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

TL;DR

This study highlights the importance of measuring the velocity dispersion of older stellar populations in spiral galaxies to accurately estimate disc surface density and avoid underestimating the galaxy's rotational velocity.

Contribution

It demonstrates that using the velocity dispersion of the older stellar population, rather than the combined population, significantly improves disc mass estimates in spiral galaxies.

Findings

The vertical velocity distribution of K-giants is best fit by two components with dispersions of 9.6 and 18.6 km/s.

Using the combined velocity dispersion underestimates the disc surface density by a factor of ~2.

Incorrect assumptions about stellar populations can lead to underestimating galaxy rotational velocities.

Abstract

In decomposing the HI 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 scale height. 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 scale heights for these galaxies are estimated statistically from near-IR surface photometry of edge-on galaxies. The scale height 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 scale height. We present an analysis of the vertical kinematics of K-giants in the solar vicinity. We find the vertical velocity distribution best fit by two components with dispersions of 9.6 +/- 0.5 km/s and 18.6 +/- 1.0 km/s, 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) with the scale height estimated for the older 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 that for the maximal disc, thus making it appear submaximal.