New treasure trove of globular clusters holds clues about galaxy evolution

By Daniel Stolte,
College Communications

Jan. 11, 2022

Centaurus A is an elliptical galaxy situated about 13 million light-years from Earth. This colour composite picture reveals the lobes and jets emanating from the energetic galaxy’s central black gap.
ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)

A survey accomplished utilizing a mix of floor and space-based telescopes yielded a treasure trove of beforehand unknown globular clusters – previous, dense teams of 1000’s of stars that each one shaped on the identical time – within the outer areas of the elliptical galaxy Centaurus A. The work presents a major advance in understanding the structure and cosmological historical past of this galaxy and presents new insights into galaxy formation generally and the distribution of darkish matter within the universe.

Allison Hughes, a doctoral pupil within the College of Arizona Division of Astronomy and Steward Observatory, is the primary writer of a peer-reviewed paper summarizing the findings, which was revealed within the Astrophysical Journal in June. She offered the research Tuesday throughout an Astronomical Society of America press briefing. Whereas the in-person AAS 239th assembly was canceled as a consequence of COVID-19 considerations, press briefings had been held nearly on Zoom.

Centaurus A, often known as NGC 5128, is a visually beautiful, elliptical galaxy that includes a relativistic jet spewing from a supermassive black gap at its middle and spectacular streams of scattered stars left behind by previous collisions and mergers with smaller galaxies orbiting Centaurus A. Situated within the constellation Centaurus, 13 million light-years from Earth, Centaurus A is simply too distant to permit astronomers to see particular person stars, however star clusters may be recognized and used as “fossil proof” of the galaxy’s tumultuous evolution.

Hughes and her colleagues current a brand new catalog of roughly 40,000 globular cluster candidates in Centaurus A, recommending follow-up observations targeted on a set of 1,900 which might be probably to be true globular clusters. The researchers surveyed globular cluster candidates out to a projected radius of roughly 150 kiloparsecs, almost half one million light-years from the galaxy’s middle. The info combines observations from the next sources: the Panoramic Imaging Survey of Centaurus and Sculptor, or PISCeS; Gaia, an area observatory of the European Area Company; and the NOAO Supply Catalog, which mixes publicly accessible photographs from telescopes in each hemispheres masking almost your entire sky.

Centaurus A has been a number one goal for extragalactic globular cluster research as a consequence of its richness and proximity to Earth, however the majority of research have targeted on the interior 40 kiloparsecs (about 130,500 light-years) of the galaxy, Hughes defined, leaving the outer reaches of the galaxy largely unexplored. Rating the candidates based mostly on the chance that they’re true globular clusters, the staff discovered that roughly 1,900 are extremely more likely to be confirmed as such and must be the highest precedence for follow-up spectroscopic affirmation.

“We’re utilizing the Gaia satellite tv for pc, which principally focuses on surveys inside our personal galaxy, the Milky Manner, in a brand new approach in that we hyperlink up its observations with telescopes on the bottom, on this case the Magellan Clay telescope in Chile and the Anglo-Australian Telescope in Australia.”

Centaurus A’s construction tells astronomers that it went via a number of main mergers with different galaxies, resulting in its glob-like look with river-like areas which have many extra stars than the encompassing areas, Hughes mentioned. Offering the closest instance of an elliptical galaxy, Centaurus A presents astronomers a chance to check up shut a galaxy that could be very not like our personal. The Milky Manner, in addition to its closest neighbor, the Andromeda Galaxy, are each spiral galaxies. With their acquainted, pinwheel-like look, spiral galaxies might seem to be the “typical” galaxy, but it surely seems that their much less orderly elliptical cousins outnumber them within the cosmos.

“Centaurus A might appear like an odd outlier, however that is solely as a result of we are able to get shut sufficient to see its nitty gritty particulars,” Hughes mentioned. “Extra doubtless than not, each elliptical and spiral galaxies just like the Milky Manner are messier than we understand as quickly as we glance a little bit bit deeper than simply on the floor.”

Globular clusters function proof of processes that occurred a very long time in the past, Hughes mentioned.

“For instance, in the event you see a line of those globular clusters that each one have related metallicity (chemical composition) and transfer with related radial velocity, we all know they will need to have come from the identical dwarf galaxy or some related object that collided with Centaurus A and is now within the means of being assimilated.”

Star clusters type from dense patches of gasoline within the interstellar medium. Virtually each galaxy has globular clusters, together with the Milky Manner, which boasts round 150 of them, however most stars should not organized in such clumps. By finding out globular clusters, astronomers can collect clues concerning the galaxy internet hosting them, comparable to its mass, its historical past of interactions with close by galaxies and even the distribution of darkish matter inside, in response to Hughes.

“Globular clusters are attention-grabbing as a result of they can be utilized as tracers of buildings and processes in different galaxies the place we will not resolve particular person stars,” Hughes mentioned. “They maintain on to chemical signatures, comparable to the fundamental composition of their particular person stars, in order that they inform us one thing concerning the atmosphere wherein they shaped.”

The researchers particularly seemed for globular clusters removed from the middle of the galaxy as a result of Centaurus A’s substructure hints at a big, undiscovered inhabitants of such clusters, Hughes defined. Earlier observations had discovered slightly below 600 clusters within the extra central areas, however the outer areas of the galaxy had remained largely uncharted.

“We seemed farther out and found greater than 100 new clusters already, and probably there are extra, as a result of we have not even completed processing the information,” Hughes mentioned. “We will then use that information to reconstruct the structure and actions in that galaxy, and in addition work out its mass. From that, we are able to ultimately subtract all its stars and see what’s left – that invisible mass have to be its darkish matter.”

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