This artist’s impression reveals how the Milky Way galaxy would look seen from virtually edge on. Shown are the disk with star-blocking mud clouds, the central bulge and globular clusters (added by the creator) in the halo. ESO/NASA/JPL-Caltech/M. Kornmesser/R.HurtOur galaxy is outdated, almost as outdated as the universe itself. But it didn’t begin as a spiral of stars round a peanut-form center. It grew over time, each accumulating stars from collisions with different galaxies and forming stars itself from inflowing gasoline.
Piecing collectively the particulars of this historical past is a serious aim of astronomers, and Diederik Kruijssen (University of Heidelberg, Germany, and Harvard) and colleagues have proposed one model of this story, now revealed in the Monthly Notices of the Royal Astronomical Society (preprint obtainable right here).
The Big Five
To peek into the previous, Kruijssen and colleagues examined globular clusters. These historic balls of densely packed stars have been pawns of historical past, accompanying satellite tv for pc galaxies as they have been subsumed into the Milky Way. Now they largely orbit outdoors our galaxy’s disk in the stellar halo. Astronomers know of no less than 150 of them.
Many globular clusters fashioned proper the place they’re, however the orbits of a subset of them counsel they’re strangers in a wierd land. These clusters don’t have as many parts heavier than hydrogen and helium, recognized to astronomers as metals, indicating their origin in smaller galaxies quite than the bigger and metallic-enriched Milky Way.
To perceive the place — and when — these globular clusters got here from, Kruijssen’s workforce relied on a set of zoomed-in cosmological simulations referred to as E-MOSAICS. These simulations present the evolution of Milky Way-like galaxies as they cannibalize smaller galaxies, subsuming their globular clusters.
“Galaxy assembly is an extremely messy process, during which the orbits of the globular clusters are completely reshuffled,” Kruijssen explains. “To make sense of the complex system that is left today, we therefore decided to use artificial intelligence.”
Kruijssen and his colleagues skilled a synthetic neural community on E-MOSAICS, instructing it to narrate the properties of globular clusters and their galaxy of origin. The result’s a historical past of the most vital mergers with different galaxies:
A historical past of collisions, this merger tree reveals when 5 completely different satellite tv for pc galaxies collided and accreted onto the Milky Way. D. Kruijssen et al. / Monthly Notices of the Royal Astronomical SocietyFirst got here a galaxy playfully dubbed “the Kraken,” which collided with our galaxy round 11 billion years in the past. Though not the most huge satellite tv for pc that the Milky Way encountered, it was the most vital with respect to our galaxy’s mass at the time. It contributed no less than 13 globular clusters to the Milky Way.A billion years later got here a smaller galaxy whose stays are seen as a rivulset of stars dubbed the Helmi streams; it introduced alongside no less than 5 globular clusters.Another two small galaxies joined our personal in fast succession, nicknamed “Sequoia” and “Gaia-Enceladus,” accompanied by no less than three and no less than 20 globular clusters, respectively.The most up-to-date acquisition was the Sagittarius dwarf, which joined us 7 billion years in the past. In addition to the seven globular clusters it introduced with it, the galaxy’s stays are additionally seen as strung-out loops of stars circling our galaxy on almost polar orbits.This artist’s impression reveals the tidal tails of stars (white) streaming from the Sagittarius Dwarf Galaxy (orange) that orbits the Milky Way. The vivid yellow circle to the proper of the galaxy’s middle is our Sun (to not scale). Sagittarius is on the different facet of the galaxy from us, however we will see its tidal tails of stars stretching throughout the sky as they wrap round our galaxy.Amanda Smith / University of CambridgeGaia-Enceladus was beforehand considered the greatest collision. “However, the merger with Kraken took place 11 billion years ago, when the Milky Way was four times less massive,” Kruijssen explains. “As a result, the collision with Kraken must have truly transformed what the Milky Way looked like at the time.”
It’s doable that many different, smaller galactic collisions occurred that didn’t contribute globular clusters, however this research wasn’t designed to catalog these.
Interestingly, Kruijssen and his colleagues additionally discover that though these galaxy collisions symbolize the most main occasions of the Milky Way’s historical past, they solely contributed a few billion stars, about the mass of the stellar halo however a drop in the bucket in comparison with the spiral-imprinted disk. Most of our stars fashioned inside our galaxy.
What Does “Average” Mean, Anyway?
Helmer Koppelman (Institute for Advanced Study, Princeton), who was not concerned in the research, calls it “a fascinating summary of the Milky Way’s merger history.”
But there are some philosophical caveats to maintain in thoughts. First, though the research relies on simulations of galaxies which might be on common like the Milky Way, Koppelman explains that “the ‘average’ galaxy might not exist – meaning that each galaxy has its own peculiarities.”
Not solely do the simulations assume an “average” Milky Way, in addition they assume a selected universe. Other simulations — and there are a variety of them — would possibly give completely different outcomes. In truth, the research of galaxies’ formation historical past is impressed in half by the need to check how properly simulated universes replicate actuality.
“In a way, the authors have provided an excellent test for this,” Koppelman says. “Independent future research that either agrees or disagrees with the merger history presented in the paper might tell us whether the simulations are accurate or not.”
There’s clearly quite a lot of work nonetheless to be finished in reconstructing our galaxy’s messy historical past, however this research gives a cohesive framework for these efforts.