Deep images from the Hubble Space Telescope have revealed a narrow structure about 6,500 light-years across in a particularly diffuse galaxy about 115 million light-years away. Models of its orbit suggest a massive halo of dark matter and open a new way to map the invisible matter in other galaxies.
Astronomers have found evidence of the first known stream of stars originating in a globular cluster outside the Milky Way. The thin, faint structure was detected in the galaxy UGC 9050-Dw1, at a distance of about 35.2 million parsecs – about 115 million light-years from Earth.
Star streams They are formed when the gravity of a galaxy gradually pulls in stars from a smaller system orbiting it. The torn stars do not scatter immediately, but stretch along their orbit, creating a narrow ribbon of light that can last for billions of years.
Such streams are well known in the Milky Way, where they are used by astronomers to examine the galaxy's gravitational field and search for invisible concentrations of dark matterSo far, however, no extragalactic stream originating from a globular cluster has been confirmed.
The study, published in the journal Nature, presents the structure as the first evidence that it is possible not only to detect such streams in other galaxies, but also to use their shape to study the distribution of dark matter far from the Milky Way.
“We discovered a stream of stars from a globular cluster in another galaxy,” said Julie Kielholm, a doctoral student at the Niels Bohr Institute inUniversity of Copenhagen and the lead author of the study. “This is the first time such a stream has been observed outside the Milky Way galaxy.”
A galaxy that looks large but is sparse in stars
UGC 9050-Dw1 belongs to a group of ultra-diffuse galaxies, or UDGs for short. These are galaxies whose stellar masses are similar to those of dwarf galaxies, but whose dimensions may approach those of a large galaxy. Their stars are spread out over a wide area, so their surface brightness is very low.
This combination makes them faint and difficult to observe, but also extremely intriguing. Some of the diffuse galaxies have been found to contain large amounts of dark matter, while others have had surprisingly low estimates. Measuring the velocities of stars or the rotation of the galaxy—standard methods for estimating mass—is particularly difficult when the light is so weak.
The new stream may provide an alternative method.
The researchers examined deep-field photographs taken using the Advanced Camera for Surveys of Hubble Space TelescopeThe structure also appeared in data from the Canada-France-Hawaii Telescope, so the researchers determined that it was not a camera defect or an artifact of image processing.
They gave the stream its name Oyashio (Oyashio), named after the cold ocean current in the North Pacific.
Thousands of light years long and only hundreds wide
The visible part of Oyashio extends about two kiloparsecs long – about 6,500 light-years – but is only about 72 parsecs wide, or about 235 light-years. It is located at a projected distance of about 2.5 kiloparsecs from the center of the galaxy.
At the edge of the structure is a compact light source that the researchers identify as a candidate for the globular cluster from which the stars were torn. The color of the source is similar to the color of the stream and the colors of spherical clusters others in the galaxy, as expected if the stars in the stream and cluster formed in the same stellar population.
The width of Oyashio also supports this explanation. Streams originating from dwarf galaxies tend to be wider, because the system from which the stars were torn is larger and more massive. Such a narrow stream is more suitable for the gradual breakup of a globular cluster.
The researchers simulated stellar populations and the motion of clusters within the galaxy's gravitational field. The models were able to reproduce the brightness, width, and curvature of the observed structure when assuming that it originated in a cluster with an initial mass of at most 2.5 million solar masses.
How a stream of stars maps dark matter
Dark matter does not emit light and does not absorb or reflect electromagnetic radiation, so it cannot be photographed directly. Its presence is inferred from the gravity it exerts on stars, gas, and light.
A stellar stream acts as a kind of line that outlines the gravitational field in which the stars have moved. The trajectory, curvature, width, and distribution of stars along the stream are affected by the total mass of the galaxy and how it is distributed.
The researchers used a computational method that fits dynamical models directly to the shape of the stream. The models indicated that UGC 9050-Dw1 resides within a massive halo of dark matter, with a total mass of hundreds of billions of solar masses. The result is consistent, within a wide range of uncertainty, with previous estimates based on the number of globular clusters in the galaxy.
“We show that a well-established tool for studying the Milky Way can be used to understand other galaxies, where it has been very difficult to measure the distribution of dark matter,” Holm said.
The measurement does not reveal what particles the dark matter is made of. Its importance is in opening up the possibility of examining its distribution and the internal structure of the halo in galaxies of different types.
Gravitational impacts that leave scars in the stream
According to the conventional model of cold dark matter, large halos should contain many dark matter subhalos, some of which are too small to form stars. Such dark subhalos cannot be observed directly, but their passage near a stream of stars may create gaps, bulges, or density changes in it.
In the Milky Way, it is sometimes difficult to determine whether such distortion is caused by a dark matter halo or by a normal object, such as a star cluster, a gas cloud, or the structure of the galaxy's disk.Diffuse galaxies In particular, there is less normal matter and fewer bright structures to disrupt the flow. Therefore, they may provide a relatively clean laboratory for testing predictions about the substructure of dark matter.
“The information we’ve been able to get so far from stellar streams in globular clusters has been limited to one galaxy – our own,” Holm said. “Being able to observe streams in different types of galaxies opens the way to a broader understanding of the behavior of dark matter.”
Still a candidate, not final approval
Despite the historical title, the researchers use cautious language and define Oyashio as a candidate for a star stream. Its color, width, shape, location, and possible connection to a globular cluster support this interpretation, and dynamical models also manage to reproduce it.
The researchers also considered alternative explanations, including a bow created by gravitational lensing, a stellar envelope from a previous collision, a dust zone, or a broader stream originating from a dwarf galaxy. None of these explanations fit the data, they said, like the possibility of a globular cluster stream.
However, a coincidental alignment of stars along the line of sight cannot yet be completely ruled out. Deeper observations by Hubble or the James Webb Space Telescope, as well as spectroscopic measurements of the stars in the stream and the putative cluster, could either confirm or refute the identification.
The Euclid and Nancy Grace Roman telescopes, designed to survey large areas of the sky with high sensitivity to faint structures, may reveal many more streams in the future. If such streams are discovered in a variety of galaxies, they will be used to compare dark matter halos under different conditions and test how well the model predictions fit the observable universe.
Questions and Answers
What is a star stream from a spherical accumulator?
It is a long, narrow structure of stars that have been gradually torn from a globular cluster by the tidal forces of the galaxy it surrounds. The stars continue to move in a similar orbit to that of the cluster, forming arms in front of and behind it.
What is new in the research?
Large stellar streams are known to exist outside the Milky Way, but until now, no narrow stream originating from a globular cluster has been confirmed outside our galaxy. The study presents the first evidence for such a structure and uses it to measure the dark matter halo of another galaxy.
How can dark matter be studied using a stream of stars?
The gravity of dark matter affects the orbits of stars, the curvature of the stream, its width, and its density. Comparing the observed structure to simulations allows us to estimate the mass of the halo and the distribution of mass within it.
Is the discovery certain?
Not quite yet. The data and models strongly support a globular cluster origin, but the researchers call the structure a candidate and note that deep observations and spectroscopy are needed to confirm the identification.
Source/Original article: Evidence for the first globular cluster stellar stream beyond the Milky Way - Nature
Research message: Breakthrough in unusual galaxy may help unravel the mystery of dark matter – University of Copenhagen
More of the topic in Hayadan:
Dark matter solves the mystery of the Milky Way's stellar streams
A “ghost galaxy” composed of 99% dark matter was discovered 300 million light-years away
This is how globular clusters are born – and perhaps a new type of star system.
Does the mysterious glow at the center of the Milky Way actually come from dark matter?
Look around - tidal forces have solved the mystery of dark matter in a distant galaxy
For the scientific article: Opening the scientific article
