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Light from the early Universe reveals that barred spiral galaxies, like our own Milky Way, are more common today than they once were
VERY LARGE TELESCOPE, 1 JANUARY 2018
In this image taken by the Very Large Telescope, the galaxy NGC 1398 sits face-on to us, revealing its intricate structure perfectly. Dark lanes of cosmic dust within its spiral arms feed the pink star-forming regions dotted throughout, and at the heart of the galaxy lies a huge bar of stars. NGC 1398 is a perfect example of a barred spiral galaxy, so-called because its spiralling arms originate not from the centre, as with other spiral galaxies, but from this bar-like structure. It’s thought that most spiral galaxies in today’s Universe are barred, but this wasn’t always the case. A 2008 study using the Hubble Space Telescope revealed that seven billion years ago, barred spiral galaxies were less common. Since then the proportion has more than tripled: while just 20 per cent of spiral galaxies had bars in the early Universe, that figure is approaching 70 per cent today.
There is still no definitive answer as to why so many spiral galaxies form this distinctive linear structure. It’s thought that the bar forms as a result of the stars in the galaxy’s inner region developing an elliptical orbit: as the stars deviate from their circular path, their elliptical orbits become more pronounced, leading to the formation of a bar. The drive behind this elliptical orbit is not yet known.
What’s becoming clear, however, is that bars can be important features in driving the evolution of their host galaxies. They force cosmic gas towards the galactic centre, fueling star formation, creating central bulges of stars and feeding supermassive black holes.
Observations of the Milky Way have revealed a central bar-like bulge, leading astronomers to conclude that our own Galaxy is a barred spiral. While we can’t yet view the Milky Way from afar, observing barred spirals such as NGC 1398 is the next best thing and doing so can help us understand the structure and evolution of our place in the Universe.