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

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

The results—released this month The Astronomical Journal – offer insight into a multitude of astrophysical domains. The results, for example, shed light on the history of our galaxy and galactic evolution, while offering 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’ve just measured with greater precision than previously possible,” says the study’s lead author, Jiwon “Jesse” Han. a Ph.D. student at the Astrophysics Center | Harvard & Smithsonian. “There are many important implications to the fact that the stellar halo is not spherical but rather shaped like a football, rugby ball or 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, Han’s advisor and professor of science. Astronomy at Harvard University and the Center for Astrophysics. “We now know that the classic image of our galaxy embedded in a spherical volume of stars must be discarded.”

The stellar halo of the Milky Way is the visible part of what is more widely called the galactic halo. This galactic halo is dominated by invisible dark matter, whose presence is only measurable through the gravity it exerts. Each galaxy has its own dark matter halo. These halos serve as a kind of scaffolding on which ordinary visible matter clings. In turn, this visible matter 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. “In order to learn more about galactic halos in general, and in particular about the galactic halo and the history of our own galaxy, the stellar halo is an excellent starting point.”

Probing the shape of the Milky Way’s stellar halo, however, has long challenged astrophysicists for the simple reason that we’re 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 outer galaxies, where we just look at them and measure their halos,” says Han, “we don’t have the same kind of aerial, outer perspective of our own galaxy’s halo.”

To complicate matters further, the stellar halo turned out to be quite diffuse, containing only about one percent of the mass of all stars in the galaxy. Yet over time, astronomers have managed to identify several thousand stars that populate this halo, which stand out from other stars in the Milky Way because of their distinctive chemical composition (measurable by studies of their starlight). , as well as by their distances and their movements across the sky. Through such studies, astronomers have realized that halo stars are not evenly distributed. The goal has since been to study patterns of star overdensities – appearing spatially as clusters and fluxes – to sort out the ultimate origins of the stellar halo.

The new study by CfA researchers and colleagues builds on two major datasets collected in recent years that have probed the stellar halo like never before.

The first set comes from Gaia, a groundbreaking spacecraft launched by the European Space Agency in 2013. Gaia has gone on to compile the most precise measurements of the positions, movements and distances of millions of stars in the Milky Way, including including some nearby stellar halo stars. .

The second set of data comes from H3 (Hectochella in the High-Resolution Halo), 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 the decidedly non-spherical halo – and the shape of the football fits perfectly with other findings to date. . The shape, for example, is independently and strongly consistent with a 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 collided 7 to 10 billion years ago with our much larger galaxy. The extinct dwarf galaxy is amusingly known as Gaia-Sausage-Enceladus (GSE), where “Gaia” refers to the aforementioned spacecraft, “Sausage” for a pattern appearing when plotting data from Gaia, and “Enceladus for the Greek mythological giant who was buried under a mountain – much like how GSE was buried in the Milky Way. Following this galactic collision event, the dwarf galaxy was torn apart and its constituent stars scattered in a scattered halo. Such an origin story explains the inherent dissimilarity of stars in the stellar halo to stars born and raised in the Milky Way.

The study results further explain how GSE and the Milky Way interacted eons ago. The shape of the football – technically called a triaxial ellipsoid – reflects observations of two star stacks in the stellar halo. The stacks apparently formed when GSE passed through two orbits of the Milky Way. During these orbits, GSE would have slowed down twice at the so-called apocenters, or the farthest points in the dwarf galaxy’s orbit from the greatest gravitational attractor, the heavy Milky Way; these breaks resulted in the loss of additional GSE stars. Meanwhile, the tilt of the stellar halo indicates that GSE encountered the Milky Way at an incident angle and not in a straight line.

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

Notably, so much time has passed since the GSE-Milky Way crash that one might have expected stars in the stellar halo to dynamically settle into the long-assumed classic spherical shape. The fact that they didn’t likely speaks to the larger galactic halo, the team says. This dark matter-dominated structure is itself likely askew and, by its gravity, also keeps the stellar halo shifted.

“The tilted stellar halo strongly suggests that the underlying dark matter halo is also tilted,” Conroy explains. “A tilt of the dark matter halo could have important ramifications for our ability to detect dark matter particles in laboratories on Earth.”

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

Finding the most plausible stellar halo configuration should advance much astrophysical research while providing basic details about our place in the universe.

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