The halo shape of stars is achieved in the Milky Way


Inclination in our stars: The halo shape of Milky Way stars is achieved

A new study has revealed the true shape of the diffuse cloud of stars surrounding the disk of our galaxy. For decades, astronomers believed that this cloud of stars — called the stellar halo — was largely spherical, like a beach ball. A new model based on recent observations shows that the stellar halo is elongated and tilted, like a soccer ball that has just been kicked.


The the findings—published this month The Astronomical Journal Provide insights into a range of astrophysics subject areas. The results, for example, shed light on the history of our galaxy and galaxy evolutionwhile also providing clues in the ongoing search for the mysterious substance known as dark matter.

“The shape of a stellar halo is a very fundamental factor that we measured more precisely than was possible before,” says study lead author Jiwon “Jesse” Han, PhD. Student at the Center for Astrophysics | Harvard and Smithsonian. “There are a lot of important implications for a star halo not being spherical but shaped like a football, rugby ball or balloon – take your pick!”

adds Charlie Conroy, study co-author, advisor to Hahn, and professor of astronomy at Harvard University and the Center for Astrophysics. “Now we know that the textbook image of our galaxy embedded in a spherical volume of stars must be discarded.”

Astronomers have discovered that the Milky Way’s stellar halo – a cloud of stars scattered around all galaxies – is balloon-shaped and tilted. This artist’s illustration shows what the three-dimensional halo surrounding our galaxy looks like. Credit: Melissa Weiss/The Astrophysical Center | Harvard and Smithsonian

The stellar halo of the Milky Way is the visible part of what is widely called the galactic halo. This galactic halo is dominated by invisible darkness issue, whose existence can only be measured by the gravity it exerts. Each galaxy has its own halo of dark matter. These halos act as a kind of scaffolding on which ordinary visible matter is attached. In turn, this visible matter forms stars and other observable galactic structure. To better understand how galaxies Stellar halos form and interact, as well as the fundamental nature of dark matter, thus are valuable astrophysical targets.

“The stellar halo is a galactic halo dynamic tracker,” Hahn says. “In order to learn more about galactic halos in general, and especially our galaxy’s halo and its history, the stellar halo is a great place to start.”

However, understanding what the Milky Way’s stellar halo looks like has long challenged astrophysicists for the simple reason that we are part of its interior. The stellar halo extends into the hundreds of thousands light years Above and below the starry plane of our galaxy, where our solar system resides.

“Unlike outer galaxies, where we just look at them and measure their halos,” Hahn says, “we lack the same kind of outer atmospheric perspective of our galaxy’s halo.”

Complicating matters, the stellar halo has been shown to be quite diffuse, containing only about one percent of the mass of all stars in the galaxy. Over time, however, astronomers have succeeded in identifying several thousand stars inhabiting this halo, which can be distinguished from other Milky Way stars by their distinct chemical composition (which can be measured through studies of the light of their stars), as well as by their distances and motions. across the sky. Through these studies, astronomers realized that halo stars are not evenly distributed. The goal since then has been to study the patterns of increased density of stars—which spatially appear as bands and streams—to sort out the final origins of the stellar halo.

The new study by CfA researchers and colleagues made use of two major data sets collected in recent years that communicated the stellar corona like never before.

The first group is from Gaia, a revolutionary spacecraft launched by the European Space Agency in 2013. Gaia has continued to collect the most accurate measurements of the positions, motions, and distances of millions of stars in the Milky Way, including some nearby stellar halo stars.

The second data set is from H3 (Hectochelle in the Halo at High Resolution), a ground-based survey conducted at MMT, located at the Fred Lawrence Whipple Observatory in Arizona, and a collaboration between CfA and the University of Arizona. H3 has collected detailed observations of tens of thousands of stellar halo stars too distant for Gaia to assess.

Combining this data into a flexible model that allowed the shape of the stellar halo to emerge from all observations yielded a decidedly non-spherical halo — and the soccer-ball shape agrees well with the other results so far. The figure, for example, is independently and strongly consistent with the leading theory regarding the formation of the Milky Way’s stellar halo.

According to this framework, the stellar halo formed when a lone dwarf galaxy 7-10 billion years ago collided with our much larger galaxy. The departing dwarf galaxy is known as Gaia-Sausage-Enceladus (GSE), with “Gaia” referring to the aforementioned spacecraft, “Sausage” for a pattern that appears when plotting Gaia data, and “Enceladus” for the Greek mythological giant that was buried under a mountain – just like how GSE is buried in the Milky Way. As a result of this galactic collision event, the dwarf galaxy was torn apart and its component stars were scattered in a sparse halo. This origin story explains the inherent variability of the stellar halo stars that were born and bred in the Milky Way.

The study results further chronicle how the GSE and the Milky Way interacted all those eons ago. The soccer ball shape — technically called a triaxial ellipsoid — reflects observations of two groups of stars in the stellar halo. The formations ostensibly formed when the GSE passed through two orbits of the Milky Way. During these orbits, GSE would have slowed twice as far at the so-called concentrators, or the outermost points in the orbit of the dwarf galaxy of its greatest gravitational pull, the massive Milky Way; These stops led to an additional fall of the GSE stars. Meanwhile, the tilt of the stellar halo indicates that GSE encountered the Milky Way at an angle of incidence rather than straight on.

“The tilt and distribution of stars in the stellar halo provides exciting confirmation that our galaxy collided with another, smaller galaxy 7-10 billion years ago,” says Conroy.

Notably, so much time has passed since the GSE-Milky Way crash that stellar halo stars were expected to dynamically settle into the long assumed classic spherical shape. The fact that it is unlikely, the team says, speaks to the galaxy’s broader halo. This dark matter-dominated structure is itself most likely deflected and, through its gravity, also keeps the stellar halo out of whack.

“The stellar halo is strongly tilted, indicating that the underlying dark matter halo is also tilted,” Conroy says. “The tilt in the dark matter halo could have major ramifications for our ability to detect dark matter particles in laboratories on Earth.”

Conroy’s last point points to multiple dark matter detector experiments in operation and now planned. These detectors could increase their chances of capturing an elusive interaction with dark matter if astrophysicists can pinpoint where the matter is most concentrated, in galactic terms. As Earth moves through the Milky Way, it will periodically encounter these regions of dense, high-velocity dark matter particles, increasing the odds of detection.

Discover the star auraThe most plausible configuration is to advance many astrophysical investigations while filling in key details about our place in the universe.

“These are interesting intuitive questions to ask our galaxy: ‘What does the galaxy look like? ” and what astral halo It seems like?

more information:
Jiwon Jesse Han et al, The stellar halo of the galaxy is tilted and doubly broken, The Astronomical Journal (2022). DOI: 10.3847/1538-3881/ac97e9

the quote: Tilt in Our Stars: Milky Way Galaxy Star Halo Shape Materialized (2022, November 18), Retrieved November 18, 2022 from https://phys.org/news/2022-11-tilt-stars-milky-halo. programming language

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