A recent study suggests the dwarf galaxy Antlia 2 had a long-ago run-in with our galaxy, rippling and warping its disk. But not everyone agrees with that scenario.

Antlia 2 simulation
A frame from Chakrabarti and colleague's simulation shows the dark dwarf galaxy Antlia 2 colliding with the Milky Way hundreds of millions of years ago.
S. Chakrabarti

Last fall, astronomers announced the discovery of a giant but incredibly faint dwarf galaxy 420,000 light-years away from Earth, on the far side of the Milky Way. That galaxy, dubbed Antlia 2, just might be the one that Sukanya Chakrabarti (Rochester Institute of Technology) has been looking for, she announced at a meeting of the American Astronomical Society in St. Louis, Missouri.

In multiple studies published between 2009 and 2014, Chakrabarti fleshed out a prediction: that a dwarf galaxy had smashed through the Milky Way several hundred million years ago, setting off ripples that decorate our galaxy’s outer disk. The theorist turned observer and set out to look for signs of the dwarf, but while she found some promising hints of its existence, there was no smoking gun.

“Then last year, I heard from my colleagues and friends about the discovery of the Antlia 2 dwarf galaxy,” Chakrabarti recalls. “I hadn’t looked at astro-ph [the astronomy preprint arXiv] that day actually. But four or five people wrote to me, having seen the discovery paper, and asked if it wasn’t the dwarf galaxy that I’d predicted 10 years ago!”

Galactic Ripples

Ripples in the Milky Way's Disk
Astronomers have long sought to explain ripples in the Milky Way's disk, shown here in an artist's conception.
Dana Berry

In 2006, astronomers mapping out the hydrogen gas in the Milky Way’s disk found something puzzling: The outer disk is warped and rippled at its edges, as if some galactic sculptor wasn’t paying attention at the pottery wheel.

Chakrabarti learned about this galactic-scale oddity as a postdoc. She started simulating th effects of dwarf galaxies sweeping by outer galactic disks, ultimately coming to the conclusion that such a gravitational interaction was the best way to recreate the Milky Way’s unique shape. But long-term Hubble Space Telescope measurements of the stars in known dwarf galaxies, including the Small and Large Magellanic Clouds and the torn-apart Sagittarius Dwarf, showed that they wouldn’t do the trick. The dwarf galaxy had to be a new one, as yet undiscovered.

“‘Round that time, I thought I would try and look for it myself,” Chakrabarti says, “though up to that point I’d been mainly a theorist.” With collaborators, she found a couple of candidate Cepheid variable stars that appeared to be 300,000 light-years away, presumably part of (or recently part of) a dwarf galaxy being torn asunder in the Milky Way’s gravitational field. However, the nature and distance of these stars remained unclear, and Chakrabarti in the meantime moved on to other studies.

Then Gabriel Torrealba (Academia Sinica, Taiwan) and colleagues discovered Antlia 2. As dwarf galaxies go, it’s an odd duck. Its stars are spread surprisingly thin, so that even though it’s about the size of the Large Magellanic Cloud, its surface brightness is a full two magnitudes fainter. It’s hard to understand how such a galaxy holds together, but perhaps it doesn’t — Torrealba’s group suggested that the Milky Way’s gravity might be tearing it apart.

As soon as the galaxy came to Chakrabarti’s attention, she began doing calculations. It wasn’t easy. The Milky Way’s mass isn’t known that well (to a factor of two, roughly) and neither is Antlia 2’s. It’s also unclear how close Antlia 2 would have come to our galaxy’s center. So Chakrabarti and colleagues explore a variety of options in their study.

Both mass and closest approach are critical factors in determining if a dwarf galaxy could have had the gravitational strength to draw up ripples from the galaxy’s disk. The Sagittarius dwarf galaxy’s mass, Chakrabarti and colleagues write, is insufficient, and the Large and Small Magellanic Clouds are too far away. Other dwarf galaxies that have been discovered are even less massive and/or farther away.

“I think at the point, when we were sure that the Sagittarius dwarf galaxy definitely could not produce the perturbations in the outer HI disk and we saw that Antlia 2 could do it — that’s when we were really sure,” Chakrabarti says.

Antlia 2 simulation
This simulation depicts how the Milky Way and Antlia 2 have interacted from 3 billion years ago to present day. The left panels show the gas distribution and the right panels show the stars. The top panels show the galaxies face on while the bottom panels show the galaxies edge-on.
S. Chakrabarti et al.

Not a Smoking Gun

But not everybody agrees with this conclusion. Vasily Belokurov (University of Cambridge, UK) cites a series of less than probable assumptions required for the Antlia 2 scenario. “Their simulations are rather simple and are produced for one realization of Antlia 2 only,” Belokurov explains. “There are already constraints on the mass of Antlia 2 and at the moment the mass appears lower than what they need.”

Moreover, he adds, the simulations where Antlia 2 can create ripples focus on the less likely scenario that Antlia 2 crashed relatively close to our galactic center, coming within roughly 30,000 light-years. The simulations also require a Milky Way on the more massive end of the spectrum.

“It may well be that Antlia 2 is very massive, the Milky Way is very massive, all other satellites are ruled out, and Chakrabarti is right,” Belokurov says. “But I am not at all convinced.”

Chakrabarti acknowledges the difficulties of dealing with the uncertainties in the data provided by Gaia so far. Those difficulties extend to the question of whether the candidate Cepheid variable stars that she had discovered actually belong to Antlia 2. “It’s quite possible that the Cepheid candidates belong to the Antlia 2 tidal debris,” Chakrabarti says, “but without better data, we can’t be sure.”

Fortunately, better data are forthcoming. The next two data releases from the Gaia mission, expected in 2020 and 2021, will improve on the accuracy of earlier releases. And since Chakrabarti and colleagues have predicted that Antlia 2 passed close by the galactic center, the improved measurements of its stars’ orbits will provide the best test yet of whether this diminutive galaxy really restructured our galaxy’s outer disk.

Comments


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Jim-Baughman

June 21, 2019 at 5:32 pm

Illustrations have to be large enough so that TEXT within them can be READ. The above image (unable to enlarge) titled "Astronomers have long sought to explain ripples in the Milky Way's disk, shown here in an artist's conception." is so small it might as well be on the head of a pin.

Why is S&T so stubborn about the tininess of images in their Latest News pages? I have asked this question before, and been ignored. I simply do not understand this obstinacy. The job of any business that wishes to avoid bankruptcy is to LISTEN TO YOUR CUSTOMERS, not find ways to childishly spite them.

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StanR

June 22, 2019 at 1:17 am

I agree, Jim. (At least, allowing readers to click on the image to pop up a larger version would be good.)

In this case, I found a workaround was to use the "Zoom" feature of my browser (Firefox, though I think they all can do it). At 240% I could read the text in the image, then reverted to 100% to finish reading the article.

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AB

June 22, 2019 at 12:38 pm

Just a formatting error where a link is missing?
"Ctrl +" will zoom your screen. "Ctrl - " (minus) will shrink it. "Ctrl 0" will reset it to default.

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Jim-Baughman

June 23, 2019 at 4:07 pm

AB...

Not an error, this is consistent policy across all articles. And when they DO bother to link, it is to an image just as small as the thumbnail.
If I can do "Ctrl+" etc., think how much easier we would all have it if the code writers for these pages did a little "<a=href..." (code for linking to a LARGE, READABLE image) instead?

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Anthony Barreiro

June 22, 2019 at 3:10 pm

Jim, you should demand a refund of every penny you've ever paid to use the S&T website! Which would be exactly zero cents. The staff of S&T work hard every week to report breaking news, research findings, observing highlights, and how-to's, and then they publish it all for free on the website. Depending on your browser settings and the size of your monitor, images might look small. As Stan and AB (no relation), said, images can be resized in your browser. Please learn how to do that.

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Jim-Baughman

June 23, 2019 at 4:18 pm

Anthony:

You can't "use" the website (i.e. post a comment) without being a subscriber, which I have been for forty years. As busywork for you, I invited you to calculate how much that has cost me.

If a coder can put an image into an article, with only a few more keystrokes he can turn it into a link. Then he can save the pattern as a template for any future article. A page that has unreadable images is exactly the same as a page with no images.

You are aware that S&T filed for bankruptcy in March?

The only way to boost the number of customers to your shop is to do things in their interest, not do what seems easiest for you but aggravates them.

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Jim-Baughman

June 23, 2019 at 4:27 pm

"invited" SB "invite"

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Anthony Barreiro

June 23, 2019 at 6:26 pm

You don't need to subscribe to Sky and Telescope magazine to log into the website and post comments. You could have had access to the website whether or not you subscribed to the magazine. You haven't paid anything to use the website.

Sky and Telescope did not declare bankruptcy. F+W Media, which owns S&T, filed for bankruptcy. The American Astronomical Society has offered to buy S&T. I think that will be a good thing.

I'm sorry you're unhappy with the size of images in website articles.

I won't post further comments in this thread.

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Jim-Baughman

June 27, 2019 at 11:51 am

When I am not logged in and try to post a comment, a message pops up requiring me to log in.

Your browser is set so that it automatically logs you in when you open the S&T website. Therefore you can always post, creating the illusion that posting is something available to anyone who wanders by. If you log OUT and then try to post, see what happens. I'll quote you, since it seems apposite (if as snarky as it was above) here... "Please learn how to do that."

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Monica Young

July 8, 2019 at 4:32 pm

Dear Jim: Registration for the website is free and does not require that you be a subscriber to Sky & Telescope magazine. Logging in is only required to post a comment in order to prevent spam — our online content is of course free!

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fif52

June 26, 2019 at 5:24 pm

It is a shame that these comments are put in this article as it has plenty of potential. A few of these images are simulations and should be larger and others could be larger.

As to the topic, milky way's mass is known, as are others. According to recent articles here and elsewhere there are distinct banding caused by interactions from other celestial objects. These objects have also interacted with other galaxies. These along with the Milky Way are moving in an arc around a yet undefined point of a super cluster. The others interact amongst themselves, so it is possible Milky Way has also interacted in a similar way.

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Monica Young

July 8, 2019 at 4:30 pm

The simulation certainly could be bigger, but we show it here at the resolution that was provided in the institution's press release: https://www.rit.edu/news/new-evidence-shows-crash-antlia-2-gave-milky-way-ripples-its-outer-disc

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