8 mins read 30 Sep 2020

# Finding the Youngest Galaxies

Astronomers are continually looking for ways to observe the very earliest galaxies to have existed at a time when the Universe was much younger, in search for answers on how they came to be. Prof. Geraint Lewis looks at some techniques employed in hunting the youngest galaxies and how Australia will play a role in this field of science.

The Universe was born about 13.8 billion years ago, and after its fiery start, it was a featureless soup of the simplest chemical elements. In the darkness, gravity was drawing matter together, with the first stars bursting into life when the universe was a couple of hundred million years old. Matter continued to pool into growing clumps, with the first proto-galaxies appearing in another few hundred million years.

The seeds of our own Milky Way were sewn at this time, but this baby galaxy was nothing like the grand spiral we see today. With only a few percent of its current mass, the Milky Way had a lot of growing to do, accreting the matter of other proto-galaxies that got too close.

## The Earliest Galaxies

Astronomers are keen to study newborn galaxies in the early universe, peering back over billions of years with their most powerful telescope. The observations are difficult, as the light from the first galaxies has been highly redshifted due to the expansion of the universe. Additionally, the presence of hydrogen gas throughout the universe eats away ultraviolet and optical light, leaving astronomers to search in the infrared.

The faintness of the youngest galaxies makes their discovery even more challenging, with astronomers investing huge resources in looking for them; the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is using more than 900 orbits of the Hubble Space Telescope in a search for baby galaxies.

## Science Check: Redshifts

Redshift is a term used by astronomers to describe how much light that has reached our detectors has been shifted, towards the red end of the spectrum. Scientists are able to determine this value by carefully measuring the wavelength of light being received.

The term itself can be misleading - the light does not have to be the colour red. More so, it has to be shifted in the direction from shorter wavelengths (higher frequencies) to longer wavelengths (of lower frequency). For example, UV light (who’s wavelength range is between 100 and 350 nm) can be ‘redshifted’ to visible light (whose wavelength range is about 350 - 700 nm).

To do this, scientists use instruments known as spectrographs to take detailed observations of the elementary signatures of light that has been split into a range of wavelengths. Whilst on Earth (and in a frame of ‘rest’) and over the years, scientists have established that each element has its own wavelength that is unique to itself.

For example, the rest wavelength of hydrogen alpha (on Earth) is 656.28 nm. But when looking in the far distant Universe, astronomers note that they measure this same element (H-alpha) to exhibit a hypothetical wavelength of 900 nm. A quick calculation yields that the H-alpha light has shifted by 243.72 nm towards a longer wavelength (lower frequency) from the time it left the source to the time it arrived in our detectors. It now falls in the infrared range (700 nm -1 mm wavelengths). So to best detect this light, we would need to use infrared instruments.

To calculate redshift, we can now take the difference of the two values, and divide it by the original wavelength:

$Redshift (z) = (Observed Wavelength - Original Wavelength) / Original Wavelength$

Plugging in these numbers, we find that in this hypothetical scenario, the z value is equal to 0.37. We can then use this number to determine how fast the object is moving away from us, and at what distance it is at. All of these values help us paint a picture of the Universe around us across not only space, but also time (the past).

It’s important to note that the light itself hasn’t changed over its course of travel across the Universe - instead, the universe has expanded and stretched the photons of light into longer wavelengths.

All of this of course presents a challenge to astronomers, in having to think of ways to best detect these distant, ancient, faint signals from space using a variety of instrumentation (on and off Earth), techniques and theories.

## Gravitational Lensing

Nature, however, can offer a helping hand. Between us and the early universe sit immense clusters of galaxies, some of the most massive objects known. This mass, dominated by immense quantities of dark matter, distorts light rays as they travel from a distant source to our telescope, with this gravitational lensing able to produce multiple images of the same background galaxy. And just like a glass lens, these gravitational lenses can magnify a distant source, making them appear larger and brighter.

In searching for baby galaxies, astronomers scour the high magnification regions of clusters, searching for spots of infrared emission that would be invisible without the presence of the gravitational lens. The results have been spectacular, with four of the five most distant galaxies boosted by lensing magnification.

## The Hunt for Cosmic Dawn

The search for galaxies at the “Cosmic Dawn” has only really begun, with a growing array of telescopes focused on the universe’s earliest epochs. In the second half of this decade, the Square Kilometre Array (SKA) - an interferometer of tens of thousands of radio frequency antennas will scan the skies from Western Australia, searching for the hydrogen gas in the early universe that condensed into the first stars and galaxies.

Hydrogen is the most abundant element found across the Universe, including stars, galaxies, the human body, water and more. It was created as a result of the Big Bang, along with Helium and small traces of Lithium. Hydrogen then went on to clump together to form stars, and in the process - heavier elements (like carbon, oxygen, silicon and so on) were created by nuclear fusion.

Hydrogen itself gives off its own radio waves at its own particular frequency (1.42 GHz, which translates to a wavelength of about 21 cm) and it has been observed in the spiral arms of our galaxy, along with other galaxies. It’s the material that goes into forming stars.

By looking at the distribution of hydrogen across the cosmos and deep time using the SKA, astronomers hope to develop a better understanding of how the first stars and galaxies came to be in the early years of the Universe’s history, and then how they evolved into the galaxies we see today.

Placed well above any distortions produced by Earth’s atmosphere, the JWST will be able to probe further into the Universe, searching for distant, redshifted galaxies from a time when the Universe was still young.

One of the key goals of JSWT will be to stare into the high magnification regions of massive clusters and use these “natural telescopes” to reveal the youngest galaxies. With this, astronomers will gain the clearest picture of just where galaxies like our own Milky Way came from.

Through the assessment of data that is produced in future observatories like the SKA and JWST, scientists hope to answer some of our most fundamental astrophysical questions, which will no doubt raise further questions to be investigated.

Galaxy formation video credit: James Webb Space Telescope / YouTube.

## Prof. Geraint F. Lewis

Born and raised in South Wales, Geraint F. Lewis is a professor of astrophysics at the Sydney Institute for Astronomy at the University of Sydney. After wanting to be a vet, and to look after dinosaur bones in a museum, he stumbled into a career in astronomy where his research focuses on cosmology, gravitational lensing, and galactic cannibalism, all with the goal of unravelling the dark-side of the universe, the matter and the energy that dominate the cosmos. He has published almost 400 papers in international journals, and, with Luke Barnes, he is the author of two books, “A Fortunate Universe: Life in a finely tuned cosmos” and “The Cosmic Revolutionary’s Handbook: or How to beat the Big Bang”. He is a Pieces and his favourite fundamental particle is the neutrino.