Earliest spiral galaxy ever observed puzzles astronomers
When astronomers used the Hubble Space Telescope to scout a remote patch of the sky and investigate the early stages of galaxy formation, they stumbled upon something which they did not expect. They realized that the distant spiral galaxy BX422, appearing to us as it was only three billion years after the Big Bang, seems to be uncharacteristically well-formed for its young age. By studying its features, which are in direct contrast with our current knowledge of galaxy formation, scientists hope to shed more light on how spiral galaxies – including our own – are formed.
The stars inside a protogalaxy develop at very different rates. This uneven pace compresses the interstellar gas and creates differences in the gravitational field that force stars, dust and gas into a rotating disc. Eventually, additional differences in gravity can also build the arms of a spiral galaxy.
This process takes billions of years to complete and so, when astronomers look at early galaxies in far-off space, they don't see well-formed spiral arms but rather an assortment of irregular shapes that will eventually evolve into a more familiar form. So, when UCLA researchers Alice Shapley and David Law first laid eyes on galaxy BX442 – a very young galaxy with well-defined spiral arms – their first reaction was to believe their eyes were being tricked.
To establish that they were looking at one rotating spiral galaxy and not, for instance, at two galaxies that happened to line up in the image, the researchers used a near-infrared spectrograph to analyze around 3,600 separate locations within the galaxy. They found that what they saw wasn't an illusion: the spiral arms do indeed belong to this galaxy, which is quite large compared to other galaxies from this time.
False color composite image of BX442 (Image: Joe Bergen/Dunlap Institute for Astronomy & Astrophisics)
Examining the data, the researchers found evidence of a supermassive black hole at the center of the galaxy. While this may have contributed to the galaxy's peculiarity, Law and Shapley believe, based on computer simulations, that the explanation for the galaxy's uniqueness is more likely linked to a companion dwarf galaxy whose gravitational interaction may have caused spirals to form.
Despite the galaxy being some ten billion light years away, understanding its nature will touch us more closely than one might expect. Just last year, a study suggested that the spirals in our very own galaxy may have been caused, at least in part, by an impact with the Sagittarius dwarf galaxy. BX422 is a unique opportunity to learn more about this process, as it represents a one-of-a-kind link between the turbulent galaxies in the early universe and the more stable spiral galaxies that we see around us.
The team will continue their investigation by taking pictures at other wavelengths, attempting to confirm their theories by mapping the mixture of stars and gas throughout the galaxy.
The findings were reported in the July 19 edition of the journal Nature.