The influence of a kinematically cold young component on disc-halo decompositions in spiral galaxies: insights from solar neighbourhood K-giants

In decomposing the H I rotation curves of disc galaxies, it is necessary to break a degeneracy between the gravitational fields of the disc and the dark halo by estimating the disc surface density. This is done by combining measurements of the vertical velocity dispersion of the disc with the disc scaleheight. The vertical velocity dispersion of the discs is measured from absorption lines (near the V band) of near-face-on spiral galaxies, with the light coming from a mixed population of giants of all ages. However, the scaleheights for these galaxies are estimated statistically from near-IR surface photometry of edge-on galaxies. The scaleheight estimate is therefore dominated by a population of older (> 2 Gyr) red giants. In this paper, we demonstrate the importance of measuring the velocity dispersion for the same older population of stars that is used to estimate the vertical scaleheight. We present an analysis of the vertical kinematics of K-giants in the solar vicinity. We find the vertical velocity distribution best fitted by two components with dispersions of 9.6 +/- 0.5 km s(-1) and 18.6 +/- 1.0 km s(-1), which we interpret as the dispersions of the young and old disc populations, respectively. Combining the ( single) measured velocity dispersion of the total young + old disc population ( 13.0 +/- 0.1 km s(-1)) with the scaleheight estimated for the older population would underestimate the disc surface density by a factor of similar to 2. Such a disc would have a peak rotational velocity that is only 70 per cent of that for the maximal disc, thus making it appear submaximal.