New treasure trove of globular clusters holds clues about galaxy
evolution

Date:
January 11, 2022
Source:
University of Arizona
Summary:
Using observations of Centaurus A, a nearby elliptical galaxy,
obtained with the Gaia space telescope and ground-based instruments
under the PISCeS survey, a team of astronomers presents an
unprecedented number of globular cluster candidates in the outer
regions of the galaxy. The findings provide astronomers with an
even more detailed picture of galactic architecture and history
of collisions and mergers.



FULL STORY ==========================================================================
A survey completed using a combination of ground and space-based
telescopes yielded a treasure trove of previously unknown globular
clusters -- old, dense groups of thousands of stars that all formed
at the same time -- in the outer regions of the elliptical galaxy
Centaurus A. The work presents a significant advance in understanding
the architecture and cosmological history of this galaxy and offers new insights into galaxy formation in general and the distribution of dark
matter in the universe.


========================================================================== Allison Hughes, a doctoral student in the University of Arizona
Department of Astronomy and Steward Observatory, is the first author of
a peer-reviewed paper summarizing the findings that was published in
the Astrophysical Journalin June. She will present the study during a
virtual press briefing at the 239th Meeting of the Astronomical Society
of America Tuesday.

Centaurus A, also known as NGC 5128, is a visually stunning, elliptical
galaxy featuring a relativistic jet spewing from a supermassive black
hole at its center and spectacular streams of scattered stars left behind
by past collisions and mergers with smaller galaxies orbiting Centaurus
A. Located in the constellation Centaurus, 13 million light-years from
Earth, Centaurus A is too far away to allow astronomers to see individual stars, but star clusters can be identified as such and used as "fossil evidence" of the galaxy's tumultuous evolution.

Hughes and her colleagues present a new catalog of approximately 40,000 globular cluster candidates in Centaurus A, recommending follow-up
observations focused on a set of 1,900 that are most likely to be true
globular clusters.

The researchers surveyed globular cluster candidates out to a projected
radius of approximately 150 kiloparsecs, nearly half a million
light-years from the galaxy's center. The data combines observations
from the following sources: the Panoramic Imaging Survey of Centaurus
and Sculptor, or PISCeS; Gaia, a space observatory of the European Space Agency, and the NOAO Source Catalog, which combines publicly accessible
images from telescopes in both hemispheres covering nearly the entire 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 inner 40 kiloparsecs (about
130,500 light-years) of the galaxy, Hughes explained, leaving the outer
reaches of the galaxy largely unexplored. Ranking the candidates based
on the likelihood that they are true globular clusters, the team found
that approximately 1,900 are highly likely to be confirmed as such and
should be the highest 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.

Providing the closest example of an elliptical galaxy, Centaurus A
offers astronomers an opportunity to study up close a galaxy that is
very unlike our own. The Milky Way, as well as its closest neighbor,
the Andromeda Galaxy, are both spiral galaxies. With their familiar, pinwheel-like appearance, spiral galaxies may seem like the "typical"
galaxy, but it turns out that their less orderly elliptical cousins
outnumber 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." Star clusters form from dense patches
of gas in the interstellar medium. Almost every galaxy has globular
clusters, including the Milky Way, which boasts around 150 of them,
but most stars are not arranged in such clumps. By studying globular
clusters, astronomers can gather clues about the galaxy hosting them,
such as its mass, its history of interactions with nearby galaxies and
even the distribution of dark matter within, according to Hughes.

"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 specifically looked for globular clusters far from the center of the
galaxy because Centaurus A's substructure hints at a large, undiscovered population of such clusters, Hughes explained. Previous observations
had found just under 600 clusters in the more central regions, but the
outer regions of the galaxy had remained largely uncharted.

"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." ========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Daniel Stolte.

Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
*
Centaurus_A_is_an_elliptical_galaxy_located_about_13_million_light-years
from_Earth.

========================================================================== Journal Reference:
1. Allison K. Hughes, David J. Sand, Anil Seth, Jay Strader, Karina
Voggel,
Antoine Dumont, Denija Crnojević, Nelson Caldwell, Duncan
A. Forbes, Joshua D. Simon, Puragra Guhathakurta, Elisa Toloba. NGC
5128 Globular Cluster Candidates Out to 150 kpc: A Comprehensive
Catalog from Gaia and Ground-based Data*. The Astrophysical Journal,
2021; 914 (1): 16 DOI: 10.3847/1538-4357/abf63c ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220111193045.htm

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