A team of astronomers reveals an unusual number of globular cluster candidates in the galaxy’s outer regions using observations of Centaurus A, a neighboring elliptical galaxy, taken with the Gaia satellite telescope and ground-based equipment as part of the PISCeS survey. The discoveries give astronomers a more precise picture of galaxy architecture and the history of collisions and mergers.
A scan done with a combination of ground and space-based observatories uncovered a treasure trove of hitherto unknown globular clusters – ancient, dense groups of thousands of stars that all formed at the same time – in the elliptical galaxy Centaurus A’s outer regions. The work represents a substantial gain in understanding the architecture and cosmic history of this galaxy, as well as fresh insights into galaxy formation and the distribution of dark matter in the universe in general.
Allison Hughes, a doctorate student in the Department of Astronomy and Steward Observatory at the University of Arizona, is the first author of a peer-reviewed study describing the findings that was published in the Astrophysical Journal in June. She will reveal the findings during a virtual press conference at the Astronomical Society of America’s 239th Meeting.
Centaurus A, commonly known as NGC 5128, is an elliptical galaxy with a relativistic jet spewing from its center and beautiful streams of scattered stars left behind by prior collisions and mergers with smaller galaxies surrounding Centaurus A. Centaurus A, located 13 million light-years from Earth, is too far away for astronomers to identify individual stars, yet star clusters can be identified as such and utilized as “fossil evidence” of the galaxy’s turbulent growth.
We’re using the Gaia satellite, which mostly focuses on surveys within our own galaxy, the Milky Way, in a new way in that we link up its observations with telescopes on the ground, in this case the Magellan Clay telescope in Chile and the Anglo-Australian Telescope in Australia.
Allison Hughes
Hughes and her colleagues provide a new catalog of roughly 40,000 Centaurus A globular cluster candidates, advising follow-up observations focusing on a subset of 1,900 that are most likely to be real globular clusters. The researchers studied globular cluster candidates out to a projected radius of around 150 kiloparsecs, or roughly 500,000 light-years from the galaxy’s center. The data comprises observations from the Panoramic Imaging Survey of Centaurus and Sculptor, or PISCeS; Gaia, a European Satellite Agency space observatory; and the NOAO Source Catalog, which aggregates publically available photos from telescopes in both hemispheres covering nearly the whole sky.
Centaurus A has been a leading target for extragalactic globular cluster studies due to its richness and proximity to Earth, but the majority of studies have focused on the galaxy’s inner 40 kiloparsecs (about 130,500 light-years), leaving the galaxy’s outer reaches largely unexplored, according to Hughes. Ranking the candidates based on their likelihood of being real globular clusters, the team discovered that roughly 1,900 are highly likely to be confirmed as such and should be prioritized for follow-up spectroscopic confirmation.
“We’re using the Gaia satellite, which mostly focuses on surveys within our own galaxy, the Milky Way, in a new way in that we link up its observations with telescopes on the ground, in this case the Magellan Clay telescope in Chile and the Anglo-Australian Telescope in Australia.”
According to Hughes, Centaurus A’s structure indicates that it went through numerous large mergers with other galaxies, resulting in its glob-like appearance with river-like parts that have considerably more stars than the surrounding areas. Centaurus A, the closest example of an elliptical galaxy, allows scientists to investigate a galaxy that is substantially different from our own up close. Both the Milky Way and its nearest neighbor, the Andromeda Galaxy, are spiral galaxies. Spiral galaxies, with their iconic, pinwheel-like appearance, may appear to be the “typical” galaxy, yet their less ordered elliptical cousins outnumber them in the universe.
“Centaurus A may appear to be a strange outlier, but that’s only because we can get near enough to examine its nitty gritty characteristics,” Hughes explained. “More likely than not, both elliptical and spiral galaxies like the Milky Way are messier than we realize when we look a little deeper than just on the surface.”
Hughes explained that globular clusters are evidence of processes that occurred a long time ago. “For example, if you see a line of these globular clusters that all have comparable metallicity (chemical composition) and move with similar radial velocity, we know they must have come from the same dwarf galaxy or another similar object that crashed with Centaurus A and is now being assimilated.”
Star clusters arise in the interstellar medium from concentrated regions of gas. Almost every galaxy, including the Milky Way, features globular clusters, although most stars are not grouped in such groupings. According to Hughes, examining globular clusters can provide astronomers with information about the galaxy that hosts them, such as its mass, history of interactions with surrounding galaxies, and even the distribution of dark matter within.
“Globular clusters are interesting because they can be utilized as tracers of structures and processes in other galaxies where individual stars cannot be resolved,” Hughes explained. “They preserve chemical traces, such as the elemental composition of their individual stars, which reveal information about the environment in which they evolved.”
The researchers deliberately looked for globular clusters far from the galaxy’s core because Centaurus A’s substructure suggests a big, unknown population of such clusters, according to Hughes. Previous scans have discovered just under 600 clusters in the galaxy’s more core parts, but the galaxy’s outer regions remained completely unexplored.
“We went further afield and uncovered more than 100 new clusters already, and there are most certainly more because we haven’t even completed processing the data,” Hughes explained. “We can then utilize that data to rebuild the architecture and movements of that galaxy, as well as its mass,” Hughes explained. “We can finally eliminate all of its stars to see what’s left — that unseen mass must be its dark matter.”
