A scan employing a mix of ground and space-based observatories discovered a wealth of previously unknown globular clusters, which are old, dense collections of thousands of stars that all formed at the same time in the elliptical galaxy Centaurus A’s outer regions.
The research advances our understanding of the galaxy’s architecture and cosmological history, as well as providing new insights into galaxy formation in general and the distribution of dark matter throughout the universe.
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 presenting the findings, which was published in the Astrophysical Journal in June. She’ll give a virtual press conference at the Astronomical Society of America’s 239th Meeting on Tuesday to present the findings.
Centaurus A, also known as NGC 5128, is a visually magnificent elliptical galaxy with a relativistic jet erupting from a supermassive black hole at its center and spectacular streams of scattered stars leftover from prior collisions and mergers with smaller galaxies.
Centaurus A, which is 13 million light-years from Earth and located in the constellation Centaurus, is too far away for astronomers to identify individual stars, but star clusters can be detected and utilized as “fossil evidence” of the galaxy’s turbulent growth.
Hughes and her colleagues provide a new catalog of almost 40,000 globular cluster candidates in Centaurus A, advising follow-up observations on a group of 1,900 that are most likely to be real globular clusters.
The researchers looked for prospects for globular clusters out to a radius of around 150 kiloparsecs, or almost half a million light-years from the galaxy’s center.
The data comes from the Panoramic Imaging Survey of Centaurus and Sculptor, or PISCeS; Gaia, a European Satellite Agency space observatory; and the NOAO Source Catalog, which includes publicly accessible images from telescopes in both hemispheres that cover nearly the entire sky.
Centaurus A may look like an odd outlier, but that’s only because we can get close enough to see its nitty-gritty details. More likely than not, both elliptical and spiral galaxies like the Milky Way are messier than we realize as soon as we look a little bit deeper than just on the surface.
Allison Hughes
Because of its richness and proximity to Earth, Centaurus A has been a popular target for extragalactic globular cluster research. However, most studies have concentrated on the galaxy’s inner 40 kiloparsecs (roughly 130,500 light-years), leaving the galaxy’s outer reaches largely unexplored, according to Hughes.
The researchers determined that about 1,900 candidates are extremely likely to be confirmed as real globular clusters and should be given priority 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. Centaurus A’s structure tells astronomers that it went through several major mergers with other galaxies, leading to its glob-like appearance with river-like regions that have many more stars than the surrounding areas,” Hughes said.
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 the Andromeda Galaxy, the Milky Way’s nearest neighbor, are spiral galaxies.
Spiral galaxies may appear to be the “typical” galaxy because of their famous pinwheel-like form, yet their less ordered elliptical cousins outweigh them in the cosmos.
“Centaurus A may look like an odd outlier, but that’s only because we can get close enough to see its nitty-gritty details,” Hughes said. “More likely than not, both elliptical and spiral galaxies like the Milky Way are messier than we realize as soon as we look a little bit deeper than just on the surface.”
“Globular clusters serve as evidence of processes that happened a long time ago,” Hughes said.
“For example, if you see a line of these globular clusters that all have similar metallicity (chemical composition) and move with similar radial velocity, we know they must have come from the same dwarf galaxy or some similar object that collided with Centaurus A and is now in the process of being assimilated.”
In the interstellar medium, concentrated regions of gas produce star clusters. Although globular clusters can be found in almost every galaxy, including the Milky Way, which has roughly 150 of them, most stars are not grouped in such groupings.
According to Hughes, astronomers may learn a lot about the galaxy that hosts globular clusters by studying them. They can learn about the galaxy’s mass, history of interactions with surrounding galaxies, and even the distribution of dark matter.
“Globular clusters are interesting because they can be used as tracers of structures and processes in other galaxies where we can’t resolve individual stars,” Hughes said. “They hold on to chemical signatures, such as the elemental composition of their individual stars, so they tell us something about the environment in which they formed.”
According to Hughes, the researchers were looking for globular clusters far from the galaxy’s center since Centaurus A’s substructure suggests a big, unknown population of such clusters. Previous investigations have discovered slightly under 600 clusters in the galaxy’s center regions, but the galaxy’s outer regions remained mostly unexplored.
“We looked farther out and discovered more than 100 new clusters already, and most likely there are more because we haven’t even finished processing the data,” Hughes said.
“We can then use that data to reconstruct the architecture and movements in that galaxy, and also figure out its mass,” Hughes said. “From that we can eventually subtract all its stars and see what’s left that invisible mass must be its dark matter.”
