THE ODD OFFSET BETWEEN THE GALACTIC DISK AND ITS BAR IN NGC 3906

Abstract

We use mid-infrared 3.6 and 4.5 μm imaging of NGC 3906 from the Spitzer Survey of Stellar Structure in Galaxies (S4G) to understand the nature of an unusual offset between its stellar bar and the photometric center of an otherwise regular, circular outer stellar disk. We measure an offset of ∼910 pc between the center of the stellar bar and photometric center of the stellar disk; the bar center coincides with the kinematic center of the disk determined from previous HI observations. Although the undisturbed shape of the disk suggests that NGC 3906 has not undergone a significant merger event in its recent history, the most plausible explanation for the observed offset is an interaction. Given the relatively isolated nature of NGC 3906 this interaction could be with dark matter substructure in the galaxy's halo or from a recent interaction with a fast moving neighbor that remains to be identified. Simulations aimed at reproducing the observed offset between the stellar bar/kinematic center of the system and the photometric center of the disk are necessary to confirm this hypothesis and constrain the interaction history of the galaxy.