How Our Largest Dwarf Galaxy Keeps the Others In Line

There’s one thing unusual about the dwarf galaxies round the Milky Way.

These smaller galaxies orbit our personal, however a lot of their orbits align alongside what astronomers time period the huge polar construction: a pancake-shaped plane that intersects our personal crepe-thin galaxy. Out of the dozens of identified satellites in the Milky Way’s retinue, about half of them, possibly much more, belong to this construction, dotting the plane like raisins in the pancake. What’s extra this pancake rotates, the blueberries whirling round the Milky Way in the identical route. 

Map of satellite galaxies around the Milky Way
The satellites round the Milky Way are in movement, as proven right here in a map overlaid on a visualization of the Milky Way Galaxy produced by Gaia information. Roughly half of the satellites, maybe extra, are in movement collectively, rotating collectively in a pancake-like construction that bisects the Milky Way plane.
Institute of Astrophysics of the Canary Islands

This alignment has puzzled astronomers since the first hints of it appeared in the Nineteen Seventies. Cosmological simulations don’t usually predict this impact. Some researchers have even questioned if the drawback is with our understanding of darkish matter or of gravity itself.

In a paper posted to the astronomy preprint arXiv, Nicolás Garavito Camargo (University of Arizona) and colleagues recommend the focus must be on the largest of all the little fish: the Large Magellanic Cloud (LMC).

The LMC is an enormous dwarf, with at the least a tenth the mass of the Milky Way. It’s swinging round our galaxy for the first time on a trajectory that aligns with the huge polar construction, and it could have introduced a half dozen smaller galaxies in with it. That can’t be a coincidence, proper? Astronomers have thought so, too, and several other have urged that the LMC has by some means impressed the mysterious polar construction.

In reality, earlier this year, Jenna Samuel (now at University of Texas, Austin) and colleagues made this case: Samuel’s led simulations to exhibit that dwarf galaxies are likely to align alongside cosmic pancakes — and keep that way, at the least for some time — when there’s an enormous dwarf like the LMC in the combine.

southern sky over San Pedro
In this view of the sky over Chile (taken throughout a Sky & Telescope tour), the Large and Small Magellanic Clouds are seen at proper, whereas the Milky Way crosses the sky at left. The Large Magellanic Cloud is the largest of our galaxy’s satellites; the Small Magellanic Cloud is in flip the largest of its retinue.
S&T / J. Kelly Beatty

The query is how: The LMC and its retinue are just a few of the Milky Way’s many satellites, so it and its attendants can’t account for the plane just by falling in. Garavito Camargo and colleagues describe how the LMC may have affected the orbits of the different galaxies.

In quick, the dwarf galaxy is throwing its weight round, affecting the Milky Way, its darkish matter halo, and its satellites, all through the easy power of gravity. Not solely does the dwarf pull on the different satellite tv for pc galaxies forward of it in its orbit, it additionally draws the material behind it into a wake. In addition, the researchers realized, they would want to account for their very own place in the galaxy.

As it falls in, the LMC has pulled the Milky Way off-center. Our galaxy is gigantic, although, and the shift in middle of mass has taken time to journey outward. While the inside areas of the galaxy and its halo are already orbiting the new middle of mass, the outer areas haven’t gotten the memo but. So, from our perch in the inside galaxy, we see the outer areas rotate.

“We’re actually in a moving car, when we thought we were just sitting still,” explains coauthor Gurtina Besla (University of Arizona). “You see all these things move by you and you think they’re moving at some speed, but in fact we’re moving along with it and they’re actually moving slightly slower.” In impact, once we see the satellite tv for pc galaxies shifting collectively in live performance, a few of that’s merely the impact of our personal movement — one thing that we hadn’t thought of earlier than.

Garavito Camargo, Besla, and colleagues mixed all of those results in a simulation of the LMC–Milky Way encounter, confirming that the LMC’s infall is able to reshaping the orbits of quite a few objects round the Milky Way.

This film exhibits satellite tv for pc galaxies round the Milky Way (higher left) and Andromeda (decrease proper). The dwarf galaxies round our personal galaxy are largely aligned alongside a plane. (A subset of Andromeda’s satellites are additionally alongside a plane, however their distribution is not fairly so compact.)
Pawlowski M., Kroupa P. & Jerjen H. / Monthly Notices of the Royal Astronomical Society 2013

Even taking all these results collectively, although, they nonetheless won’t absolutely clarify the unusual alignment of Milky Way satellites. Then once more, they don’t need to: After all, cosmological simulations do create planes of satellites, simply not ones as organized as the one round the Milky Way.

“The idea is that, after correcting for all these effects, the remaining plane may better resemble the structures predicted in cosmological simulations,” Garavito Camargo says. “All this together can create something that’s kind-of weird and statistically more improbable than what you might find in a generic cosmological simulation.”

“The new work offers a fresh approach and one that is a very thorough and convincing piece of research, which naturally got me and others working on this issue very excited,” says Marcel Pawlowski (Leibniz Institute for Astrophysics Potsdam), who wasn’t concerned in the new examine. He maintains, nonetheless, that there’s nonetheless work to be carried out to know the construction’s origin.


Leave a Comment

Your email address will not be published. Required fields are marked *

Shopping Cart