The tilt of our stars: the shape of the Milky Way's halo of stars is realized

The tilt of our stars: the shape of the Milky Way’s halo of stars is realized

A new study has revealed the true shape of the diffuse star cloud that surrounds our galaxy’s disk. For decades, astronomers have assumed that this cloud of stars – called the star halo – is largely spherical, like a beach ball. Now a new model based on modern observations shows that the star halo is elongated and tilted, much like a soccer ball that has just been kicked.

The results – published this month The Astronomical Journal — provide insights into a variety of astrophysical topics. The results shed light on the history of our galaxy and galactic evolution, for example, while also providing clues to the ongoing hunt for the mysterious substance known as dark matter.

“The shape of the stellar halo is a very fundamental parameter that we have just measured with greater accuracy than was previously possible,” says the study’s lead author, Jiwon “Jesse” Han, a graduate student at the Center for Astrophysics | Harvard & Smithsonian. “There are many important implications for the star halo not being spherical but shaped like a football, rugby ball or blimp – 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, Hans’ advisor and professor of astronomy at Harvard University and the US Center for Astrophysics. “We now know that the textbook picture of our galaxy embedded in a spherical volume of stars must be jettisoned.”

Astronomers have discovered that the Milky Way’s stellar halo – a cloud of diffuse stars surrounding all galaxies – is zeppelin-shaped and tilted. This artist’s illustration emphasizes the shape of the three-dimensional halo that surrounds our galaxy. Photo credit: Melissa Weiss/Center for Astrophysics | Harvard & Smithsonian

The stellar halo of the Milky Way is the visible part of what is more commonly called the galactic halo. This galactic halo is dominated by invisible dark matter, whose presence can only be measured by the gravity it exerts. Each galaxy has its own dark matter halo. These halos serve as a kind of scaffolding from which ordinary, visible matter hangs. This visible matter, in turn, forms stars and other observable galactic structures. Accordingly, to better understand how galaxies form and interact, as well as the underlying nature of dark matter, stellar halos are valuable astrophysical targets.

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

However, fathoming the shape of the Milky Way’s stellar halo has long challenged astrophysicists for the simple reason that we are embedded in it. The stellar halo extends several hundred thousand light-years above and below the star-filled plane of our galaxy, where our solar system resides.

“Unlike external galaxies, where we just look at them and measure their halos,” Han says, “we lack the same kind of outside aerial perspective of our own galaxy’s halo.”

To make matters worse, the stellar halo has proven to be fairly diffuse, containing only about one percent the mass of all the stars in the galaxy. But over time, astronomers have been able to identify many thousands of stars populating this halo, which differ from other Milky Way stars because of their distinctive chemical composition (which can be measured through studies of their starlight), as well as their distances and motions across them distinguish the sky. Through such studies, astronomers have realized that halo stars are not evenly distributed. The goal since then has been to study the overdensity patterns of stars spatially appearing as bundles and streams to clarify the ultimate origins of the stellar halo.

The new study by CfA researchers and colleagues uses two large data sets collected over the past few years that have plumbed the stellar halo like never before.

The first sentence is from Gaia, a revolutionary spacecraft launched by the European Space Agency in 2013. Gaia has continued to compile 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 dataset is from H3 (Hectochelle in the Halo at High Resolution), a ground-based survey conducted at the MMT at the Fred Lawrence Whipple Observatory in Arizona and a collaboration between the CfA and the University of Arizona. H3 has collected detailed observations of tens of thousands of stellar halo stars too distant for Gaia to judge.

Combining this data in a flexible model that allowed the stellar halo shape to emerge from all observations yielded the decidedly non-spherical halo – and the football shape fits well with other previous findings. The shape, for example, independently and strongly agrees with a leading theory about the formation of the Milky Way’s stellar halo.

According to this scheme, the stellar halo formed when a lone dwarf galaxy collided with our much larger galaxy 7 to 10 billion years ago. The defunct dwarf galaxy is amusingly known as Gaia-Sausage-Enceladus (GSE), where “Gaia” refers to the spacecraft mentioned above, “Sausage” to a pattern that appears when plotting the Gaia data, and “Enceladus” to the Greek mythological giant who was buried under a mountain – similar to how GSE was buried in the Milky Way. As a result of this galactic collision event, the dwarf galaxy was torn apart and its individual stars scattered into a scattered halo. Such an origin story explains the inherent dissimilarity of stellar halo stars to stars born and bred in the Milky Way.

The results of the study further show how GSE and the Milky Way interacted all these eons ago. The football shape – technically called a triaxial ellipsoid – reflects observations of two clusters of stars in the stellar halo. The pile-ups supposedly formed as GSE passed through two orbits of the Milky Way. During these orbits, GSE would have slowed down twice at so-called apocentres, or the most distant points in the orbit of the dwarf galaxy of the larger gravitational attractor, the powerful Milky Way; These pauses resulted in an additional loss of GSE stars. The tilt of the stellar halo indicates that GSE struck the Milky Way at an angle of incidence rather than directly.

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

Remarkably, so much time has passed since the collapse of the GSE Milky Way that it was expected that the stellar halo stars would dynamically settle into the classic, long-held spherical shape. The fact that they likely didn’t do so speaks to the broader galactic halo, the team says. This dark-matter-dominated structure itself is likely tilted, and its gravity also unbalances the stellar halo.

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

Conroy’s final point alludes to the numerous dark matter detector experiments currently underway and planned. These detectors could increase their chances of capturing an elusive dark matter interaction if astrophysicists can assess where the substance is more concentrated galactically. As Earth moves through the Milky Way, it will periodically encounter these regions of dense and faster dark matter particles, increasing the likelihood of detection.

Discovering the most plausible configuration of the stellar halo will advance many astrophysical studies while filling in fundamental details about our place in the Universe.

“These are such intuitively interesting questions about our galaxy: ‘What does the galaxy look like?’ and ‘What does the stellar halo look like?'” says Han. “In particular, with this research and study direction, we’re finally answering those questions.”

More information:
Jiwon Jesse Han et al, The Stellar Halo of the Galaxy is Tilted and Double Broken, The Astronomical Journal (2022). DOI: 10.3847/1538-3881/ac97e9

Provided by the Harvard-Smithsonian Center for Astrophysics

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