Dearth of dark matter or massive dark halo? Mass-shape-anisotropy degeneracies revealed by NMAGIC dynamical models of the elliptical galaxy NGC 3379

Abstract

Recent results from the Planetary Nebula Spectrograph (PNS) survey have revealed a rapidly falling velocity dispersion profile in the nearby elliptical galaxy NGC 3379, casting doubts on whether this intermediate-luminosity galaxy has the kind of dark matter (DM) halo expected in Λ cold dark matter (ΛCDM) cosmology. We present a detailed dynamical study of this galaxy, combining ground based long-slit spectroscopy, integral-field data from the Spectrographic Areal Unit for Research on Optical Nebulae (SAURON) instrument and PNS data reaching to more than seven effective radii. We construct dynamical models with the flexible χ2-made-to-measure (χ2M2M) particle method implemented in the nmagic code. We fit spherical, axisymmetric and some triaxial models to the photometric and combined kinematic data in a sequence of gravitational potentials whose circular velocity curves at large radii vary between a near-Keplerian decline and the nearly flat shapes generated by massive haloes. We find that models with a range of halo masses, anisotropies, shapes and inclinations are good representations of the data. In particular, the data are consistent both with near-isotropic systems dominated by the stellar mass and with models in moderately massive haloes with strongly radially anisotropic outer parts (β ≳ 0.8 at 7Re). Formal likelihood limits would exclude (at 1σ) the model with stars only, as well as halo models with vcirc(7Re) ≳ 250 km s-1. All valid models fitting all the data are dynamically stable over gigayears, including the most anisotropic ones. Overall the kinematic data for NGC 3379 out to 7Re are consistent with a range of mass distributions in this galaxy. NGC 3379 may well have a DM halo as predicted by recent merger models within ΛCDM cosmology, provided its outer envelope is strongly radially anisotropic.