A search for non-circular flows in the extended HI disc of NGC 2915

TL;DR

This study uses new HI observations of NGC 2915 to detect non-circular flows, revealing elliptical streaming, a spherical dark matter halo, and radial outflows, enhancing understanding of galaxy dynamics and dark matter distribution.

Contribution

First detailed analysis of non-circular flows in NGC 2915's extended HI disc using two independent methods, revealing complex kinematic features and dark matter halo shape.

Findings

Evidence for elliptical streaming linked to spiral and bar structures

Detection of a spherical dark matter halo

Identification of axisymmetric radial outflows of 5-17 km/s

Abstract

NGC 2915 is a nearby blue compact dwarf with a differentially rotating HI disc extending out to ~ 5 R-band R_{25} radii. This disc serves as an ideal tracer of the system's gravitational potential in regions of the galaxy that are dominated by dark matter. We use new HI synthesis observations of NGC 2915, obtained with the Australia Telescope Compact Array, to search for non-circular flows within the outer HI disc. Two independent methods are used, and the results of each interpreted in the context of relevant axisymmetric and non-axisymmetric perturbations of the potential. We find evidence for: (1) elliptical streaming associated with the spiral structure of the HI disc and the central bar-like feature in the mass distribution, (2) a spherical dark matter halo, and (3) an axisymmetric radial outflow of ~ 5-17 km/s (~ 6-20 percent of the circular speed). A possible bar-like perturbation of the potential hinders attempts to unambiguously detect kinematic signatures of radial flows in the HI velocity field. The radial outflows are inconsistent with the plausible disc formation scenario in which gas from the surrounding inter-galactic medium is deposited on the outer HI disc and then transported towards the centre of the galaxy. They are, however, consistent with the possibility of some material being re-distributed towards the outer disc in order to conserve angular momentum as material flows inwards along a bar.