Free Astronomy Magazine January-February 2020

36 JANUARY-FEBRUARY 2020 an analogue of galaxies which existed at a much earlier time in the history of the Universe: a period known as the epoch of reionisation — an era which began only 150 million years after the Big Bang. The epoch of reionisation was a key era in the early Universe, one which ended the “dark ages”, the epoch before the first stars were created when the Universe was dark and filled with neutral hydrogen. Once the first stars formed, they started to radiate light, producing the high-energy photons required to ionise the neutral hydrogen. This converted the intergalactic matter into the mostly ionised form in which it exists today. However, to ionise intergalactic hydrogen, high-energy radia- tion from these early stars would have had to escape their host galaxies without first be- ing absorbed by interstellar matter. So far only a small number of galaxies have been found to “leak” high-en- ergy photons into deep space. How this light escaped from the early galaxies remains a mystery. Hubble captures a dozen sunburst arc doppelgangers T his new image from the NASA/ESA Hubble Space Telescope shows an astronomical object whose image is multiplied by the effect of strong gravi- tational lensing. The galaxy, nicknamed the Sunburst Arc, is almost 11 billion light-years away from Earth and has been lensed into multiple images by a massive clus- ter of galaxies 4.6 billion light-years away. The mass of the galaxy cluster is large enough to bend and magnify the light from the more distant galaxy be- hind it. This process leads not only to a deformation of the light from the object, but also to a multiplication of the image of the lensed galaxy. In the case of the Sunburst Arc the lensing effect led to at least 12 images of the galaxy, distributed over four major arcs. Three of these arcs are visible in the top right of the image, while one counterarc is visible in the lower left — partially obscured by a bright foreground star within the Milky Way. Hubble uses these cosmic magnifying glasses to study objects otherwise too faint and too small for even its extraordinarily sensitive instruments. The Sunburst Arc is no exception, despite being one of the brightest gravitationally lensed galaxies known. The lens makes various images of the Sunburst Arc be- tween 10 and 30 times brighter. This allows Hubble to view structures as small as 520 light-years across — a rare detailed observation for an object that distant. This compares reasonably well with star forming regions in galaxies in the local Universe, allowing astronomers to study the galaxy and its environment in great detail. Hubble’s observations showed that the Sunburst Arc is by NASA/ESA