Milky Way’s Halo Is Not the Flat Sphere Astronomers Expect: ScienceAlert

Milky Way's Halo Is Not the Flat Sphere Astronomers Expect: ScienceAlert
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Step outside the Milky Way for a moment and you can see the bright disc of stars we call home has a strange reverseπŸ‡§πŸ‡· Now it seems that the rest of our galaxy is also a little far away.

A new map of stars above and below the galactic plane shows its galactic halo – a diffuse globe of gas. dark matter, and stars surrounding spiral galaxies are also uncertain. Rather than the expected nice round astrospheres, the Milky Way’s halo is an undulating ellipsoid with all three axes of different lengths.

“For decades, the general assumption was that the stellar halo was more or less spherical and isotropic, or the same in all directions.” says astronomer Charlie Conroy Harvard and the Smithsonian Center for Astrophysics (CfA).

“We now know that we need to discard the textbook picture of our galaxy embedded in globular stars.”

Determining the shape of our galaxy is really difficult. Imagine trying to figure out the shape of a huge lake as you wade through it. Only in recent years, with the launch of the European Space Agency’s Gaia telescope in 2013, have we gained detailed information about the three-dimensional shape of our galaxy.

Gaia shares Earth’s orbit around the Sun. Changes in the telescope’s position in the solar system allow it to measure parallax number of objects in the Milky Way that have obtained the most precise measurements to date to calculate the positions and motions of thousands of distant stars.

Thanks to this information, we now know that the Milky Way disk exists bent and bentπŸ‡§πŸ‡· We also know that the Milky Way is busy many times acts of galactic cannibalismone of the most prominent of these has been the collision with the galaxy we call a galaxy. Gaia sausageor Gaia Enceladus, about 7-10 billion years ago.

This collision, scientists believe, It created the star halo of the Milky WayπŸ‡§πŸ‡· Gaia’s sausage broke apart as it collided with our galaxy, its disparate stars scattered across the Milky Way’s halo.

A team of scientists led by CfA astronomer and PhD student Jiwon “Jesse” Han set out to better understand the galactic halo and the Gaia sausage’s role in it.

“The stellar halo is a dynamic tracer of the galactic halo” Han saysπŸ‡§πŸ‡· “To learn more about galactic halos in general and our own galaxy’s galactic halos and history in particular, stellar halos are a great place to start.”

Unfortunately, Gaia data on the chemical abundances of halo stars beyond certain distances are not very reliable. Stellar populations can be correlated by their chemical abundance, making it an important piece of information for mapping the relationship between the stars of a halo.

So the researchers added data from a survey called Halo at High Resolution, or Hectochelle H3πŸ‡§πŸ‡· ground-based survey that collects chemical abundance data on thousands of stars in the Milky Way’s stellar halo, among other features.

With these data, the researchers inferred the density profile of the stellar population of the Milky Way’s halo. They found that the best fit for their data was a football-shaped halo tilted 25 degrees relative to the galactic plane.

Estimated dimensions of the halo. (Han et al., AJ, 2022)

It fits previous studies He found that the stars in the Milky Way’s halo are in a triaxial ellipsoidal formation (although the specifications vary slightly). It also fits the theory that Gaia’s sausage created, or at least played a major role in creating, the Milky Way’s halo. The curved shape of the halo indicates that two galaxies are colliding at an angle.

The researchers also discovered two star clusters at significant distances from the galactic center. They found that these collections represent the apocenters of the first stellar orbits around the galactic centerβ€”the farthest the stars travel in their elongated, elliptical orbits.

Just like an object in orbit accelerates about to reach the point closest to the center of gravity or “pericenter”., the apocenter is at the deceleration point. When Gaia sausage encountered the Milky Way, its stars were thrown into two wild orbits, slowed to a standstill at the apocenters, and simply made this place its new home.

But that was a long time ago, long enough that the strange form should have long ago resolved itself and settled back into a sphere. The strong tilt suggests that the dark matter halo surrounding the Milky Way, the mysterious mass responsible for the extreme gravity in the Universe, is also highly tilted.

So it looks like we have some new and interesting answers, but also some new and interesting questions. According to the researchers, ongoing and future studies should provide even stronger constraints on the shape of the halo to understand how our galaxy evolved.

“These are very interesting questions to ask about our galaxy: ‘What does the galaxy look like?’ and ‘What does a stellar halo look like?'” Han saysπŸ‡§πŸ‡·

“Especially with this line of research and research, we’re finally answering those questions.”

The study was published Journal of AstronomyπŸ‡§πŸ‡·

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