Resolving the Disc-Halo Degeneracy II: NGC 6946

TL;DR

This study measures the stellar velocity dispersion in NGC 6946 to accurately determine its disc mass density, revealing the galaxy's disc is closer to maximal, with baryons dominating the inner gravitational field.

Contribution

It introduces a method to distinguish kinematically distinct stellar populations for precise disc mass measurements, reducing previous biases from mismatched tracers.

Findings

NGC 6946's disc is closer to maximal with baryonic dominance in inner regions.

Identification of two kinematically distinct stellar populations.

Improved surface mass density estimates using matched dynamical tracers.

Abstract

The mass-to-light ratio (M/L) is a key parameter in decomposing galactic rotation curves into contributions from the baryonic components and the dark halo of a galaxy. One direct observational method to determine the disc M/L is by calculating the surface mass density of the disc from the stellar vertical velocity dispersion and the scale height of the disc. Usually, the scale height is obtained from near-IR studies of edge-on galaxies and pertains to the older, kinematically hotter stars in the disc, while the vertical velocity dispersion of stars is measured in the optical band and refers to stars of all ages (up to ~10 Gyr) and velocity dispersions. This mismatch between the scale height and the velocity dispersion can lead to underestimates of the disc surface density and a misleading conclusion of the sub-maximality of galaxy discs. In this paper we present the study of the stellar velocity dispersion of the disc galaxy NGC 6946 using integrated star light and individual planetary nebulae as dynamical tracers. We demonstrate the presence of two kinematically distinct populations of tracers which contribute to the total stellar velocity dispersion. Thus, we are able to use the dispersion and the scale height of the same dynamical population to derive the surface mass density of the disc over a radial extent. We find the disc of NGC 6946 to be closer to maximal with the baryonic component contributing most of the radial gravitational field in the inner parts of the galaxy (Vmax(bar) = 0.76($\pm$0.14)Vmax).