First ground-based CCD proper motions for Fornax II: Final results

TL;DR

This paper reports the first ground-based CCD measurement of Fornax's proper motion, compares it with previous data, and analyzes its orbital dynamics within the Milky Way, revealing potential hypervelocity infall scenarios.

Contribution

It provides the first entirely ground-based astrometric proper motion measurement for Fornax and analyzes its orbital properties using this new data.

Findings

Proper motion measurements vary, indicating possible systematic effects.

Fornax's orbit suggests recent perigalacticon at ~150 kpc.

One orbital solution implies Fornax as a hypervelocity satellite with a large apogalactic distance.

Abstract

We present the first entirely ground-based astrometric determination of the proper motion for the Fornax Local Group Dwarf Spheroidal satellite galaxy of the Milky Way, using CCD data acquired with the ESO 3.5 m NTT telescope at La Silla Observatory in Chile. Our unweighted mean from five Quasar fields in the background of Fornax, used as fiducial reference points, leads to $\mu_\alpha \cos \delta=0.62 \pm 0.16$ $\masy$, and $\mu_\delta=-0.53 \pm 0.15$ $\masy$. A detailed comparison with all previous measurements of this quantity seems to imply that there is still no convincing convergence to a single value, perhaps indicating the existence of unnacounted systematic effects in (some of) these measurements. From all available proper motion and radial velocity measurements for Fornax, we compute Fornax's orbital parameters and their uncertainty using a realistic Galactic potential and a Monte Carlo simulation. Properties of the derived orbits are then compared to main star formation episodes in the history of Fornax. All published proper motion values imply that Fornax has recently (200-300 Myr ago) approached perigalacticon at a distance of $\sim$150 kpc. However, the derived period exhibits a large scatter, as does the apogalacticon. Our orbit, being the most energetic, implies a very large apogalactic distance of $\sim 950$ kpc. If this were the case, then Fornax would be a representative of an hypervelocity MW satellite in late infall.