Gas and dark matter in the Sculptor group: NGC 55

Abstract

We present new, sensitive HI observations of the Sculptor group galaxy NGC 55 with the Australia Telescope Compact Array. We achieve a 5 sigma HI column density sensitivity of 10^19 cm^-2 over a spectral channel width of 8 km/s for emission filling the 158" x 84" synthesised beam. Our observations reveal for the first time the full extent of the HI disc of NGC 55 at this sensitivity and at a moderately high spatial resolution of about 1 kpc. The HI disc of NGC 55 appears to be distorted on all scales. There is a strong east-west asymmetry in the column density distribution along the major axis, suggesting that the disc is under the influence of ram-pressure forces. We also find evidence of streaming motions of the gas along the bar of NGC 55. The fitting of tilted rings to the velocity field reveals a strong warping of the outer gas disc which could be the result of tidal interaction with either NGC 300 or a smaller satellite galaxy. Finally, we find a large number of distinct clumps and spurs across the entire disc, indicating that internal or external processes, such as satellite accretion or gas outflows, have stirred up the gas disc. We also detect several isolated HI clouds within about 20 kpc projected distance from NGC 55. Their dynamical properties and apparent concentration around NGC 55 suggest that most of the clouds are forming a circum-galactic population similar to the high-velocity clouds of the Milky Way and M31, although two of the clouds could be foreground objects and part of the Magellanic Stream. While it is difficult to determine the origin of these clouds, our data seem to favour either tidal stripping or gas outflows as the source of the gas.