Brightest early galaxy not so early after all

A distant galaxy thought to have been the brightest ever found in the early universe has been revealed as a fraud, researchers reveal.

The discovery implies that young ultra-bright galaxies may be less common than previously believed, challenging theories describing the development of the universe in the first billion years following the Big Bang.

The research is part of an ongoing project called the Brightest of Reionising Galaxies (BoRG) survey, led by the University of Melbourne’s Michele Trenti, that seeks to identify bright early galaxies.

To do this, the project uses a secondary camera on the Hubble Space Telescope, known as Wide Field Camera 3. The camera is set to observe a random patch of sky for several hours at a time. The exercise has so far been repeated more than 100 times, building up a detailed dataset and maximising the chances of seeing one of the rare ultra-bright clusters.{%recommended 1532%}

In 2016, the team identified two galaxies – dubbed BoRG 0116+1425 747 and BoRG 0116+1425 630 – as dating from about 13 billion years ago, when the universe was about 5% its current age.

One of the key determinants for making the identification was the colour of both galaxies, which is governed by a phenomenon known as redshift. Essentially, this describes the change in colour observed from a given point as an astronomical object moves away. The further away something is, the redder its observed light becomes.

The redshift observed for BoRG 0116+1425 747 and BoRG 0116+1425 630 indicated that both were a very long way away indeed, and therefore the images captured were of light that had been emitted by them a very long time ago – hence the tentative dating of 13 billion years. Indeed, BoRG 0116+1425 630 was estimated to be the oldest bright young galaxy ever detected.

To test the findings, Trenti, astrophysicist Rachael Livermore and colleagues from the BoRG team again utilised Hubble to revisit the galaxies and take more detailed measurements of their colours.

The results were a surprise. BoRG 0116+1425 747 was confirmed as dating from 13 billion years ago. BoRG 0116+1425 630, in contrast, turned out to be much more recent. 

“Looking really far away basically allows us to take baby pictures of galaxies, so we can see how they started and then figure out how they grew into the types of galaxies we see today,” Livermore says.

“Now that we have a better measurement of the colours, it now looks as though the brightest galaxy is actually relatively nearby – we see it only nine billion years back in time, whereas it was previously thought to be 13 billion.”

In a paper published in the Astrophysical Journal Letters and previously lodged on the preprint server ArXiv, the scientists describe the second galaxy as an “interloper”.

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