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 line-of-sight velocity distribution (LOSVD) out to five 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_⊙, and higher than expected from our stellar population synthesis fits with either a canonical Kroupa (Υ_⋆ = 1.3 Υ_⊙) or Salpeter (Υ_⋆ = 1.7 Υ_⊙) 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. Parametrized as a spherical NFW profile, the dark halo mass is 11.5 ≤ log(M_DM/M_⊙) ≤ 11.8 and the stellar M/L is 0.6 ≤ Υ_⋆/Υ_⊙ ≤ 1.1. However, this M/L is lower than predicted by its old stellar population. Moreover, the halo mass within the kinematic extent is 10 times larger than expected based on Λ-cold-dark-matter predictions, and an extrapolation yields cluster-sized dark halo masses. Adopting Υ_⋆ = 1.7 Υ_⊙ 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 M/L, which are disfavoured according to our parameter estimation.