Gas and dark matter in the Sculptor group: NGC 300

Abstract

We used the Australia Telescope Compact Array to map a large field of approximately 2°× 2° around the Sculptor group galaxy NGC 300 in the 21-cm line emission of neutral hydrogen. We achieved a 5σ H i column density sensitivity of 1019 cm-2 over a spectral channel width of 8 km s-1 for emission filling the 180 × 88 arcsec2 synthesized beam. The corresponding H i mass sensitivity is 1.2 × 105 M⊙, assuming a distance of 1.9 Mpc. For the first time, the vast H i disc of NGC 300 has been mapped over its entire extent at a moderately high spatial resolution of about 1 kpc.NGC 300 is characterized by a dense inner H i disc, well aligned with the optical disc of 290° orientation angle, and an extended outer H i disc with a major axis of more than 1° on the sky (equivalent to a diameter of about 35 kpc) and a different orientation angle of 332°. A significant fraction (about 43 per cent) of the total detected H i mass of 1.5 × 109 M⊙ resides within the extended outer disc. We fitted a tilted ring model to the velocity field of NGC 300 to derive the rotation curve out to a radius of 18.4 kpc, almost twice the range of previous rotation curve studies. The rotation curve rises to a maximum velocity of almost 100 km s-1 and then gently decreases again in the outer disc beyond a radius of about 10 kpc. Mass models fitted to the derived rotation curve yield good fits for Burkert and NFW dark matter halo models, whereas pseudo-isothermal halo models and Modified Newtonian Dynamics (MOND)-based models both struggle to cope with the declining rotation curve.We also observe significant asymmetries in the outer H i disc of NGC 300, in particular near the edge of the disc, which are possibly due to ram-pressure stripping of gas by the intergalactic medium (IGM) of the Sculptor group. Our estimates show that ram-pressure stripping can occur under reasonable assumptions on the density of the IGM and the relative velocity of NGC 300. The asymmetries in the gas disc suggest a proper motion of NGC 300 towards the south-east. At the same time, our data exclude IGM densities of significantly higher than 10-5 cm-3 in the vicinity of NGC 300, as otherwise the outer gas disc would have been stripped. © 2010 CSIRO. Journal compilation © 2010 RAS.