young Component on Disc-Halo Decompositions in Spiral Galaxies: Insights from Solar Neighbourhood K-giants