The dark matter content of the blue compact dwarf NGC 2915

TL;DR

This study investigates the dark matter distribution in NGC 2915, revealing it is heavily dark-matter dominated with a core-dominated halo, using new HI observations and mass modeling.

Contribution

The paper provides detailed HI observations and mass modeling of NGC 2915, demonstrating its extreme dark matter dominance and characterizing its dark matter halo properties.

Findings

NGC 2915 is dark-matter-dominated at nearly all radii.

The total mass to light ratio is approximately 140.

The dark matter halo is best described by a pseudo-isothermal sphere.

Abstract

NGC 2915 is a nearby blue compact dwarf with the HI properties of a late-type spiral. Its large, rotating HI disk (extending out to R ~ 22 B-band scale lengths) and apparent lack of stars in the outer HI disk make it a useful candidate for dark matter studies. New HI synthesis observations of NGC 2915 have been obtained using the Australian Telescope Compact Array. These data are combined with high-quality 3.6 $\mu$m imaging from the Spitzer Infrared Nearby Galaxies Survey. The central regions of the HI disk are shown to consist of two distinct HI concentrations with significantly non-Gaussian line profiles. We fit a tilted ring model to the HI velocity field to derive a rotation curve. This is used as input for mass models that determine the contributions from the stellar and gas disks as well as the dark matter halo. The galaxy is dark-matter-dominated at nearly all radii. At the last measured point of the rotation curve, the total mass to blue light ratio is ~ 140 times solar, making NGC 2915 one of the darkest galaxies known. We show that the stellar disk cannot account for the steeply-rising portion of the observed rotation curve. The best-fitting dark matter halo is a pseudo-isothermal sphere with a core density $\rho_0\sim 0.17 \pm 0.03$ \msun pc$^{-3}$ and a core radius $r_c\sim 0.9 \pm 0.1$ kpc.