Light warped by galaxy cluster reveals distant dwarf galaxies

Distant dwarf galaxies from when the universe was producing stars at its fastest rate have been spotted, thanks to a trick of the light predicted by Einstein.

Anahita Alavi from the University of California, Riverside and colleagues from the US, South Africa and Europe used the Hubble Space Telescope to detect these small, faint objects as their light bent and brightened around massive clusters of galaxies.

The work, published in The Astrophysical Journal, provides a snapshot of the universe when it was only a few billion years old.

Tracing the universe’s evolution through its youth is, unsurprisingly, tricky.

It’s thought the early universe was dark and opaque. But a few hundred million to a billion years after the Big Bang, it entered what’s known as the reionisation period, where neutral hydrogen was transformed to ionised plasma.

So it emerged from the “dark ages” to a bright and transparent universe – as it is today – and astronomers think dwarf galaxies played a significant role at this point.

They tend to glow brightly in ultraviolet wavelengths, a sign of recent star formation. But their size and relative faintness mean they’re hard to see, even with telescope technology today.

There is a way around it – a gravitational lens.

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A gravitational lens is formed by an object so massive that it creates a dent spacetime – a prediction of Einstein’s general theory of relativity. 

If a gravitational lens is in the direct line of sight to another distant object – such as a dwarf galaxy – it can magnify the light from the background source.

This is what Alavi and her colleagues did. Using the Wide Field Camera 3 on the Hubble Space Telescope, they took deep images of three clusters of galaxies.

Behind those clusters, the astronomers saw the smeared light of a large population of dwarf galaxies from when the universe was between two and six billion years old – an age they checked with the W M Keck Observatory.

There are many more dwarf galaxies than their brighter counterparts, suggesting they were the most numerous galaxies during that period.

And though faint, dwarf galaxies from that epoch produce around 60% of the ultraviolet light at the time, meaning they host a significant fraction of newly forming stars.