First Ground-Based Charge-Coupled Device Proper Motions for Fornax. Ii. Final Results

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 (MW), using charge-coupled device data acquired with the ESO 3.5 m New Technology 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 μ α cos δ = 0.62 ± 0.16 mas yr −1 and μ δ = −0.53 ± 0.15 mas yr −1 . 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 unaccounted 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 ∼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 ∼950 kpc. If this were the case, then Fornax would be a representative of a hypervelocity MW satellite in late infall.

Insights from the outskirts: Chemical and dynamical properties in the outer parts of the Fornax dwarf spheroidal galaxy

“…Most of these studies furthermore predict an orbital period of ∼ 6 Gyr, which implies that Fornax experienced at least two full orbits around the MW during its evolution. In contrast to these studies, Méndez et al (2011) derive a significantly larger orbital period of 21 Gyr paired with an extremely high eccentricity. While the orbital information may play an important role on the evolution of dSphs, the evident discrepancies illustrate the large uncertainty in the orbital properties of Fornax, in particular for large look-back times.…”

“…Recent proper motion studies with both groundbased telescopes (Walker et al 2008;Méndez et al 2011) and the Hubble Space Telescope (Dinescu et al 2004;Piatek et al 2007) agree that the current orbital position of Fornax is close to perigalacticon, which it passed less than 1 Gyr ago. Most of these studies furthermore predict an orbital period of ∼6 Gyr, which implies that Fornax experienced at least two full orbits around the MW during its evolution.…”

The Fornax Dwarf Galaxy as a Remnant of Recent Dwarf-Dwarf Merging in the Local Group

“…In all cases, Fornax is conceived to have been at perigalacticon in the last ∼100-300 Gyr. Méndez et al (2011) propose that this triggered an enhanced star formation at this period, and perhaps a similarly induced starburst >10 Gyr ago, thus supporting their hyper-extended orbit (if the merger scenario can explain the remaining intermediate subpopulation). By contrast, the minimum galactocentric radial distance of 131-148kpc leads Coleman et al (2005) to assert the relative insignificance of tidal forces at this separation.…”

“…In both debates, the influence of the Milky Way (MW) potential has been contemplated with respect to the assumed orbit of Fornax. Successive measurements for the present day proper motion of Fornax have been conducted recently by Dinescu et al (2004), Piatek et al (2007), Walker et al (2008), andMéndez et al (2011), hereafter D04, P07, W08 and M11 respectively. M11 gives a high-energy orbit with an orbital period ∼21 Gyr, while D04, P07 and W08 imply a low-energy orbit with orbital periods ranging between 5-8 Gyr.…”

The effect of tides on the Fornax dwarf spheroidal galaxy

“…In this work, we take under consideration the Fornax dSph. There is consensus that this object is moving on a rather external orbit around the MW (Dinescu et al 2004;Piatek et al 2007, hereafter P07;Méndez et al 2011), so Fornax is a good candidate of dSph that might be close to equilibrium. Our aim is to quantify the effects of tides on Fornax's structure and kinematics, by means of N -body simulations following the evolution in the MW of a satellite dwarf galaxy, similar in structure to Fornax, on orbits that are consistent with the current observational constraints, including Fornax's proper motions.…”