Home Science The Shape Of The Milky Way’s Halo Of Stars Is Realized – Eurasia Review

The Shape Of The Milky Way’s Halo Of Stars Is Realized – Eurasia Review

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A new study has revealed the true shape of the star-studded cloud that surrounds our galaxy’s disk. I’ve thought of it as mostly spherical, like a beach ball. Now, a new model based on the latest observations shows that the stellar halo is oblong and tilted like a football that has just been kicked.

Survey Results — Published This Month astronomical journal — Provides insight into many astrophysical subject areas. For example, the results shed light on our galaxy and its evolutionary history, while also providing clues to the ongoing tracking of a mysterious substance known as dark matter.

“The shape of the stellar halo is a very fundamental parameter, and we have been able to measure it more accurately than before,” says Jiwon “Jesse,” a Ph.D. student at the Center for Astrophysics and lead author of the study. ” Han said. Harvard & Smithsonian. “It has many important implications that the stellar halo is shaped like a football, rugby ball, or Zeppelin instead of being spherical.

“For decades there was a common assumption that the stellar halo was more or less spherical and isotropic, or the same in all directions,” said Han’s adviser and Harvard University astronomy expert. Professor and study co-author Charlie Conroy adds. Astrophysics Center. “It turns out that textbook pictures of our galaxy buried in spherical stars must be discarded.”

The Milky Way’s stellar halo is the visible part of what is more broadly called the galactic halo. The galaxy’s halo is dominated by invisible dark matter, whose presence is measurable only by the gravitational force it exerts. Every galaxy has its own dark matter halo. These halos act as a kind of scaffolding from which normal visible matter hangs. That visible material then forms stars and other observable galactic structures. To better understand how galaxies form and interact, as well as the underlying properties of dark matter, stellar halos are valuable astrophysical targets.

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

But speculating about the shape of the Milky Way’s stellar halo has long challenged astrophysicists for the simple reason that we are embedded in it. It extends hundreds of thousands of light-years above and below the star-filled plane of the galaxy.

“Unlike outer galaxies, we just look at them and measure the halo,” says Han.

To further complicate matters, stellar halos have proven to be very diffuse, containing only about 1% of the mass of all stars in the galaxy. But over time, astronomers have managed to identify thousands of stars in this halo. These stars can be distinguished from other Milky Way stars by their characteristic chemical composition (which can be measured by studying starlight) and by their distance and motion. Sky. Through such studies, astronomers have noticed that halo stars are not evenly distributed. Since then, the goal has been to study patterns of stellar excess density. This appears spatially as a bundle or stream to sort out the ultimate origin of the star’s halo.

A new study by CfA researchers and colleagues leverages two major datasets collected in recent years to form stellar halos like never before.

The first set is from the revolutionary spacecraft Gaia, launched by the European Space Agency in 2013. Gaia continues to compile the most accurate measurements of the positions, motions and distances of millions of stars in the Milky Way, including the nearby halo stars. .

The second dataset is from H3 (Hectoshell in High Resolution Halo), a ground-based survey conducted at MMT at the Fred Lawrence Whipple Observatory in Arizona, a collaboration between CfA and the University of Arizona. is. H3 collected detailed observations of tens of thousands of halo stars too far for Gaia to assess.

By combining these data into a flexible model, all observations revealed the shape of the stellar halo, yielding a distinctly non-spherical halo. The football shape fits nicely with other findings so far. For example, this shape is independently and strongly consistent with the prevailing theory of the formation of the Milky Way’s stellar halo.

According to this framework, the stellar halo formed when an isolated dwarf galaxy collided with our much larger galaxy 7-10 billion years ago. The departed dwarf galaxy is interestingly known as Gaia-Sausage-Enceladus (GSE). Here, “Gaia” refers to the aforementioned spacecraft, “Sausage” to the pattern that appears when plotting Gaia’s data, and “Enceladus” to the giant of Greek mythology. It was buried under the mountain in the same way that the GSE was buried in the Milky Way. As a result of this galactic collision event, the dwarf galaxy was torn apart and its constituent stars scattered into dispersed halos. I’m explaining.

The results of this study further document how the GSE and the Milky Way interacted thousands of years ago. The football shape, technically called the triaxial ellipsoid, reflects the observed stacking of her two stars in the stellar halo. The pileup was ostensibly formed when the GSE passed her two orbits in the Milky Way. During these orbits, the GSE would have slowed him down by 2 degrees at the farthest point in the orbit of the so-called apocentre, or massive Milky Way dwarf galaxy, which is the larger gravitational attractor. These suspensions led to extra dropouts for GSE stars. On the other hand, the tilt of the stellar halo indicates that the GSE encountered the Milky Way at an angle of incidence rather than straight on.

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

In particular, since so much time has passed since the GSE-Milky Way collision, stellar halo stars were expected to dynamically settle into their classical, long-supposed spherical shape. This dark-matter-dominated structure is itself likely tilted, and its gravity similarly keeps the stellar halo unstable, the team says. I’m here.

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

Conroy’s latter point alludes to multiple dark matter detector experiments currently underway and planned. These detectors could increase the chances of catching interactions with the elusive dark matter if astrophysicists can determine where dark matter is galactically concentrated. As the Earth passes through the Milky Way, it regularly encounters regions of dense, high-velocity dark matter particles, increasing the likelihood of detection.

The discovery of the most plausible configuration of stellar halos will advance many astrophysical investigations, filling in fundamental details about our position in the universe.

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

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