Quantifying Resonant Structure in NGC 6946 from Two-dimensional Kinematics

TL;DR

This study measures the pattern speeds of different structures in NGC 6946 using two-dimensional kinematics, revealing multiple pattern speeds and resonance locations that inform the galaxy's dynamical evolution.

Contribution

It provides the first detailed measurement of multiple pattern speeds in NGC 6946 using the Tremaine-Weinberg method on two-dimensional data.

Findings

Main pattern speed Omega_p=22 km/s/kpc

Additional pattern speed Omega_p=47 km/s/kpc near the nucleus

Confirmation of nested bars and resonance locations

Abstract

We study the two-dimensional kinematics of the H-alpha-emitting gas in the nearby barred Scd galaxy, NGC 6946, in order to determine the pattern speed of the primary m=2 perturbation mode. The pattern speed is a crucial parameter for constraining the internal dynamics, estimating the impact velocities of the gravitational perturbation at the resonance radii, and to set up an evolutionary scenario for NGC 6946. Our data allows us to derive the best fitting kinematic position angle and the geometry of the underlying gaseous disk, which we use to derive the pattern speed using the Tremaine-Weinberg method. We find a main pattern speed Omega_p=22 km/s/kpc, but our data clearly reveal the presence of an additional pattern speed Omega_p=47 km/s/kpc in a zone within 1.25 kpc of the nucleus. Using the epicyclic approximation, we deduce the location of the resonance radii and confirm that inside the outer Inner Lindblad Resonance radius of the main oval, a primary bar has formed rotating at more than twice the outer pattern speed. We further confirm that a nuclear bar has formed inside the Inner Lindblad Resonance radius of the primary bar, coinciding with the inner Inner Lindblad Resonance radius of the large-scale m=2 mode oval.