HAVE WE FOUND GALAXY X?
Are astronomers any closer to pinpointing the mysterious object packed with dark matter?
We like to think of our galaxy as a lone wanderer through the cosmos, solitarily spinning in the intergalactic gloom. But we are not alone in the universe – we have company along for the ride. Dwarf galaxies surround our own galactic swirl. Staying within 1.4 million light years of our galaxy’s rim, dozens of these petite galaxies have already been discovered. But astronomers believe there is at least one more out there, a cluster of unknown matter hidden at the edge of the Milky Way: Galaxy X.
Theorised for years, Galaxy X is thought to be around one per cent the mass of the Milky Way, 300,000 light years from its centre and consisting almost entirely of dark matter, the mysterious substance said to make up 23 per cent of the universe’s mass. But it’s still just a theory. While astronomers have the mathematic calculations to believe the secretive galaxy exists, they’ve yet to find any concrete evidence. Despite their best efforts, it has stubbornly remained theoretical. But all that might soon change in the weeks to come…
“I started looking for Galaxy X about ten years ago, so I feel quite old talking about this,” quips Sukanya Chakrabarti, an assistant professor of physics at the Rochester Institute of Technology.
After authoring a paper on the potential existence of the galaxy in 2009, Chakrabarti has since become the world’s lead investigator in the hunt for Galaxy X. And as she admits, it’s been a long quest.
Back in 2009, she and fellow astronomer Leo
Blitz analysed variations in the distribution of gas found at the edge of the Milky Way, and they couldn’t easily explain what they found. “It showed very large perturbations on the outskirts of the edges of our galaxy,” says Chakrabarti. “This is well beyond the solar circle, so you wouldn't expect such large perturbations.” Now, at this stage in any other hunt for a celestial object, Chakrabarti might have turned to telescopic data and looked for the disturbance herself. But with Galaxy X, nothing is ever simple.
“I didn't make life easy for myself,” she jokes.
“The calculations preferred a co-planar merger – that is, the dwarf galaxy was orbiting roughly in the same plane as the Milky Way. And that means that if you were trying to look for it, you'd be having to deal with the dust obscuration close to the plane of the galaxy.” Unlike many dwarf galaxies, Galaxy X didn’t seem to be hovering above or below the Milky Way’s plane, which would make it easier to see. Instead, the elusive system appeared to be
“Even after including the gravitational effects of all the known players of the Milky Way, you still couldn't explain the disturbances”
Sukanya Chakrabarti
hidden right at the edge of the
Milky Way’s disc, and so would be completely obscured by the giant expanse of dust and gas between it and Earth.
“So, if you look above the plane, you'll see other dwarf galaxies which are easily visible because they're well off the plane of the galaxy,” says Chakrabarti. “But if you're looking through dust, it's like looking through a smoke screen. So that's why a lot of these surveys for dwarf galaxies have largely avoided the galactic plane. And that's also why most of the discoveries of dwarf galaxies so far have been well above it.”
Undeterred, Chakrabarti opted for a different search tactic. Instead of looking for Galaxy X directly, she studied everything else in its supposed region and scraped information about the dwarf galaxy’s behaviour through what couldn’t be seen. These experiments began with computer simulations of the Milky Way and its known dwarf galaxies, and perturbations at the simulated galaxy’s edge soon showed up. “I included the gravitational effects of the three most massive dwarf galaxies of the Milky Way – these are the Large Magellanic and Small Magellanic Clouds and the Sagittarius Dwarf Galaxy,” she explains. Floating less than 200,000 light years from Earth, these satellite galaxies and their millions of stars can be seen from the Southern Hemisphere and even helped early Polynesian cultures navigate the seas. “I gave them those orbits to see how close a perturbing dwarf galaxy would have to get to produce the observed disturbances in the outer gas disc of the Milky Way. They were the most realistic simulations that I could do,” she adds.
“And I found that even after including the gravitational effects of all the known players of the Milky Way, you still couldn't explain the observed disturbances in the outer gas disc. So I concluded that we needed a new dwarf galaxy that had not yet been identified in 2009. And that's really when I put on my observer’s hat and started to look for it myself.”
Her observer’s hat donned and curiosity piqued, Chakrabarti spent the next few years hunting
“To be convinced that a distant galaxy is real, a sample of stars at a particular distance on the sky must be convincing”
Alice Quillen
through spectroscopic surveys of space, searching for any clue that could lead her to the missing phenomena. And, in 2015, she and her colleagues found just that: four bright, variable stars right where Galaxy X should be. It was just the kind of sign they needed.
“To find a dwarf galaxy, you try to find stars that are in it,” explains Alice Quillen, a professor of physics and astronomy at the University of Rochester. Lending her significant expertise to the Galaxy X quest, Quillen helped write the 2015 paper that further pinpointed the dwarf galaxy on the map. “Dwarf galaxies are very diffuse,” she adds.
“So to be convinced that a distant galaxy is real, a sample of stars at a particular distance on the sky must be convincing.” And fortunately for Quillen and her colleagues, the stars found in 2015 were very convincing, at least for them.
“We found that they were all coherently receding,” says Chakrabarti. “That is, they were all moving away from us with roughly the same velocity, which was comparable to what you expected from the dynamical models. So we believed that these stars could either be part of Galaxy X or part of a tidal stream that was produced by it.”
Perturbations in outer space, a hypothesised cause and calculations to prove its existence; in many ways,
Chakrabarti’s quest for Galaxy X is a mirror story to Urbain Le Verrier’s discovery of Neptune in the 1800s.
Like Chakrabarti, the French astronomer first observed discrepancies with celestial orbits. Uranus’ elliptical path didn’t seem to make sense if there wasn't another hidden planet affecting its trajectory. Sure
“The kind of dwarf galaxies that we've been finding lately are much fainter than we had ever hoped we'd be able to find”
Sukanya Chakrabarti
enough, Le Verrier’s prediction was right, and Neptune was found within one degree of his calculations. It was the first time in history that any hidden object had been discovered just through analysis of its gravitational imprints on another celestial body.
But Galaxy X is different; proof of its existence has yet to be published. And without this hard evidence, the scientific community has been a little less accepting than it was for Le Verrier. Some astronomers have even rebuked the existence of the galaxy altogether. Writing in the Astrophysical Journal Letters in 2015, one group of scientists discredited the stars found by Chakrabarti and Quillen as “spotted objects” and claimed that there was no evidence for a dwarf galaxy at all. Suffice to say, Chakrabarti was less than pleased about the community’s scepticism.
“I can't say that I was thrilled about it,” she jokes. “Scientists are sceptical in general and discoveries are hard to prove, so this was probably no different in that sense.” Healthy scepticism or not, the challenge quickly deepened Chakrabarti’s resolve. Now she and her colleagues had both a galaxy to prove real, and a paper to prove wrong. More evidence was desperately needed and, in 2017, they found it.
“We did refine several things in our 2017 paper,” Chakrabarti explains. “In addition to getting light curves to identify the variable star candidates, we also got spectra [more data from the near-infrared spectrum]. And by looking at the spectra we got a completely independent piece of information, independent to the light curves. And that is really why we did it, because we wanted it to be found in a very unbiased way.”
With this added information Galaxy X was brought back to the realms of possibility, and the quest reinvigorated. The 2015 challenger paper had rocked the hunt’s foundations, but they were now rebuilt stronger than ever. “That's just the nature of science,” Chakrabarti admits. “You are continually trying to refine what you did.” And so the search marches on, ten years after it first began. But is there any hope that Galaxy X’s smoking gun could be found before the pursuit reaches its next decade? Without giving much away, Chakrabarti hints that such a revelation could be just around the corner.
“There might be, on the horizon, some more news about Galaxy X soon,” she confides. “We're
at this stage right now where we’ve gone through a particular candidate and we're in the process of writing the paper, which is about 50 per cent done.” But while a new candidate could lend the quest some more credibility, a photograph would be the trophy that could finish the hunt altogether. Unfortunately, any dark-matter object obscured by half the Milky Way would be a challenge to image. But as the recently released first picture of a black hole demonstrates, even one of the most elusive, light-devouring objects in the universe can now be visualised. Of course, that achievement was no simple feat. The black hole image was constructed from radio waves that travelled 54 million light years to be sourced from eight different observatories across the world. But, time and cost aside, is there anything stopping Galaxy X from receiving the same sort of photo shoot?
After posing the question to Chakrabarti, the astronomer clearly becomes animated with classified knowledge, and stifles any announcements before they slip out. “I'm having difficulty answering the question, because that’s one of the things I've been writing about right now. So what I want to say at the moment is that the chances are very good.
“Seeing it in radio would mean that it has to have gas, but seeing it in the optical is very possible now with the new surveys that are ongoing. And the kind of dwarf galaxies that we've been finding lately are much fainter than we had ever hoped we'd be able to find. I think the chances of getting an optical image are really, really good, so stay tuned on that. There will probably be some news on that very, very shortly – some big news.”
While Chakrabarti remains tight-lipped, it’s clear that the quest for Galaxy X is about to get a lot more interesting. And as the dwarf system is thought to consist almost entirely of dark matter, any information about its nature could have far-reaching revelations about the mysterious substance and its role in the universe. But for Chakrabarti and her colleagues, the most tantalising prospect of Galaxy X is the galaxy itself. Sought after for over a decade, any subsequent facts and figures from its discovery will undoubtedly pale in comparison to the actual find. X may mark the spot for future research and astronomical studies, but after years of searching, just the confirmation of another companion in our endless drift through the universe would be enough to make both journeys worthwhile.