The shape of the halo of stars in the Milky Way is realized

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

The findings – published this month The astronomical magazine – provide insight into a large number of astrophysical topics. For example, the results shed light on the history of our galaxy and galactic evolution, 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 just measured with greater accuracy than previously possible,” said lead author Jiwon “Jesse” Han, a Ph.D. student at the Center for Astrophysics | Harvard & Smithsonian. “There are many important implications of the stellar halo not being spherical, but instead being shaped like a football, rugby ball or zeppelin — take your pick!”

“For decades, the common assumption has been that the stellar halo is more or less spherical and isotropic, or the same in every direction,” adds co-author Charlie Conroy, Han’s advisor, and a professor of astronomy at Harvard University and the Center for Astrophysics. “We now know that the textbook image of our galaxy embedded in a spherical volume of stars should be thrown out.”

The Milky Way’s stellar halo is the visible portion of what is more broadly referred to as the galactic halo. This galactic halo is dominated by invisible dark matter, the presence of which is measurable only by the gravitational pull it exerts. Each galaxy has its own halo of dark matter. These halos serve as a kind of scaffolding on which ordinary, visible matter hangs. That visible matter, in turn, forms stars and other observable galactic structures. To better understand how galaxies form and interact, as well as the underlying nature of dark matter, stellar halos are therefore 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 and history of our own galaxy in particular, the stellar halo is a great place to start.”

However, understanding 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 and measure their halos,” says Han, “we don’t have the same kind of sky perspective, beyond the perspective of our own galaxy’s halo.”

Complicating matters, the stellar halo has turned out to be quite diffuse, containing only about one percent of the mass of all the stars in the galaxy. Yet over time, astronomers have managed to identify many thousands of stars populating this halo, which are distinguishable from other stars in the Milky Way due to their distinctive chemical composition (measurable by studies of their starlight), as well as their distances and movements around the world. the sky. Through such studies, astronomers have realized that halo stars are not evenly distributed. The goal has since been to study the patterns of overly high densities of stars — appearing spatially as clusters and streams — to pinpoint the ultimate origin of the stellar halo.

The new study by CfA researchers and colleagues uses two major datasets collected over the past few years that have explored the stellar halo like never before.

The first set 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 survey conducted at the MMT, located 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 assess.

Combining this data into a flexible model that allowed the shape of the star’s halo to emerge from all observations created the decidedly non-spherical halo — and the football shape ties in nicely with other findings so far. For example, the shape is independently and strongly consistent with a leading theory of the formation of the Milky Way’s stellar halo.

According to this framework, the stellar halo formed when a lone dwarf galaxy collided with our much larger galaxy 7-10 billion years ago. The deceased dwarf galaxy is amusingly known as Gaia-Sausage-Enceladus (GSE), with “Gaia” referring to the aforementioned spacecraft, “Sausage” for a pattern that appears when plotting the Gaia data, and “Enceladus” for the Greek mythological giant that was buried under a mountain – much like GSE was buried in the Milky Way. As a result of this galactic collision, the dwarf galaxy was ripped apart and its constituent stars scattered in a scattered halo. Such an origin story explains the inherent dissimilarity of the stellar halo stars to stars born and raised in the Milky Way.

The results of the study further reveal how GSE and the Milky Way interacted all those centuries ago. The football shape — technically called a triaxial ellipsoid — reflects observations of two clusters of stars in the stellar halo. The clusters apparently formed as GSE passed through two orbits of the Milky Way. During these orbits, GSE would have slowed down twice in so-called apocenters, or the furthest points in the dwarf galaxy’s orbit from the larger gravitational puller, the hefty Milky Way; these pauses led to additional shedding of GSE stars. Meanwhile, the tilt of the stellar halo indicates that GSE encountered the Milky Way at an angle and not straight ahead.

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

In particular, so much time has elapsed since the collapse of the GSE Milky Way that the stellar halo stars were expected to dynamically settle into the classical long-assumed spherical shape. The fact that they probably didn’t speak to the wider galactic halo, the team says. This dark matter-dominated structure is likely itself skewed, and its gravity also keeps the stellar halo off balance.

“The tilted stellar 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 last point refers to the multiple dark matter detector experiments now underway and planned. These detectors could increase their chances of capturing an elusive interaction with dark matter if astrophysicists can assess where the substance is more highly concentrated, galactically speaking. As Earth moves through the Milky Way, it will periodically encounter these higher-velocity regions of dense dark matter particles, increasing the likelihood of detection.

The discovery of the most plausible configuration of the stellar halo will advance many astrophysical studies while filling in basic details about our place in the universe.

“These are such intuitively interesting questions to ask about our galaxy: ‘What does the galaxy look like?’ and ‘What does the stellar halo look like?'” says Han. “With this line of research and study in particular, we are finally answering those questions.”