Abstract
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 submaximality 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 that 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(±0.14)Vmax).
Highlights
The ‘disc–halo’ degeneracy is an important issue when decomposing H I rotation curves of galaxies, where the contribution from the baryonic matter is mostly determined by the stellar mass-to-light ratio (M/L)
A rather direct observational technique of measuring the disc M/L is from its surface mass density, which can be estimated from the vertical velocity dispersion and the vertical scale height of the disc
We use absorption line spectra in the inner regions and planetary nebulae (PNe) in the outer regions of the galaxy NGC 6946 to trace the kinematics of the disc
Summary
The ‘disc–halo’ degeneracy is an important issue when decomposing H I rotation curves of galaxies, where the contribution from the baryonic matter is mostly determined by the stellar mass-to-light ratio (M/L). A rather direct observational technique of measuring the disc M/L is from its surface mass density, which can be estimated from the vertical velocity dispersion and the vertical scale height of the disc (van der Kruit & Freeman 1984; Bottema, van der Kruit & Freeman 1987; Herrmann et al 2008; Bershady et al 2010a). The scale heights obtained from observations of edge-on discs are primarily for the kinematically hotter stars that are above the dust lane in the galaxies This mismatch can lead to the surface mass density being underestimated which can in turn lead to galaxies’ discs being incorrectly classified as submaximal.
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