A small galaxy orbiting the Milky Way may not be what we thought: ScienceAlert

A small galaxy orbiting the Milky Way may not be what we thought: ScienceAlert

It is difficult to measure distances in space. Unless you know how bright something is intrinsically, knowing how far away it is gets a bit complicated.

However, knowing the distance can make a big difference in how we interpret the data. It is not surprising that astronomers would have to revise their results based on a new distance measurement for an object.

What’s unusual is when it happens with something humanity has been staring at for thousands of years. Astronomers have just discovered something shocking about one of the most famous objects in Earth’s skies. A new analysis suggests that the Small Magellanic Cloud is not just a small galaxy orbiting the Milky Way, but two galaxies.

How could we make this mistake? The two separate star clusters are overlapping along our line of sight, says a team led by astronomer Claire Murray of the Space Telescope Science Institute. Their data indicate that the stars’ backpoint is located about 16,000 light-years behind the other.

Image of the Small Magellanic Cloud released in 2020. (CTIO/NOIRLab/NSF/AURA/SMASH/D. Nidever, Montana State University)

Results, accepted in Astrophysical Journal And uploaded to the preprint resource arXiv, it makes a compelling case for the dual nature of what we previously interpreted as a single organism.

The Small Magellanic Cloud is one of several dwarf galaxies orbiting (and slowly falling into) the Milky Way. It is about 200,000 light-years away from us, about 7,000 light-years across, and its mass is about 3 billion suns. It is also associated with another galaxy visible near the sky, the Large Magellanic Cloud, which is twice the size of the Small Magellanic Cloud. The two orbit each other as they orbit the Milky Way.

In fact, hints that the Small Magellanic Cloud may not be what it seems have been emerging since the 1980s. The way the star haze moves is strange: the interstellar gas environment does not seem to match other properties of the dwarf galaxy, and there appear to be at least two distinct groups of stars within it.

Previous research thought the Small Magellanic Cloud might be strange because it was gravitationally disrupted by interactions with the Large Magellanic Cloud, but the shape and dynamics of the dwarf galaxy have remained inconclusive.

Murray and her colleagues conducted a comprehensive investigation of the space cloud to try to find out once and for all. They studied data from the Gaia Survey, a project to map the positions and velocities of stars in the Milky Way in 3D with the highest resolution yet. They used data from a galactic survey conducted with the Australian Square Kilometer Array Pathfinder radio telescope to study in detail the composition of the gas filling the Small Magellanic Cloud in interstellar space.

Magellanic clouds in the sky of Australia. (Ed Donens/Flickr, CCB 2.0)

Their study found that the Small Magellanic Cloud consists of two different groups of stars with similar gas masses, separated by a large distance. Each cluster has its own signature interstellar gas, and the way the stars move in each is also distinct.

The team’s measurements indicate that the closest distance between the two groups is about 199,000 light-years away. The other is 215,000 light-years away, which is roughly half the distance between the Sun and the center of the Milky Way. The researchers say this is largely consistent with previous estimates of the Small Magellanic Cloud’s line-of-sight structure, but it is also the most convincing evidence to date.

The reason we couldn’t tell them apart with certainty previously is because one lies directly behind the other along our line of sight, close enough to each other that they almost — but not quite — look like a single group of stars in the night sky.

The Small Magellanic Cloud is a well-known and beloved feature of the southern sky. It has been observed for at least thousands of years by indigenous astronomers in Australia, South America and Africa.

He, along with his big brother, will continue to shine in the sky for eons to come; But its demise is imminent. It is gradually falling into the Milky Way, as many other galaxies have done before. This is an important part of how galaxies grow slowly, over billions of years.

Thanks to the Magellan Tow, we have a front-row seat to this process in action.

The research was accepted in Astrophysical Journalwhich is available on arXiv.

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