The Phoenix stellar stream, a thin more than-density of stars in the halo of our Milky Way Galaxy initially identified by the Dark Energy Survey, is composed of the debris of a quite ancient globular cluster that was torn apart by the Milky Way’s gravity two billion years ago, according to new investigation led by the University of Sydney.
An artist’s impression of the thin stream of stars torn from the Phoenix globular cluster, wrapping about the Milky Way (left). Wan et al targeted vibrant red giant stars (suitable) to measure the chemical composition of the disrupted Phoenix globular cluster. Image credit: James Josephides, Swinburne Astronomy Productions / S5 Collaboration.
Globular clusters are agglomerations of quite ancient stars, held with each other by their mutual gravitational attraction in a spherical shape about 100-200 light-years across.
They include hundreds of thousands or maybe a million stars. The huge mass in the wealthy stellar center of a cluster pulls the stars inward to kind a ball of stars. The word globulus, from which these objects take their name, is Latin for smaller sphere.
They are amongst the oldest bound stellar structures observed in the Universe and are relics of the initial epochs of galaxy formation.
It is believed that each galaxy has a population of globular clusters. Some, like our Milky Way Galaxy, have a handful of hundred, whilst elliptical galaxies can have numerous thousand globular clusters.
The parent cluster of the Phoenix stellar stream had a lifecycle that was quite unique from the globular clusters we see right now.
“The globular cluster remnants that make up the Phoenix stream were disrupted many billion years ago, but luckily retain the memory of its formation in the very early Universe, which we can read from the chemical composition of its stars,” mentioned Dr. Ting Li, an astronomer at the Observatories of the Carnegie Institution for Science, Princeton University, Fermi National Accelerator Laboratory and the Kavli Institute for Cosmological Physics at the University of Chicago.
An artist’s representation of the Phoenix stellar stream. Image credit: Geraint F. Lewis / S5 Collaboration.
Using the Anglo-Australian Telescope, the astronomers measured the metallicity — the abundance of components heavier than hydrogen and helium — of vibrant red giant star in the Phoenix stellar stream.
“We were really surprised to find that the Phoenix stream has a very low metallicity, making it distinctly different to all of the other globular clusters in the Galaxy,” mentioned Zhen Wan, a Ph.D. student at the University of Sydney.
Because other recognized globular clusters are enriched by the presence of heavy components forged by stellar earlier generations, it was theorized that there was a minimum abundance of heavier components necessary for a globular cluster to kind.
But the Phoenix stream progenitor is properly under this predicted minimum metallicity, posing a considerable dilemma for prior suggestions about how globular clusters are born.
“This stream comes from a cluster that, by our understanding, shouldn’t have existed,” mentioned Dr. Daniel Zucker, an astronomer at Macquarie University.
“One possible explanation is that the Phoenix stream represents the last of its kind, the remnant of a population of globular clusters that was born in radically different environments to those we see today,” Dr. Li mentioned.
While potentially many in the previous, this population of globular clusters was steadily depleted by the gravitational forces of the Galaxy, which tore them to pieces, absorbing their stars into the major physique of the galactic program.
This implies that the stream is a comparatively short-term phenomenon, which will dissipate in time.
“We found the remains of this cluster before it faded forever into the Galaxy’s halo,” Wan mentioned.
As however, there is no clear explanation for the origins of the Phoenix stream progenitor cluster and exactly where it sits in the evolution of galaxies remains unclear.
“There is plenty of theoretical work left to do. There are now many new questions for us to explore about how galaxies and globular clusters form, which is incredibly exciting,” mentioned Professor Geraint Lewis, an astronomer at the University of Sydney.
The discovery is reported in a paper in the journal Nature.
Z. Wan et al. 2020. The tidal remnant of an unusually metal-poor globular cluster. Nature 583, 768-770 doi: 10.1038/s41586-020-2483-six