Journal Article

Gas and dark matter in the Sculptor group: NGC 300

T. Westmeier, R. Braun and B. S. Koribalski

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 4, pages 2217-2236
Published in print February 2011 | ISSN: 0035-8711
Published online January 2011 | e-ISSN: 1365-2966 | DOI: https://dx.doi.org/10.1111/j.1365-2966.2010.17596.x
Gas and dark matter in the Sculptor group: NGC 300

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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.

Keywords: galaxies: individual: NGC 300; galaxies: kinematics and dynamics; galaxies: structure; radio lines: galaxies

Journal Article.  15254 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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