A team of astronomers using the Gemini-South telescope has discovered one of the oldest stellar clusters in the Milky Way. The team’s results date the globular cluster, named HP 1, at about 12.8 billion years, making its stars among the oldest ever found in our Galaxy.
Color composite image of the globular cluster HP 1 obtained using the Gemini-South telescope in Chile. Image credit: Gemini Observatory / AURA / NSF / Mattia Libralato, Space Telescope Science Institute.
Globular clusters are ancient groups of thousands or even millions of stars, gravitationally bound into a single structure about 100-200 light-years across.
These massive stellar systems are among the oldest known objects in the Universe and are relics of the first epochs of galaxy formation.
Most of them are thought to have coalesced out of the primordial gas cloud that later collapsed to form the Milky Way’s spiral disk, while others appear to be the cores of dwarf galaxies consumed by the Galaxy.
Of about 150-180 globular clusters known in the Milky Way, about a quarter are located within the Galaxy’s greatly obscured and tightly packed central bulge region. This spherical mass of stars some 10,000 light years across forms the central hub of the Milky Way which is made primarily of old stars, gas, and dust.
Among the clusters within the bulge, those that are the most metal-poor (lacking in heavier elements) — which includes HP 1 — have long been suspected of being the oldest.
HP 1 then is pivotal, as it serves as an excellent tracer of our Galaxy’s early chemical evolution.
“This star cluster is like an ancient fossil buried deep in our Galaxy’s bulge, and now we’ve been able to date it to a far-off time when the Universe was very young,” said team member Dr. Stefano Souza, a PhD student at the Universidade de São Paulo, Brazil.
“These are also some of the oldest stars we’ve seen anywhere.
“HP 1 is one of the surviving members of the fundamental building blocks that assembled our Galaxy’s inner bulge,” said team leader Dr. Leandro Kerber, an astronomer at the Universidade de São Paulo and Universidade Estadual de Santa Cruz, Brazil.
Until a few years ago, astronomers believed that the oldest globular star clusters were only located in the outer parts of the Milky Way, while the younger ones resided in the innermost Galactic regions.
However, this study, as well as other recent work based on data from the Gemini Observatory and the NASA/ESA Hubble Space Telescope, have revealed that ancient star clusters are also found within the Galactic bulge and relatively close to the Galactic center.
“HP 1 is playing a critical role in our understanding of how the Milky Way formed. It is helping us to bridge the gap in our understanding between our Galaxy’s past and its present,” Dr. Kerber said.
The astronomers used the exquisitely deep high-resolution adaptive optics images from Gemini Observatory as well as archival optical images from Hubble to identify faint cluster members, which are essential for age determination.
With this rich data set they confirmed that HP 1 is a fossil relic born less than a billion years after the Big Bang, when the Universe was in its infancy.
To determine the cluster’s distance, they used archival ground-based data to identify 11 RR Lyrae variable stars within HP 1.
The observed brightness of these stars indicate that HP 1 is at a distance of about 21,500 light years, placing it approximately 6,000 light years from the Galactic center, well within the Galaxy’s central bulge region.
The team also used data from Gemini, Hubble, ESO’s Very Large Telescope, and ESA’s Gaia mission to refine the orbit of HP 1 within the Milky Way.
This analysis shows that during HP 1’s history, the cluster came as close as about 400 light years from the Galactic center — less than one-tenth of its current distance.
The findings appear in the Monthly Notices of the Royal Astronomical Society.
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L.O. Kerber et al. 2019. A deep view of a fossil relic in the Galactic bulge: the Globular Cluster HP 1. MNRAS 484 (4): 5530-5550; doi: 10.1093/mnras/stz003
