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Hidden in plain sight behind the disc of dust and gas that makes up the Milky Way there is a ghostly galaxy that looks like no other we鈥檝e seen before. Or, at least, that鈥檚 what one group of astronomers claims 鈥� but another researcher has questioned their discovery.

The newly detected dwarf galaxy is a third the size of the Milky Way. It鈥檚 located in the Antlia constellation. It is known as Antlia 2 鈥� or Ant 2.

鈥淚t has a brightness similar to some of the galaxies around the Milky Way that we have known for years now. But being ten times larger than those, its brightness distributes over a larger area in the sky, making it very diffuse and hence more difficult to find,鈥� says Gabriel Torrealba at Academia Sinica in Taiwan.

He and his team hunted down Ant 2 using bright stars called RR Lyrae variables, which pulse at regular intervals, brightening and dimming over the course of half a day. We know that almost every smaller galaxy orbiting the Milky Way has at least one of these beacons that moves in concert with the stars around it.

Enormous ghost

Torrealba and his collaborators used four RR Lyrae stars observed by the Gaia satellite, which tracks the position and movement of nearby stars to make a map of our galaxy and its environs. The team found that this group of pulsing stars was moving together.

They then measured the light coming from 100 red giant stars around the four RR Lyrae stars and found that they were also moving at the same rate, which means they鈥檙e part of the same galaxy. But these stars also revealed that the galaxy they live in is odd.

Based on its distance, the team calculated that Ant 2 is enormous, spanning about 9500 light years. But it鈥檚 extremely dim for something that large. That鈥檚 because it鈥檚 100 times more diffuse than even so-called ultra-diffuse galaxies. This means Ant 2 has the lowest known surface brightness of any stellar system we鈥檝e seen.

Torrealba suggests that might be down to the galaxy鈥檚 dark matter. It may be spread evenly throughout Ant 2, instead of clumped together at the centre as it is in most galaxies. That might make it possible for Ant 2 to maintain its vast size while losing mass due to the Milky Way鈥檚 gravity pulling at it.

Calculation errors

But before we can really know for sure how unusual Ant 2鈥檚 properties are, there are questions to answer, according to Gisella Clementini at the National Astrophysical Institute in Italy. She studies RR Lyrae stars observed by the Gaia satellite, and says there is an error in the formula Torrealba鈥檚 team used to calculate the distance to Ant 2.

When she noticed this, she asked her collaborators in Bologna and Naples to check it as well, and they all agreed that the distance measurement is in question. That鈥檚 a significant problem because the galaxy鈥檚 extremely unusual traits are only that unusual if the galaxy is as far from us as Torrealba鈥檚 team calculated.

Clementini says she has alerted the team to the problem and that they agreed they would need to redo the analysis. 鈥淭hey have discovered something, but at this stage, I could not bet on any property of the system unless we can confirm the calculations,鈥� she says.

The RR Lyrae stars may not belong to the galaxy, says Clementini, but could be sitting in front of it. And if that鈥檚 the case, it was a lucky strike that the team found Ant 2 at all. If the galaxy is much farther away than first thought, it would be even more extreme in its dimensions. But if it鈥檚 far closer, it would look more like a usual dwarf galaxy.

Torrealba says the error in their distance formula puts the RR Lyrae stars at about 260,000 light years away, instead of the original measurement of 424,000 light years. But they confirmed the distance of the rest of the stars in Ant 2 with two other methods. He says a galaxy like Ant 2 should hold hundreds of these variable stars, and this group is just the closest of them. He adds that the new calculation doesn鈥檛 change the bizarre nature of Ant 2.

鈥淚t would need to be twice as close for the unusual aspects of this galaxy to look less unusual. Changing it by 10 or 20 per cent, it would still fall into this class of very bizarre objects,鈥� says Alan McConnachie at the University of Victoria in Canada.

He says we鈥檝e long known that there are 鈥渕issing鈥� galaxies hiding behind the gas and dust of our own Milky Way. There are a few other galaxies that are similarly vast and diffuse 鈥� Crater 2 and Andromeda 19. 鈥淲hether or not they鈥檙e just a few extreme objects, or they鈥檙e the tip of the iceberg to a new class of galaxies, is something we desperately need to know,鈥� he says.

If galaxies are more diverse than we once thought, and the Milky Way is surrounded by a sea of puffy dwarf galaxies, we may need to rewrite our understanding of how dark matter is spread throughout the universe and the mechanisms by which it collects stars.

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