Surprise! The Small Magellanic Cloud is not one galaxy, but two

2024-01-07 05:46:36

The first to leave a written record of its existence was the Italian Antonio Pigafetta, a born nobleman, geographer, adventurer and member of Magellan’s expedition that circumnavigated the world between 1519 and 1522. In his writings, Pigafetta refers to a ‘ fog cloud’ in the night skies of the southern hemisphere. Today we know that region as the ‘Magellanian Clouds’ and we know that they are two small satellite galaxies of our Milky Way. But is it really like that?

A study led by astronomer Claire Murray, from the Space Telescope Science Institute in Maryland, in fact, seems to indicate that we have been wrong for more than 500 years. And that, in reality, the cloud described by Pigafetta does not correspond to two, but to three galaxies. Quite a surprise for objects that are among the most observed, studied and analyzed in the entire sky.

In a study that will soon be published in ‘The Astrophysical Journal’ but that can already be consulted on the server arXivMurray and his colleagues explain that the Small Magellanic Cloud, just 199,000 light years from Earth, is not one galaxy, but two different ones, and that it seems like one to us only because we see them one after the other.

To reach this conclusion, Murray and his team tracked the movements of gas clouds and younger stars around the Small Magellanic Cloud. And they discovered that the small galaxy, which is about 18,900 light years in diameter (five times smaller than the Milky Way) actually contains two stellar arrays, different and separated by several thousand light years.

Both the Small and Large Magellanic Clouds are dwarf galaxies whose ultimate destiny is to be ‘devoured’ by the Milky Way, which attracts them with its immense gravity. The Large Magellanic Cloud is disk-shaped, like a miniature Milky Way, and has a mass equivalent to 21 billion suns. But the Small Magellanic Cloud, two-thirds less massive, is irregular in shape and, although it has long been suspected of consisting of multiple components, its view is obscured by dense clouds of dust and gas, making it difficult to distinguish its components. details clearly.

A galaxy disaster

In previous studies, Murray had already determined that the Small Magellanic Cloud, full of gas and buffeted by the combined gravity of the Milky Way and the Large Magellanic Cloud, is “a galaxy disaster.” But in this new research, carried out with the 36 antennas of the Australian Square Kilometer Array Pathfinder radio telescope, the astronomer and her colleagues amplified the radio waves emitted by the hydrogen of the dwarf galaxy and then used data from the European Gaia mission, which records data from more than a billion stars, to track the speed and direction of thousands of young stars in the Small Magellanic Cloud, all of them born less than ten million years ago.

Assuming that these young stars are moving alongside the large gas clouds where they formed, the researchers were able to detect two distinct ‘patches’ of gas and dust generating stars. The two clouds have different abundances of ‘metals’ – that is, elements heavier than hydrogen or helium – and one of them appears to be more distant from Earth than the other, although their exact separation is still unclear.

What is not yet known is whether the two objects are independent of each other but have both been attracted by gravity or if, on the contrary, it is a single object ‘split in two’ by gravitational forces. In favor of the first option is the fact that the two clouds appear to have similar masses. If they were two parts of a larger cloud, the researchers reason, there would undoubtedly be one larger than the other. Therefore, and if it is confirmed, as the study suggests, that both clouds are not related, then the Small Magellanic Cloud is two objects, and not just one as we believed until now. In which case there will be no choice but to change the name.