"Young galaxies are rich in neutral hydrogen which is transparent to normal optical light. But far-ultraviolet, ionizing light, emitted by extremely hot and massive stars is efficiently absorbed by hydrogen and rarely finds its way out of the galaxy. In this galaxy, we are looking straight down into a narrow hole in the galaxy's interstellar hydrogen, allowing us a direct look at the galaxy's central stars even in the ionizing wavelengths. This direct view of the stars through holes in cloud coverage has inspired the nickname for the galaxy, 'the Sunburst arc'," says lead author Thøger Emil Rivera-Thorsen.
A natural telescope
Finding a hole in the such a galaxy is rare, but the researchers' luck did not end there. The light of the Sunburst galaxy, which has traveled 11 billion light years, has been bent and focused by the tremendous mass of a foreground galaxy cluster acting as a natural lens, magnifying it a hundred times. This has made it possible to study the galaxy in a level of detail which is not usually possible for an object that is so far away.
However, a galaxy cluster is not a perfect lens. Instead of seeing just one magnified image of the Sunburst galaxy, the team found no less than 12 different, distorted images of it, similar to what one might see through irregularly shaped glass. Some of these images are complete, some are only partial. In yet another stroke of luck, the researchers found that the bright, ionizing region is seen in all the images, including the incomplete ones.
A key to the early Universe
The Sunburst arc with its thick but perforated gas cover is nothing like a typical galaxy in our local Universe -- but perhaps it is more typical of galaxies at earlier times. When our Universe was less than a billion years old, vast amounts of ionizing radiation poured out of the first infant galaxies and ionized the intergalactic gas over a few hundred million years. Scientists are still trying to figure out how so much radiation could escape through the thick gas cover of these nascent galaxies. If systems like the Sunburst galaxy are more common at very early times, this could help balance the ledger.
The gas-deprived channel to the central regions of the galaxy also offers a unique window to study the ionizing properties of its massive stars, something which cannot be done in local galaxies, not even inside the Milky Way. Like the first galaxies, our own Galaxy contains neutral hydrogen which absorbs ionizing light. The only way to observe this energetic light directly is by observing galaxies that are so far away that the cosmic expansion of space stretches the light to longer wavelengths that are not absorbed by the Milky Way's gas. But, such distant galaxies are generally far too faint to be observed in detail. The Sunburst Arc is sufficiently far away for its ionizing photons to reach us and the gravitational lensing effect magnifies it enough to offer a data quality usually only available from the local Universe.
A detailed study of the Sunburst Arc is now published in the journal Science.
