The tidal evolution of the Fornax dwarf spheroidal and its globular clusters

TL;DR

This study uses N-body simulations to investigate how Galactic tides influence the dark matter content and globular cluster dynamics of the Fornax dwarf galaxy, suggesting tidal stripping can reconcile observations with LCDM predictions.

Contribution

It demonstrates that tidal stripping can significantly reduce Fornax's dark matter halo mass and affect globular cluster dynamics, aligning models with observations.

Findings

Tidal stripping reduces dark matter mass within Fornax's half-mass radius by about half.

Globular cluster distribution is consistent with a tidally-stripped cuspy halo.

Pericentric distance estimates vary with Fornax's assumed distance, affecting tidal impact predictions.

Abstract

The dark matter (DM) content of the Fornax dwarf spheroidal galaxy inferred from its kinematics is substantially lower than expected from LCDM cosmological simulations. We use N-body simulations to examine whether this may be the result of Galactic tides. We find that, despite improved proper motions from the Gaia mission, the pericentric distance of Fornax remains poorly constrained, mainly because its largest velocity component is roughly anti-parallel to the solar motion. Translating Fornax's proper motion into a Galactocentric velocity is thus sensitively dependent on Fornax's assumed distance: the observed distance uncertainty, $\pm 8\%$, implies pericentric distances that vary between $r_{\rm peri}\sim 50$ and $r_{\rm peri}\sim 150$ kpc. Our simulations show that for $r_{\rm peri}$ in the lower range of that estimate, a LCDM subhalo with maximum circular velocity $V_{\rm max}=40$ km s$^{-1}$ (or virial mass $M_{200}\approx 10^{10} M_\odot$, as expected from LCDM) would be tidally stripped to $V_{\rm max} \sim 23$ km s$^{-1}$ over $10$ Gyr. This would reduce the DM mass within the Fornax stellar half-mass radius to about half its initial value, bringing it into agreement with observations. Tidal stripping affects mainly Fornax's DM halo; its stellar component is affected little, losing less than $5\%$ of its initial mass in the process. We also explore the effect of Galactic tides on the dynamical friction decay times of Fornax's population of globular clusters (GC) and find little evidence for substantial changes, compared with models run in isolation. A population of GCs with initial orbital radii between $1$ and $2$ kpc is consistent with the present-day spatial distribution of Fornax GCs, despite assuming a cuspy halo. Neither the DM content nor the spatial distribution of GCs seem inconsistent with a simple model where Fornax inhabits a tidally-stripped cuspy cold DM halo.