The Milky Way's star halo has been realized

New research has revealed the exact form of the hazy cloud of stars that surrounds our galaxy's disk. For decades, astronomers assumed that the stellar halo, or cloud of stars, was mostly spherical, like a beach ball. A new model based on recent measurements suggests that the star halo is oblong and slanted, much like a freshly booted football. The results published this month in The Astronomical Journal provide new insights on a variety of astrophysical topics. The findings, for example, provide insight into the history of our galaxy and galactic development, while also providing hints in the continuing search for dark matter, a mystery element.


The geometry of the star halo is a very basic characteristic that we've recently measured to more accuracy than was previously achievable, according to the study's lead author Jiwon Jesse Han, a Ph.D. student at the Center for Astrophysics | Harvard & Smithsonian. There are several major consequences to the star halo not being spherical but rather shaped like a football, rugby ball, or zeppelin, depending on your preference.


For decades, the conventional idea has been that the star halo is more or less spherical and isotropic, or the same in every direction, according to study co-author Charlie Conroy, Han's adviser and a professor of astronomy at Harvard University and the Center for Astrophysics. The textbook notion of our galaxy enclosed within a spherical container of stars must now be abandoned.


The stellar halo of the Milky Way is the visible section of what is known as the galactic halo. This galaxy halo is dominated by unseen dark matter, the presence of which can only be measured by the gravity it exerts. Every galaxy has its own dark matter halo. These halos function as a kind of scaffolding from which regular, visible stuff hangs. That visible stuff, in turn, generates stars and other observable galaxy structures. Stellar haloes are therefore useful astrophysical targets for a better understanding of how galaxies develop and interact, as well as the underlying nature of dark matter.


The stellar halo, according to Han, is a dynamic tracer of the galactic halo. The stellar halo is a fantastic location to start learning more about galactic haloes in general, and specifically our own galaxy's galactic halo and history.


Understanding the geometry of the Milky Way's star halo, on the other hand, has long been a source of consternation for astrophysicists, for the simple reason that we are entrenched inside it. The stellar halo stretches several hundred thousand light years above and below our galaxy's star-filled plane, where our Solar System is located.


Unlike with exterior galaxies, where scientists simply observe and quantify their halos, Han explained. We don't have the same aerial, outside perspective as our own galaxy's halo.


To make matters worse, the stellar halo has been shown to be highly diffuse, comprising just around 1% of the mass of all the stars in the galaxy. Yet, throughout time, astronomers have identified many thousands of stars that fill this halo, which can be distinguished from other Milky Way stars by their specific chemical makeup (as determined by studies of their luminosity) as well as their distances and movements across the sky. Astronomers have discovered that halo stars are not uniformly dispersed as a result of such research. Since then, the objective has been to investigate the patterns of star over-densities that appear as bunches and streams in space to determine the ultimate origins of the stellar halo.


The current study by CfA researchers and collaborators draws on two recent large datasets that have delved into the stellar halo like never before. The first set comes from Gaia, the European Space Agency's breakthrough satellite launched in 2013. Gaia has kept track of the most exact measurements of the locations, movements, and distances of millions of stars in the Milky Way, including some close stellar halo stars. The second dataset is from H3 (Hectochelle in the Halo at High Resolution), a ground-based survey undertaken at the MMT, which is located at the Fred Lawrence Whipple Observatory in Arizona and is a cooperation between the Center for Astrophysics and the University of Arizona.


H3 has collected comprehensive measurements of tens of thousands of stellar halo stars that are too far for Gaia to evaluate. Combining this data in a flexible model that enabled the star halo form to emerge from all of the observations resulted in a distinctly non-spherical halo and a football shape that fits in neatly with other recent discoveries. The shape, for example, independently and firmly coincides with a major hypothesis of how the Milky Way's star halo formed.

According to this model, the star halo originated when a lone dwarf galaxy collided with our much bigger galaxy 7-10 billion years ago. The departed dwarf galaxy is amusingly referred to as Gaia-Sausage-Enceladus (GSE), where "Gaia" refers to the aforementioned spacecraft, "Sausage" refers to a pattern seen when plotting the Gaia data, and "Enceladus" refers to the Greek mythological giant who was buried under a mountain, much like GSE was buried in the Milky Way. The dwarf galaxy was split apart as a result of this galactic collisional event, and its constituent stars were flung out into a dispersed halo. This genesis tale explains the stellar halo stars' fundamental differences from stars born and raised in the Milky Way.


The findings of the study add to our understanding of how GSE and the Milky Way interacted all those millennia ago. The football form, known technically as a triaxial ellipsoid, reflects observations of two-star pileups in the stellar halo. The pileups allegedly developed as GSE passed across two Milky Way orbits. During these cycles, GSE would have slowed twice at apocenters, or the farthest places in the dwarf galaxy's orbit of the bigger gravitational attractor, the massive Milky Way; these pauses resulted in the additional shedding of GSE stars. Meanwhile, the tilt of the star halo suggests that GSE collided with the Milky Way at an incidence angle rather than directly on.


According to Conroy, the tilt and distribution of stars in the stellar halo give compelling evidence that our galaxy collided with a smaller galaxy 7-10 billion years ago.


Notably, so much time has passed after the GSE-Milky Way collision that the stellar halo stars should have dynamically settled into the long-assumed spherical form. The fact that they haven't points to the larger galactic halo, according to the scientists. This dark matter-dominated structure is most likely tilted, and its gravity is tilting the star halo as well.


The slanted star halo, according to Conroy, strongly implies that the underlying dark matter halo is similarly skewed. A tilt in the dark matter halo might have serious consequences for our capacity to detect dark matter particles in Earth-based experiments.


Conroy's final statement refers to the numerous dark matter detector tests that are either underway or in the planned stages. If astrophysicists can determine where the material is more densely packed, these detectors may have a better chance of recording an elusive interaction with dark matter. As Earth travels through the Milky Way, it will meet these regions of dense and faster-moving dark matter particles, increasing the chances of detection. The identification of the most likely configuration of the star halo has the potential to advance numerous astrophysical inquiries while also filling in basic information about our location in the universe.


These are such intuitively intriguing questions to ask about our galaxy, according to Han: 'What does the galaxy look like?' and 'How does the stellar halo appear?' We are finally answering such questions with this area of inquiry and study in particular.


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