Neutral hydrogen gas in interacting galaxies: The NGC 6221/6215 galaxy group

Abstract

Neutral hydrogen observations of the spiral galaxies NGC 6221 and 6215 with the Australia Telescope Compact Array (ATCA) reveal a wide, two-stranded bridge of at least 3 × 108 M⊙ which can be traced between the two galaxies over a projected distance of 100 kpc. The velocity gradient of the H I bridge provides a rough estimate for the time since the encounter of 500 Myr. For NGC 6221, the brightest and most massive galaxy of the group, we derive a dynamical mass of Mtot = 8 × 1010 M⊙, while its companion NGC 6215 has a mass of only Mtot ∼ 2 × 109 M⊙. Further, we find three low-surface-brightness dwarf galaxies (Dwarfs 1, 2 and 3) in the neighbourhood of NGC 6221/15 with Hi masses of 3.3, 0.6 and 0.3 × 10 8 M⊙, respectively. The smallest, previously uncatalogued galaxy, Dwarf 3, lies between NGC 6221 and 6215, and may have formed out of bridge material. The brightest part of the Hi bridge lies roughly halfway between the interacting galaxies, indicating that bridge gas close to NGC 6221 and 6215 may have fallen back to the galaxies. The asymmetric extensions to the Hi envelope of NGC 6221 are likely to be reaccreted gas, still settling in. Also, the peculiar velocity field of NGC 6215 may be explained by accreted bridge material settling into a plane offset from the old disc.