Measuring the radial velocities of neutral hydrogen in about 130 galaxies, ranging from small irregular dwarf galaxies to huge spiral galaxies, a team of astronomers reports that all galaxies, regardless of size, take about a billion years to rotate.
A complete rotation in about a billion years; a well-oiled celestial mechanics, therefore, despite its chaotic appearance. This means that the material on the outer edge of a galaxy takes about a billion years to complete an orbit around the center of the galaxy. In the case of the Milky Way, this center is Sagittarius A *, a supermassive black hole about 4 million times larger than the Sun. “It’s not the precision of a Swiss watch,” says lead researcher Gerhardt Meurer from the University of Western Australia. “But it does not matter if a galaxy is very big or very small, if you could sit on the extreme edge of its disk while it’s spinning, it would take you about a billion years to go around.”
Interestingly, as all the stars in our Galaxy circle around the center at about the same speed, objects closer to the center of the Galaxy do not take as long to orbit around its center. That’s why a cosmic year – the time it takes for our solar system to complete an orbit – is only 225 to 250 million years. This is where the theory of dark matter comes from: there is simply not enough observable mass in the Milky Way to explain this effect, unless we have a misguided understanding of gravity or maybe there is an invisible mass at play, which scientists refer to as dark matter.
This new discovery means that in smaller galaxies, the material must travel more slowly because it has less distance to travel in the same amount of time. The team also notes that their study may be subject to some revisions. Indeed, of all the galaxies of the Universe, perhaps they would have simply studied those which have the same characteristics. Further studies will be needed to determine whether these results are universal.