The shape of the stellar halo of the Milky Way has been clarified

The shape of the stellar halo of the Milky Way has been clarified
The shape of the stellar halo of the Milky Way has been clarified

A new study has revealed the true shape of the rarefied cloud of stars surrounding the disk of our galaxy. For decades, astronomers believed that this cloud of stars, called a stellar halo (or halo), was spherical in shape, like a beach ball. Now, a new model based on modern observations shows that the stellar halo has an oblong shape and tilt, like a football that has just been kicked.

This was reported at the Harvard-Smithsonian Center for Astrophysics (CfA).

The findings, published this month in the Astronomical Journal, provide insight into many astrophysical fields. For example, the results shed light on our galaxy’s history and evolution, and provide clues in the ongoing hunt for the mysterious substance known as dark matter.

“The shape of a stellar halo is a very fundamental parameter that we have just measured with greater precision than has been possible before,” says lead study author Jivon “Jesse” Hahn, a CfA graduate student. “There are many important implications of the star’s halo not being spherical, but shaped like an American football, a rugby ball, or a zeppelin—take your pick!”

“For decades, the general assumption has been that the stellar halo is more or less spherical and isotropic, or the same in all directions,” adds study co-author Charlie Conroy, Hahn’s advisor, professor of astronomy at Harvard University and CfA. “We now know that the textbook picture of our galaxy trapped in a spherical volume of stars must be rejected.”

The Milky Way’s stellar halo is the visible part of what is more broadly called the galactic halo. This galactic halo is dominated by invisible dark matter whose presence can only be measured by the gravity it exerts. As CfA notes, each galaxy has its own dark matter halo. These halos serve as a kind of scaffolding on which ordinary, visible matter hangs. In turn, this visible matter forms stars and other observable galactic structures. For a better understanding of how galaxies form and interact, as well as the deep nature of dark matter, stellar halos are, accordingly, valuable astrophysical objects.

“The stellar halo is a dynamic sensor of the galactic halo,” says Han. “To learn more about galactic halos in general, and specifically about galactic halos and the history of our own galaxy, stellar halos are a great place to start.”

However, the study of the shape of the stellar halo of the Milky Way has long troubled astrophysicists for the simple reason that we are inside it. A stellar halo extends several hundred thousand light-years above and below the star-filled plane of our galaxy, where our solar system is located.

“Unlike external galaxies, when we just look at them and measure their halos,” Hahn says, “we don’t have that aerial, outside perspective of our own galaxy’s halo.”

Everything is complicated by the fact that the stellar halo turned out to be quite rare and contains only about one percent of the mass of all the stars in the galaxy. However, astronomers have since been able to identify many thousands of stars inhabiting this halo, which differ from other stars in the Milky Way by their chemical composition (which can be determined by studying their starlight), as well as their distance and motion across the sky. Thanks to such studies, astronomers realized that the stars in the halo are unevenly distributed. Since then, the goal has been to study the patterns of excessive density of stars — spatial clusters and flows — to find out the ultimate origin of the stellar halo.

The new study by CfA scientists and their colleagues builds on two large data sets collected in recent years that have made it possible to study the stellar halo like never before.

The first data set came from the Gaia spacecraft, launched by the European Space Agency in 2013. Gaia continues to collect the most accurate data on the position, motion and distance of millions of stars in the Milky Way, including some nearby stars in the stellar halo.

The second data set was obtained during H3 (High-Resolution Hectochel in the Halo), a ground-based survey conducted at the MMT Observatory in collaboration between CfA and the University of Arizona. H3 collected detailed observations of tens of thousands of stars in the stellar halo that are too far away for Gaia to measure.

Combining this data into a flexible model that allows the shape of the stellar halo to be determined based on all the observations has yielded a distinctly non-spherical halo—and this football-like shape is in good agreement with other results obtained to date. For example, this shape is independently and strongly consistent with the leading theory of formation of the stellar halo of the Milky Way.

According to this theory, the stellar halo formed when a lone dwarf galaxy collided with our larger galaxy 7-10 billion years ago. That dwarf galaxy is amusingly known as Gaia-Sausage-Enceladus (GSE), where “Gaia” refers to the aforementioned spacecraft, “Sausage” is a pattern revealed when plotting the Gaia data, and “Enceladus” is a giant from Greek mythology , which was buried under the mountain — in much the same way that the GSE was buried in the Milky Way. As a result of this galactic collision, the dwarf galaxy was torn apart, and the components of its star scattered into a diffuse halo. Such an origin story explains the dissimilarity of halo stars to stars born and raised in the Milky Way.

The results of the study once again confirm how the GSE and the Milky Way interacted all those eons ago. The shape of an American football — technically called a triaxial ellipsoid — reflects the observation of two clusters of stars in a stellar halo. These clusters likely formed when the GSE completed two orbits around the Milky Way. During these orbits, the GSE slowed down twice at the so-called apocenters, or the farthest points in the dwarf galaxy’s orbit from the more powerful gravitational attractor — the heavy Milky Way; these pauses resulted in additional GSE star resets. Meanwhile, the tilt of the stellar halo indicates that the GSE collided with the Milky Way at an angle rather than head-on.

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

It is noteworthy that so much time has passed since the collision of the GSE and the Milky Way that the halo stars should have dynamically assumed the classical, long-predicted spherical shape. The fact that they didn’t most likely points to a wider galactic halo, the team says. This dark matter-dominated structure is probably tilted itself, and due to its gravity it also keeps the stellar halo tilted.

“The tilt of the stellar halo suggests that the underlying dark matter halo is also tilted,” says Conroy. “The tilt of the dark matter halo could have significant implications for our ability to detect dark matter particles in laboratories on Earth.”

Conroy’s last point relates to the many dark matter detection experiments currently underway and planned. These detectors could increase their chances of picking up the elusive interaction with dark matter if astrophysicists can determine where the substance is most concentrated, galactically speaking. As Earth moves through the Milky Way, it will periodically encounter these regions of dense and high-speed dark matter particles, increasing the chances of detection.

Discovering the most likely configuration of a stellar halo will advance much astrophysical research and clarify key details about our place in the universe.

“These are intuitively interesting questions about our galaxy: ‘What does a galaxy look like?’ and ‘What does a stellar halo look like?’” Hahn says. “Thanks to this line of research in particular, we are finally answering those questions.”

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The article is in Ukrainian

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