UGC 3995: A Close Pair of Spiral Galaxies

UGC 3995 is a close pair of spiral galaxies whose eastern component hosts a Seyfert 2 nucleus. The object was selected for observation because a bright filamentary structure that apparently connects the nuclei of the two galaxies made it a good candidate to investigate a possible connection between galaxy interactions and active galactic nuclei. We present a detailed analysis of this system using long-slit spectroscopy and narrowband (Hα+[N II] lambdalambda6548, 6583), as well as broadband (B, R), imaging and an archival Wide Field Planetary Camera 2 image. The component galaxies reveal surprisingly small signs of interaction considering their spatial proximity and almost identical recession velocities, as the bright filament is probably an optical illusion due to the superposition of the bar of the Seyfert galaxy and of the spiral arms of the companion. The broadband morphology, a B-R color map, and a continuum-subtracted Hα+[N II] lambdalambda6548, 6583 image demonstrate that the western component UGC 3995B is in front of the Seyfert-hosting component UGC 3995A, partly obscuring its western side. The small radial velocity difference leaves the relative motion of the two galaxies largely unconstrained. The observed lack of major tidal deformations, along with some morphological peculiarities, suggests that the galaxies are proximate in space but may have recently approached each other on the plane of the sky. The geometry of the system and the radial velocity curve at P.A.~106 deg suggest that the encounter may be retrograde or, alternatively, prograde before perigalacticon. The partial overlap of the two galaxies allows us to estimate the optical thickness of the disk of component B. We derive an extinction of ~0.18 visual magnitudes in the interarm parts of the foreground galaxy disk, and >~1-1.5 visual magnitudes corresponding to the spiral arms.