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Gemini Tracks Distant Star Cluster with Adaptive Optics

May 11, 2017

Figure 1: Gemini Multi-Object Spectrograph (GMOS-South) of the Pyxis field (left image), with the center of the cluster marked with a red star. A zoom of the pseudo color image of Pyxis observed with the Gemini South Adaptive Optics Imager (GSAOI) used with the Gemini Multi-conjugate adaptive optics System (GeMS) is shown at right. The field of view of GMOS is 5 x 5 arcminutes, 85 x 85 arcseconds for GeMS.

Researchers combine images from Gemini South’s wide-field adaptive optics system (GeMS/GSAOI) with data from the Hubble Space Telescope (HST) to determine the proper motion of a distant cluster of stars. The observations, the first to use ground-based adaptive optics to precisely measure the motion of a cluster at such a large distance, allowed astronomers to set a lower limit for the mass of our Milky Way while providing clues about the cluster’s origin.

A study of the proper motion (apparent motion in the sky due to an object's motion around our galaxy) of several substructures across the Milky Way’s halo is underway at Gemini South. As part of this study the team used Adaptive Optics (AO) at Gemini South, along with data from HST, to focus on a distant cluster called Pyxis. The work allowed the team to set a lower limit for the Milky Way’s mass of 950 million solar masses. This value is consistent with most, but not all, previous determinations.

The wide-field Gemini Multi-conjugate adaptive optics System (GeMS) combined with the Gemini South Adaptive Optics Imager (GSAOI) provided the Gemini data. “We used GeMS/GSAOI to estimate the proper motion for halo objects because normal (seeing limited) ground-based telescopes need a time baseline of more than 15 years for this measurement,” says Tobias Fritz (University of Virginia) who leads the research team. “GeMS/GSAOI with its better spatial resolution can make that measurement in five years, the same types of baselines required from space-based proper motions (like HST),” continues Fritz. The team was able to measure absolute proper motions of Pyxis using GeMS/GSAOI, which provided a resolution of 0.08 arcsecond and combined that with archival HST images, with a resolution of ~ 0.1 arcsecond. Fritz adds, “The study of motions for halo objects, like Pyxis, can constrain the mass distribution of our Galaxy at large distances and thus the mass of the Milky Way.”

Pyxis, a densely packed collection of ancient stars, is one of the most distant examples of a globular clusters, dense clusters of stars which orbit our galaxy. The cluster is located some 130,000 light years away and is thought to be about 2 billion years younger than other globular clusters with the same ratio of heavier elements (metallicity). Together, these characteristics imply Pyxis did not form with other Milky Way clusters. Instead, it is likely that Pyxis was formed in a massive dwarf galaxy that was then accreted by the Milky Way. Thus, Pyxis has an extragalactic origin. However, the orbits of the known massive dwarf galaxies are inconsistent with the orbit of Pyxis, which is derived from the new proper motion measurements.

The paper, titled: The Proper Motion of Pyxis: The First Use of Adaptive Optics in Tandem with HST on a Faint Halo Object is published in The Astrophysical Journal. The work is part of a Large and Long program at Gemini that is also targeting other clusters, dwarf galaxies, and individual stars in stellar streams.

Figure 2: Absolute velocity of Pyxis. Each blue dot stands for a velocity derived from a single background galaxy. The red box shows the weighted average velocity derived from all galaxies. Only galaxies with small errors are shown for clarity.


We present a proper motion measurement for the halo globular cluster Pyxis, using HST/ACS data as the first epoch, and GeMS/GSAOI Adaptive Optics data as the second, separated by a baseline of ∼ 5 years. This is both the first measurement of the proper motion of Pyxis and the first calibration and use of Multi-Conjugate Adaptive Optics data to measure an absolute proper motion for a faint, distant halo object. Consequently, we present our analysis of the Adaptive Optics data in detail. We obtain a proper motion of µα cos(δ) =1.09±0.31 mas yr−1 and µδ =0.68±0.29 mas yr−1. From the proper motion and the line-of-sight velocity we find the orbit of Pyxis is rather eccentric with its apocenter at more than 100 kpc and its pericenter at about 30 kpc. We also investigate two literature-proposed associations for Pyxis with the recently discovered ATLAS stream and the Magellanic system. Combining our measurements with dynamical modeling and cosmological numerical simulations we find it unlikely Pyxis is associated with either system. We examine other Milky Way satellites for possible association using the orbit, eccentricity, metallicity, and age as constraints and find no likely matches in satellites down to the mass of Leo II. We propose that Pyxis probably originated in an unknown galaxy, which today is fully disrupted. Assuming that Pyxis is bound and not on a first approach, we derive a 68% lower limit on the mass of the Milky Way of 0.95×1012 M⊙.