Small galaxy reveals secrets
AMENTION of the word galaxy typically creates a mental image of a giant spiral-shaped assemblage of stars.
There are certainly a lot of galaxies like that, and our own Milky Way galaxy is one.
However, galaxies have many different shapes and sizes, and recently one of the smallest of all has given astronomers some important information about the early universe.
It is the galaxy known as Tucana II, which was discovered in 2015 and whose stars are among the oldest known. A study led by Anirudh Chiti, from the Massachusetts Institute of Technology, has supported astronomers’ ideas that galaxies are built up from “building blocks’’ of smaller ones.
Tucana II is named that way because it is in the direction of the constellation of Tucana, the Toucan, in our far southern sky, and it is the second dwarf galaxy to be found in that constellation.
Tucana II’s description as a dwarf galaxy means what it says: a structure that is very small, and contains very few stars, compared with galaxies like ours. Even though it was discovered only a few years ago, it is one of the closest galaxies to the Milky Way galaxy.
Our galaxy, the Andromeda Galaxy (also called M31) and the Triangulum Galaxy (M33) dominate our “local group” of galaxies, but there are dozens of dwarf galaxies in the group, too.
Tucana II is part of our group, and is only 163,000 light years away. (The Andromeda Galaxy is about 2.5 million light years away.) Tucana II is at a distance comparable to that of the galaxies called the Magellanic Clouds, which are clearly visible with the unaided eye from Tasmania when light pollution is not a problem.
The Magellanic Clouds are dwarf galaxies, too, but Tucana II is so much less bright that it was not spotted until the feeble light of its stars was noticed in images made during the Dark Energy Survey project. This used a 4m-diametre telescope in Chile equipped with a specially designed 570-megapixel camera. (Comparing this with a modern digital camera with “only’’ a few tens of megapixels, it’s easy to see how significant this camera is!)
The interest in the ages of the stars in Tucana II is not just because these ages are very great, but also because the stars in the outlying regions of the galaxy are even older than the ones closer to its centre. It seems, therefore, that there are two distinct populations of stars in the galaxy.
Learning of these differences in stars that are already billions of years old involves great science. A major way in which relative ages of stars can be determined is by analysing their contents, which is done by examining the spectra of their light: breaking up the light into its constituent colours in far more detail than we see the colours of a rainbow.
Elements more massive than hydrogen and helium were almost non-existent when the universe was young, because they, called metals for astronomical purposes, are formed inside stars that later spread the metals around.
In a sense, stars “pollute’’ the universe with heavier elements. However, I’m not complaining. We, and the things we see around us, are made of material from the stars — including the computer I am using to write this article.
Spectra of the stars in Tucana II reveal a very low proportion of metals, consistent with conditions in the early universe. Not only that, its outlying stars have significantly less metal content than the central ones, so the central stars are very old, but the outer ones are even older.
The existence of these two groups, and the extreme ages in general, are quite suggestive that Tucana II, small though it is, formed from a merger of two even smaller assemblages in the very young universe.
Tucana II has also been found to contain a very high proportion of dark matter. In fact, it’s made mostly of dark matter, whose nature is still a mystery. Hopefully, further studies of galaxies like this one, and other intensive work that is proceeding around the world, will help us to understand dark matter, too.