A Robust Tip of the Red Giant Branch Distance to the Fireworks Galaxy (NGC 6946)

Abstract: Archival HST data taken in F606W+F814W of two different fields in the outer regions of NGC 6946 is used to measure a tip of the red giant branch (TRGB) distance to the galaxy. We employ a Bayesian maximum-likelihood modeling method that incorporates the completeness of the photometry, and allows us to model the luminosity function of the RGB population. Our two fields provide us with distances of 7.74 $\pm$ 0.42 Mpc and 7.69 $\pm$ 0.50 Mpc, respectively. Our final distance of 7.72 $\pm$ 0.32 Mpc is higher than most values published previously in the literature. This has important implications for supernova measurements, as NGC 6946 is host to the most observed supernovae (10) of any galaxy to date. We find that the supernovae in NGC 6946 are on average $\sim$ 2.3 times more luminous than previous estimates. Our distance gives NGC 6946 a peculiar velocity of $v_{pec}$ = $-229$ $\pm$ $29$ km/s in the Local Sheet frame. This velocity is the projected component of a substantial negative SGZ motion, indicating a coherent motion along with the other members of the M101 wall toward the plane of the Local Sheet. The M101 wall, a spur off the Local Sheet into the Local Void, is experiencing the expansion of the Local Void.