The massive dark halo of the compact, early-type galaxy NGC 1281

Abstract

We investigate the compact, early-type galaxy NGC 1281 with integral field unit observations to map the stellar LOSVD out to 5 effective radii and construct orbit-based dynamical models to constrain its dark and luminous matter content. Under the assumption of mass-follows-light, the H-band stellar mass-to-light ratio (M/L) is Υ = 2.7(+-0.1) Υ_{sun}, higher than expected from our stellar population synthesis fits with either a canonical Kroupa (Υ = 1.3 Υ_{sun}) or Salpeter (Υ = 1.7 Υ_{sun}) stellar initial mass function. Such models also cannot reproduce the details of the LOSVD. Models with a dark halo recover the kinematics well and indicate that NGC 1281 is dark matter dominated, making up ~ 90 per cent of the total enclosed mass within the kinematic bounds. Parameterised as a spherical NFW profile, the dark halo mass is 11.5 < log(M_{DM}/M_{sun}) < 11.8 and the stellar M/L is 0.6 < Υ < 1.1. However, this stellar M/L is lower than predicted by its old stellar population. Moreover, the halo mass within the kinematic extent is ten times larger than expected based on ΛCDM predictions, and an extrapolation yields cluster sized dark halo masses. Adopting Υ = 1.7 Υ_{sun} yields more moderate dark halo virial masses, but these models fit the kinematics worse. A non-NFW model might solve the discrepancy between the unphysical consequences of the best-fitting dynamical models and models based on more reasonable assumptions for the dark halo and stellar mass-to-light ratio, which are disfavoured according to our parameter estimation.