Quantifying Resonant Structure in NGC 6946 from Two-dimensional Kinematics

Abstract

We study the two-dimensional kinematics of the Hα-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 setting up an evolutionary scenario for NGC 6946. Our data allow 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 Ω = 22 km s-1 kpc-1, but our data clearly reveal the presence of an additional pattern speed Ω = 47 km s-1 kpc-1 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.