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 .