The low dark matter content of the lenticular galaxy NGC 3998

TL;DR

This study uses integral-field spectroscopy and orbit modeling to reveal that the lenticular galaxy NGC 3998 contains a surprisingly low amount of dark matter within its inner regions, challenging typical galaxy models.

Contribution

The paper provides detailed kinematic analysis and orbit modeling of NGC 3998, demonstrating its low dark matter content and complex stellar structure, including a bulge, disc, and counter-rotating component.

Findings

Dark matter fraction within half-light radius is about 7%.

Galaxy is nearly face-on, fast-rotating, and axisymmetric.

Mass-to-light ratio aligns with previous spectral studies.

Abstract

We observed the lenticular galaxy NGC 3998 with the Mitchell Integral-Field Spectrograph and extracted line-of-sight velocity distributions out to 3 half-light radii. We constructed collisionless orbit models in order to constrain NGC 3998's dark and visible structure, using kinematics from both the Mitchell and SAURON instruments. We find NGC 3998 to be almost axisymmetric, seen nearly face on with a flattened intrinsic shape - i.e., a face-on fast-rotator. We find an I-band mass-to-light ratio of $4.7_{-0.45}^{+0.32}$ in good agreement with previous spectral fitting results for this galaxy. Our best-fit orbit model shows a both a bulge and a disc component, with a non-negligible counter-rotating component also evident. We find that relatively little dark matter is needed to model this galaxy, with an inferred dark mass fraction of just $(7.1^{+8.1}_{-7.1})\%$ within one half-light radius.