Accounting for approximately 1% of the total stellar mass, the stellar halo — the diffuse cloud of stars surrounding the Galactic disk — plays an outsized role in our understanding of the Milky Way. For decades, the general assumption has been that the halo is more or less spherical and isotropic, or the same in every direction. Now new research led by the Harvard & Smithsonian’s Center for Astrophysics shows the halo is oblong and tilted.
This artist’s illustration emphasizes the shape of the stellar halo surrounding our Milky Way Galaxy. Image credit: Melissa Weiss / Harvard & Smithsonian’s Center for Astrophysics.
The Milky Way’s stellar halo is the visible portion of what is called the Galactic halo.
This larger halo is dominated by invisible dark matter, whose presence is only measurable through the gravity that it exerts.
To better understand how galaxies form and interact, as well as the underlying nature of dark matter, stellar halos are accordingly valuable astrophysical targets.
“The stellar halo is a dynamic tracer of the Galactic halo,” said Jiwon Han, a Ph.D. student at the Harvard & Smithsonian’s Center for Astrophysics.
“In order to learn more about galactic halos in general, and especially our own Galaxy’s Galactic halo and history, the stellar halo is a great place to start.”
In their new study, Han and colleagues used data from ESA’s Gaia mission and the H3 (Hectochelle in the Halo at High Resolution) survey.
Combining these two datasets in a flexible model that allowed for the stellar halo shape to emerge from all the observations yielded the decidedly non-spherical halo.
This shape independently and strongly agrees 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, known as Gaia-Sausage-Enceladus (GSE), collided 7-10 billion years ago with our far-larger galaxy.
As a consequence of this galactic collisional event, the dwarf galaxy was ripped apart and its constituent stars strewn out into a dispersed halo.
Such an origin story accounts for the stellar halo stars’ inherent unlikeness to stars born and bred in the Milky Way.
“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,” said Professor Charlie Conroy, an astronomer at Harvard University and the Harvard & Smithsonian’s Center for Astrophysics.
“Notably, so much time has passed since the GSE-Milky Way smashup that the stellar halo stars would have been expected to dynamically settle into the classical, long-assumed spherical shape.”
“The fact that they haven’t likely speaks to the broader Galactic halo. This dark matter-dominated structure is itself probably askew, and through its gravity, is likewise keeping the stellar halo off-kilter.”
“The tilted stellar halo strongly suggests that the underlying dark matter halo is also tilted.”
“A tilt in the dark matter halo could have significant ramifications for our ability to detect dark matter particles in laboratories on Earth.”
The team’s findings appear in the Astronomical Journal.
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Jiwon Jesse Han et al. 2022. The Stellar Halo of the Galaxy is Tilted and Doubly Broken. AJ 164, 249; doi: 10.3847/1538-3881/ac97e9
