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.
Abstract
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⊙.
Source: Gemini Observatory
