Hubble identifies one of darkest known galaxies
The elusive object dubbed CDG-2 may be composed of 99% dark matter
Most galaxies in the nearby Universe are
quite luminous, but some are so faint they’re nearly invisible.
Astronomers, using the NASA/ESA Hubble Space Telescope in combination
with other observatories, identified a galaxy that appears to be almost
entirely dominated by dark matter with only a smattering of stars. The
galaxy, known as Candidate Dark Galaxy-2 (CDG-2), appears to contain
just four globular star clusters (compared to the Milky Way’s 150-plus),
and dimly shines with the light of only about 1 million Suns.
In the vast tapestry of the Universe, most galaxies shine brightly
across cosmic time and space. Yet a rare class of galaxies remains
nearly invisible — low-surface-brightness galaxies dominated by dark
matter and containing only a sparse scattering of faint stars.
One such elusive object, dubbed CDG-2, may be among the most heavily dark matter-dominated galaxies ever discovered. Dark matter is an invisible form of matter that does not reflect, emit, or absorb light. The science paper detailing this finding was published in The Astrophysical Journal Letters.
Detecting such faint galaxies is extraordinarily difficult. Using
advanced statistical techniques, David Li of the University of Toronto,
Canada, and his team identified 10 previously confirmed
low-surface-brightness galaxies and two additional dark galaxy
candidates by searching for tight groupings of globular clusters —
compact, spherical star groups typically found orbiting normal galaxies.
These clusters can signal the presence of a faint, hidden stellar
population.
To confirm one of the dark galaxy candidates, astronomers employed a trio of observatories: the Hubble Space Telescope, ESA’s Euclid space observatory, and the ground-based Subaru Telescope
in Hawaii. Hubble’s high-resolution imaging revealed a close collection
of four globular clusters in the Perseus galaxy cluster, 300 million
light-years away. Follow-up studies using Hubble, Euclid, and Subaru
data then revealed a faint, diffuse glow surrounding the star clusters —
strong evidence of an underlying galaxy.
“This is the first galaxy detected solely through its globular cluster population,” said Li. “Under conservative assumptions, the four clusters represent the entire globular cluster population of CDG-2.”
Preliminary analysis suggests CDG-2 has the luminosity of roughly 1
million Sun-like stars, with the globular clusters accounting for 16% of
its visible content. Remarkably, 99% of its mass, which includes both
visible matter and dark matter, appears to be dark matter. Much of its
normal matter to enable star formation (primarily hydrogen gas) was
likely stripped away by gravitational interactions with other galaxies
inside the Perseus cluster.
“The Euclid data clearly confirm
the presence of the extremely faint, diffuse light of CDG-2, revealing
the galaxy behind the globular clusters for the first time,” says Francine Marleau from the Institute for Astro- and Particle Physics at the University of Innsbruck in Austria.
“The Euclid images of the Perseus cluster demonstrate the mission’s
unique capability to detect new low-surface-brightness galaxies,
including extremely faint ones, while also revealing their globular
clusters, nuclear star clusters, internal structures, and surrounding
environments.”
Globular clusters possess immense stellar density and are
gravitationally tightly bound. This makes the clusters more resistant to
gravitational tidal disruption, and therefore reliable tracers of such
ghostly galaxies.
Galaxy field of low-surface-brightness galaxy CDG-2
The low-surface-brightness galaxy CDG-2, shown in this image from the NASA/ESA Hubble Space Telescope, is dominated by dark matter and contains only a sparse scattering of stars. This galaxy is nearly invisible, but by using advanced statistical techniques, scientists identified it by searching for tight groupings of stars called globular clusters at the centre of this image.
The Hubble observations include those from programme 15235 (W. Harris).
[Image description: A field of space with a dozen white foreground stars and a number of small, yellow background galaxies.]
Credit:
NASA, ESA, D. Li (Utoronto), Image Processing: J. DePasquale (STScI)
Dark galaxy CDG-2 near Perseus Cluster (annotated)
The low-surface-brightness galaxy CDG-2, shown in this image from the NASA/ESA Hubble Space Telescope, is dominated by dark matter and contains only a sparse scattering of stars. This galaxy is nearly invisible, but by using advanced statistical techniques, scientists identified it by searching for tight groupings of stars called globular clusters. At left, the white box marks the area that was examined. At right is a magnified view of that area. The circle marked with a dashed red boundary indicates the location of the dark-matter dominated galaxy. Within the dashed circle are four globular clusters outlined by small, blue circles. Several background galaxies also appear within the red circle, but these are not related to the galaxy CDG-2.
[Image description: At left, a field of space with a dozen white foreground stars and a number of small, yellow background galaxies. An unremarkable area at centre is outlined with a dashed circle surrounded by a white box. Lines extend from the box to a pullout at right containing faint, grainy white light surrounded by a circle labeled “Candidate dark galaxy – diffuse emission.” Four white dots are circled in blue and labeled globular clusters.]
The Hubble observations include those from programme 15235 (W. Harris).
Credit:
NASA, ESA, D. Li (Utoronto), Image Processing: J. DePasquale (STScI)
Dark galaxy CDG-2 near Perseus Cluster (annotated compass image)
This image of dark galaxy CDG-2 was captured by the Hubble Space Telescope’s ACS (Advanced Camera for Surveys) with additional data from the European Space Agency’s Euclid space mission.
The image shows a scale bar, compass arrows, and colour key for reference.
The scale bar is labeled in light-years along the top, which is the distance that light travels in one Earth-year. (It takes about 36,000 years for light to travel a distance equal to the length of the scale bar.) One light-year is equal to about 9.46 trillion kilometers.
The scale bar is also labeled in arcseconds, which is a measure of angular distance on the sky. One arcsecond is equal to an angular measurement of 1/3600 of one degree. There are 60 arcminutes in a degree and 60 arcseconds in an arcminute. (The full Moon has an angular diameter of about 30 arcminutes.) The actual size of an object that covers one arcsecond on the sky depends on its distance from the telescope.
The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above).
The colour key shows which ACS filters were used when collecting the light. The colour of each filter name is the visible-light colour used to represent the light that passes through that filter.
[Image description: Image labeled “CDG 2 near Perseus Cluster, HST ACS WFC”. At left, a field of space with a dozen white foreground stars and a number of small, yellow background galaxies. An unremarkable area at center is outlined with a dashed red circle surrounded by a white box. Lines extend from the box to a pullout at right containing faint, grainy white light surrounded by a red circle labeled “Candidate dark galaxy – diffuse emission.” Four white dots are circled in blue and labeled globular clusters. Compass arrows at lower left show east pointing to 1 o’clock and north pointing to 4 o’clock. A scale bar is labeled 36,000 light-years and 30 arcseconds. It extends about one-eighth of the way across the image. A colour key shows F475W in blue, F814W in orange, and Euclid in gray.]
Credit:
NASA, ESA, D. Li (Utoronto), Image Processing: J. DePasquale (STScI)
VIDEOS
Pan video: Galaxy field of low-surface-brightness galaxy CDG-2
The low-surface-brightness galaxy CDG-2, shown in this image from the NASA/ESA Hubble Space Telescope, is dominated by dark matter and contains only a sparse scattering of stars. This galaxy is nearly invisible, but by using advanced statistical techniques, scientists identified it by searching for tight groupings of stars called globular clusters at the centre of this image.
The Hubble observations include those from programme 15235 (W. Harris).
Credit:
NASA, ESA, D. Li (Utoronto), Image Processing: J. DePasquale (STScI), N. Bartmann (ESA/Hubble)
Music: Stellardrone - Ascent
Location of the low-surface-brightness galaxy CDG-2
The low-surface-brightness galaxy CDG-2, shown in this image from the NASA/ESA Hubble Space Telescope, is dominated by dark matter and contains only a sparse scattering of stars. This galaxy is nearly invisible, but by using advanced statistical techniques, scientists identified it by searching for tight groupings of stars called globular clusters. At left, the white box marks the area that was examined. The pullout shown is a magnified view of that area. The circle marked with a dashed red boundary indicates the location of the dark-matter dominated galaxy. Within the dashed circle are four globular clusters outlined by small, blue circles. Several background galaxies also appear within the red circle, but these are not related to the galaxy CDG-2.
Credit:
NASA, ESA, D. Li (Utoronto), Image Processing: J. DePasquale (STScI), N. Bartmann (ESA/Hubble)
Music: Stellardrone - The Night Sky in Motion
Fuente: ESA/Hubble Information Centre
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