Mounting proof signifies that our galaxy smashed up one other smaller galaxy roughly 10 billion years in the past.
We reside in an enormous disk galaxy, a whirligig pancake that’s enshrouded in an enormous halo of outdated stars. And more and more, astronomers suspect that very early in our cosmic pancake’s historical past, a collision tousled the serene stellar disk and donated the detritus that makes up a lot of the halo.
The disk has two overlapping sections: the thin disk and the thick disk. The skinny disk is roughly three,000 light-years thick and embedded within the thick disk, which is twice as thick as the skinny one (therefore the names). The entire thing is type of like an Oreo, besides that the cookie goes by means of the crème. (Bear in mind, there’s quite a lot of house between stars, so interweaving is straightforward.) Younger stars comparatively wealthy in components heavier than helium dance within the skinny disk; outdated, chemically anemic stars make up the thick one.
Each theorists and observers have struggled to elucidate why our stellar disk appears this fashion. Making a skinny disk is straightforward: As gasoline first collapses to type a galaxy, it naturally flattens out into a pleasant, skinny saucer. And gasoline continues to rain down over time, repeatedly fueling a skinny disk of star formation. However one thing has to puff the outdated stars as much as make the thick disk, explains Puragra GuhaThakurta (College of California, Santa Cruz). That one thing may very well be something from the galaxy’s central bar fluffing the disk up like an eggbeater does whipping cream, to a fender-bender with one other galaxy.
To unravel this thriller, astronomers are utilizing bigger and bigger surveys, together with that being executed by the European Gaia satellite, which is mapping the positions and motions of some billion stars. These investigations are turning up a attainable reply not within the disk itself, however within the halo.
Current work utilizing Gaia and different information has discovered that halo stars inside a number of thousand light-years of the Solar rotate around the galactic center in the opposite, or retrograde, direction as the disk does. These stars even have completely different chemical compositions than these within the disk. Mixed, the unusual traits recommend that these stars aren’t indigenous to the Milky Manner — somewhat, they’re in all probability crumbs from when our galaxy ate a galactic snack very early in its historical past.
Amina Helmi (College of Groningen, The Netherlands) and colleagues have now taken a more in-depth have a look at the retrograde stars’ motions and compositions. Reporting within the November 1st Nature, they confirmed that these halo stars are one thing fairly uncommon. First, they transfer collectively as an enormous unit. Second, they’ve a definite sample of heavy components, and a variety of their heavy aspect ranges that implies these stars did not all type in a single burst however in an prolonged interval of star formation, creating extra heavy components and more and more infecting themselves with time. Third, the celebs have a variety of ages, which matches the chemistry predictions.
Taken collectively, these quirks set the celebs aside from these born within the Milky Manner. They very probably come from elsewhere.
Based mostly on the chemical traits, the group may infer how lengthy all the celebs took to type and the way huge their father or mother galaxy was: roughly 600 million photo voltaic lots, or roughly the identical because the Small Magellanic Cloud dwarf galaxy — fairly a decent contender, however no match for the Milky Manner’s roughly 100 billion stars.
So many are the retrograde stars (about 30,000), that they type an enormous swarm across the disk for 1000’s of light-years across the Solar (possibly farther, we don’t know but). Helmi estimates that roughly 80% of our galaxy’s halo may very well be from this single historic collision. That will jibe with one other current research that couldn’t find signs of a homemade inner halo, in addition to a well-liked speculation that halos round galaxies just like the Milky Manner are made with the leftovers of smaller galaxies that the large ones tore up and ate. The Milky Manner’s sibling galaxy, Andromeda (M31), for instance, has a jumbled halo stuffed with stellar streams, whereas the Pinwheel Galaxy (M101) appears to have no halo at all.
Helmi and her colleagues additionally used simulation outcomes to substantiate merger with such a large small galaxy roughly 10 billion years in the past may clarify the retrograde stars, in line with other recent work and making a “putting” parallel to the observations, writes Kim Venn (College of Victoria, Canada) in a Nature opinion piece.
And the Milky Manner’s disk? The merger would naturally puff up the disk that existed at the moment, creating the thick disk we have now at this time. Different current research make the identical case.
Whether or not this putative merger actually did puff up the thick disk stays to be seen — astronomers are all the time digging for extra information. In the meantime, Helmi’s group is pursuing extra complicated simulations that may embrace gasoline physics to see what sort of star formation the merger may need spurred.
Beneath, you’ll discover a video simulation of the merger. Credit score: H.H. Koppelman, A. Villalobos and A. Helmi
A. Helmi et al. “The Merger that Led to the Formation of the Milky Way’s Inner Stellar Halo and Thick Disk.” Nature. November 1, 2018.
V. Belokurov et al. “Co-formation of the Disc and the Stellar Halo.” Month-to-month Notices of the Royal Astronomical Society. July 21, 2018.
M. Haywood et al. “In Disguise or Out of Reach: First Clues about In Situ and Accreted Stars in the Stellar Halo of the Milky Way from Gaia DR2.” Astrophysical Journal. August 20, 2018.
Ok. Venn. “Evidence of Ancient Milky Way Merger.” Nature. November 1, 2018.