Study of the interacting system NGC 6845

Abstract

We present optical spectroscopy, B, V , R, and I CCD photometry, and VLA neutral hydrogen observations of the interacting quartet NGC 6845, also know as Klemola 30. NGC 6845 A, the dominant component, sports a broad and bright tidal bridge and a faint tidal tail, which bifurcates. The tidal bridge has a B-I color bluer than that of the inner disk of NGC 6845 A. Five strong condensations, identified as H II regions brighter than the brightest one known in our Galaxy, are found along the tidal bridge, with the two most luminous located at the bridge tip. Two giant H II regions, comparable to 30 Dor, are located where the tidal bridge and the tidal tail join the disk of NGC 6845 A. Since the age of the H II regions is 3-8 Myr, star formation continued to occur along the tidal bridge and in the tidal arm well after they began to be torn apart (≥100 Myr). Satoh model fitting to the optical rotation curve of the A component reveals a kinematical mass of (4.4±1.2)]x1011 M๏ inside the central 12 kpc (H0=75 km s-ˡ Mpc-ˡ). The H I emission consist of two components, a more massive one that belongs to NGC 6845 A and a second one associated with NGC 6845 B. We do not detect gas associated with galaxies C and D. The total amount of H I is 1.4x1010 M๏, 5 time higher than the H I content of the Milky Way. The H I kinematics indicates an amount of dark matter associated with the A component 2 times higher than the mass inside its central 12 kpc. The group kinematics indicates M/L≈43(±2)(M/L)๏ or M/L≈66(±2)(M/L)๏ according to two di†erent prescriptions for the internal absorption correction. In spite of this large di†erence, both values furnish a similar group mass (≈1x1013 M๏). Although preliminary results on N-body simulations indicate that either B or C might well create a tidal feature like the bridge of the A component, the collision with B appears to be more favorable.