A treasure trove of previously unknown globular clusters – old, dense groups of thousands of stars that all formed at the same time hold “fossil evidence” of the galaxy’s turbulent growth, according to the new study.
A team of astronomers led by the University of Arizona reports an unparalleled number of globular clusters using observations of Centaurus A, a neighboring elliptical galaxy, taken with the Gaia satellite telescope and ground-based sensors as part of the PISCeS survey.
In the outer reaches of the elliptical galaxy Centaurus A, a probe using a mix of ground and space-based telescopes revealed a treasure trove of hitherto unknown globular clusters – old, dense groups of thousands of stars that all formed at the same time. 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.
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.
About 40,000 new globular cluster candidates in Centaurus A have been catalogued by Dr. Allison Hughes, a doctoral student in the University of Arizona Department of Astronomy and Steward Observatory, the first author and her colleagues, and they urge that astronomers focus their follow-up studies on the 1,900 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.
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. 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,” said the study authors.
According to Hughes, Centaurus A’s structure indicates that it underwent multiple large mergers with other galaxies, resulting in its glob-like appearance with river-like parts with 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 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 added. “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.”
According to Hughes, globular clusters serve as proof of processes that occurred a long time ago.
“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.”
Star clusters emerge from concentrated regions of gas in the interstellar medium. 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.”
The researchers deliberately searched 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 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 added.
“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.”
Image Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)