ar X iv : a st ro - p h / 99 06 43 5 v 1 2 8 Ju n 19 99 Study of the interacting system NGC 6845

We present optical spectroscopy, B,V,R and I CCD photometry and VLA neutral hydrogen observations of the interacting quartet NGC6845, also know as Klemola 30. NGC6845A, the dominant component, sports a broad and bright tidal bridge and a faint tidal tail, which bifurcate. The tidal bridge has a (B-I) color bluer than that of NGC6845A inner disk. Five strong condensations, identified as Hii 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 Hii regions, comparable to 30Dor, are located where the tidal bridge and the tidal tail joint the disk of NGC6845A. Since the age of the Hii regions is 3-8Myr, star formation has been occurring along the tidal bridge and in the tidal arm well after they had begun to be torn apart (≥100Myr). Satoh model fitting to the optical rotation curve of the A component reveals a kinematical mass of 4.4 (±1.2) 10 M⊙, inside the central 12 kpc (H0 = 75 km s −1 Mpc). The Hi emission consist of two components, a more massive one that belongs to NGC6845A, and a second one associated to NGC6845B. We do not detect gas associated to galaxies C and D. The total amount of Hi is 1.4× 10M⊙, five time higher than the Hi content of the Milky Way. The HI kinematics indicates an amount of dark matter associated to the A component two times higher than the mass inside its central 12 kpc. The group kinematics indicates an M/L ≈ 43(±2) (M/L) ⊙ or M/L ≈ 66(±2) (M/L) ⊙ , according to two different prescriptions for the internal absorption correction. In spite of this large difference, both values furnish similar group mass (≈ 1× 10 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 favourable. Subject headings: galaxies: fundamental parameters — galaxies: individual (NGC6845) — galaxies: interactions — galaxies: kinematics and dynamics — galaxies: photometry Brazilian CNPq fellow. Visiting astronomer, Complejo Astronómico El Leoncito, operated under agreement between the Consejo Nacional de Investigaciones Cient́ıficas y Técnicas de la República Argentina and the Universities of La Plata, Córdoba y San Juan.