ハーバードの天文学者は、天の川の星の暈の真の形を明らかにしました

天文学者は、天の川銀河の恒星ハロー (すべての銀河の周りの拡散した星の雲) がツェッペリン型で傾いていることを発見しました。 このアーティストのイラストは、私たちの銀河を取り囲む 3 次元のハローの形を強調しています。 クレジット: Melissa Weiss/天体物理学センター | ハーバードとスミソニアン

最近の研究により、恒星ハローとして知られる、私たちの銀河系の円盤を取り囲む星の拡散雲の真の形状が明らかになりました。 以前は、ビーチ ボールのように大部分が球形であると考えられていましたが、最新の観測に基づく新しいモデルは、恒星のハローが、蹴られたフットボールのように、横長で傾いていることを示しています。

これらの調査結果は、 天文ジャーナル、銀河の歴史と進化、暗黒物質の探索の手がかりなど、さまざまな天体物理学の主題への洞察を提供します。

恒星のハローの形状は非常に基本的なパラメーターであり、これを測定したところ、[{” attribute=””>accuracy than was possible before,” says study lead author Jiwon “Jesse” Han, a Ph.D. student at the Center for Astrophysics | Harvard & Smithsonian. “There are a lot of important implications of the stellar halo not being spherical but instead 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 every direction,” adds study co-author Charlie Conroy, Han’s advisor, and a professor of astronomy at Harvard University and the Center for Astrophysics. “We now know that the textbook picture of our galaxy embedded within a spherical volume of stars has to be thrown out.”

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“The stellar halo is a dynamic tracer of the galactic halo,” says Han. “In order to learn more about galactic haloes in general, and especially our own galaxy’s galactic halo and history, the stellar halo is a great place to start.”

Fathoming the shape of the Milky Way’s stellar halo, though, has long challenged astrophysicists for the simple reason that we are embedded within it. The stellar halo extends out several hundred thousand light years above and below the star-filled plane of our galaxy, where our Solar System resides.

“Unlike with external galaxies, where we just look at them and measure their halos,” says Han, “we lack the same sort of aerial, outside perspective of our own galaxy’s halo.”

Complicating matters further, the stellar halo has proven to be quite diffuse, containing only about one percent of the mass of all the galaxy’s stars. Yet over time, astronomers have succeeded in identifying many thousands of stars that populate this halo, which are distinguishable from other Milky Way stars due to their distinctive chemical makeup (gaugeable by studies of their starlight), as well as by their distances and motions across the sky. Through such studies, astronomers have realized that halo stars are not evenly distributed. The goal has since been to study the patterns of over-densities of stars — spatially appearing as bunches and streams — to sort out the ultimate origins of the stellar halo.

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According to this framework, the stellar halo formed when a lone dwarf galaxy collided 7-10 billion years ago with our far-larger galaxy. The departed dwarf galaxy is amusingly known as Gaia-Sausage-Enceladus (GSE), where “Gaia” refers to the aforementioned spacecraft, “Sausage” for a pattern appearing when plotting the Gaia data, and “Enceladus” for the Greek mythological giant who was buried under a mountain — rather like how GSE was buried in the Milky Way. As a consequence of this galactic collisional event, the dwarf galaxy was ripped apart and its constituent stars were 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 study’s results further chronicle just how GSE and the Milky Way interacted all those eons ago. The football shape — technically called a triaxial ellipsoid — reflects the observations of two pileups of stars in the stellar halo. The pileups ostensibly formed when GSE went through two orbits of the Milky Way. During these orbits, GSE would have slowed down twice at so-called apocenters, or the furthest points in the dwarf galaxy’s orbit of the greater gravitational attractor, the hefty Milky Way; these pauses led to the extra shedding of GSE stars. Meanwhile, the tilt of the stellar halo indicates that GSE encountered the Milky Way at an incident angle and not straight-on.

“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,” says Conroy.

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, the team says. 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,” says Conroy. “A tilt in the dark matter halo could have significant ramifications for our ability to detect dark matter particles in laboratories on Earth.”

Conroy’s latter point alludes to the multiple dark matter detector experiments now running and planned. These detectors could increase their chances of capturing an elusive interaction with dark matter if astrophysicists can adjudge where the substance is more heavily concentrated, galactically speaking. As Earth moves through the Milky Way, it will periodically encounter these regions of dense and higher-velocity dark matter particles, boosting the odds of detection.

The discovery of the stellar halo’s most plausible configuration stands to move many astrophysical investigations forward while filling in basic details about our place in the universe.

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

Reference: “The Stellar Halo of the Galaxy is Tilted and Doubly Broken” by Jiwon Jesse Han, Charlie Conroy, Benjamin D. Johnson, Joshua S. Speagle (沈佳 士), Ana Bonaca, Vedant Chandra, Rohan P. Naidu1, Yuan-Sen Ting (丁源 森), Turner Woody and Dennis Zaritsky, 15 November 2022, The Astronomical Journal.
DOI: 10.3847/1538-3881/ac97e9

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