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Australia Telescope Compact Array H I observations of the NGC 6845 galaxy group
We present the results of Australia Telescope Compact Array (ATCA) HIline and 20-cm radio continuum observations of the galaxy quartet NGC6845. The HI emission extends over all four galaxies but can only beassociated clearly with the two spiral galaxies, NGC 6845A and B, whichshow signs of strong tidal interaction. We derive a total HI mass of atleast 1.8 × 1010 Msolar, most of which isassociated with NGC 6845A, the largest galaxy of the group. Weinvestigate the tidal interaction between NGC 6845A and B by studyingthe kinematics of distinct HI components and their relation to the knownHII regions. No HI emission is detected from the two lenticulargalaxies, NGC 6845C and D. A previously uncatalogued dwarf galaxy, ATCAJ2001-4659, was detected 4.4 arcmin NE from NGC 6845B and has an HI massof ~5 × 108 Msolar. No HI bridge is visiblebetween the group and its newly detected companion. Extended 20-cm radiocontinuum emission is detected in NGC 6845A and B as well as in thetidal bridge between the two galaxies. We derive star formation rates of15-40 Msolar yr-1.
| Study of the Interacting System NGC 6845
We present optical spectroscopy, B, V, R, and I CCD photometry, and VLAneutral hydrogen observations of the interacting quartet NGC 6845, alsoknow as Klemola 30. NGC 6845 A, the dominant component, sports a broadand bright tidal bridge and a faint tidal tail, which bifurcates. Thetidal bridge has a B-I color bluer than that of the inner disk of NGC6845 A. Five strong condensations, identified as H II regions brighterthan the brightest one known in our Galaxy, are found along the tidalbridge, with the two most luminous located at the bridge tip. Two giantH II regions, comparable to 30 Dor, are located where the tidal bridgeand the tidal tail join the disk of NGC 6845 A. Since the age of the HII regions is 3-8 Myr, star formation continued to occur along the tidalbridge and in the tidal arm well after they began to be torn apart(>=100 Myr). Satoh model fitting to the optical rotation curve of theA component reveals a kinematical mass of (4.4+/-1.2)x10^11 M_solar,inside the central 12 kpc (H_0=75 km s^-1 Mpc^-1). The H I emissionconsist of two components, a more massive one that belongs to NGC 6845 Aand a second one associated with NGC 6845 B. We do not detect gasassociated with galaxies C and D. The total amount of H I is 1.4x10^10M_solar, 5 time higher than the H I content of the Milky Way. The H Ikinematics indicates an amount of dark matter associated with the Acomponent 2 times higher than the mass inside its central 12 kpc. Thegroup kinematics indicates M/L~43(+/-2)(M/L)_solar orM/L~66(+/-2)(M/L)_solar, according to two different prescriptions forthe internal absorption correction. In spite of this large difference,both values furnish a similar group mass (~1x10^13 M_solar). Althoughpreliminary results on N-body simulations indicate that either B or Cmight well create a tidal feature like the bridge of the A component,the collision with B appears to be more favorable.
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