Astronomers use gravitational lensing “cosmic telescope”


Physicists estimate that the Big Bang was around 13.8 billion years ago. Not long after the Big Bang, massive clouds of mostly hydrogen gas known as Damped Lyman-α systems (DLAs) slowly began condensing into the first stars. Large enough groups of stars and other matter would coalesce to form galaxies.

DLAs act as galactic nurseries and can be observed today. In research just published by an international team in Nature, the results of a new method for detecting DLAs are presented. The technique involves a novel instrument and a little help from the universe itself in the form of a funky consequence of Einstein’s Theory of General Relativity.

The existing method for observing DLA clouds uses quasars as a stand in for a kind of cosmic “backlight”. Quasars are supermassive black holes that emit gamma radiation. As the radiation spat out by the quasar passes through DLAs before reaching Earth, some of the gamma rays are absorbed in the dense atomic gas that makes up the DLA, leaving a fingerprint in the form of absorption lines.

But this method tells us nothing about the geometry or size of the galactic nurseries, giving us only information about the section that the quasar’s gamma burst passed through.

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“DLAs are a key to understanding how galaxies form in the universe, but they are typically difficult to observe since the clouds are too diffuse and do not emit any light themselves,” says the new paper lead author Rongmon Bordoloi, assistant professor of physics at North Carolina State University, US.

Instead of the decades-old quasar method, the team – which includes contributors from Australia’s Swinburne University of Technology and the Australian National University – have found a way to use gravitational lensing to zoom in on two 11-billion-year-old DLAs and the host galaxies within.

“Gravitationally lensed galaxies refers to galaxies that appear stretched and brightened,” Bordoloi says. “This is because there is a gravitationally massive structure in front of the galaxy that bends the light coming from it as it travels towards us. So we end up looking at an extended version of the object – it’s like using a cosmic telescope that increases magnification and gives us better visualization.

“The advantage to this is twofold. One, the background object is extended across the sky and bright, so it is easy to take spectrum readings on different parts of the object. Two, because lensing extends the object, you can probe very small scales. For example, if the object is one light year across, we can study small bits in very high fidelity. ”

Diagram showing grav lensing
An artist’s rendering shows how a cluster of galaxies acts as a gravitational lens that magnifies and extends the light from a galaxy background. This results in a projected image (marked in the rectangle panel) that is brighter and easier to detect with a telescope. Credit: WM Keck Observatory / Adam Makarenko.

Once the cosmic telescope does its job and stretches the DLA across the sky, the hard part is gaining readings. This is normally a very difficult and time-consuming task. The team got around this by performing integral field spectroscopy with the Keck Cosmic Web Imager – an instrument for the Keck II telescope at the WM Keck Observatory in Kamuela, Hawaii. This kind of spectroscopy allowed the researchers to obtain a spectrum at every single pixel on the 2D map of the sky being observed.

Not only was the team able to figure out how big the two DLAs they found are, they also detected host galaxies in both of them.

“I’ve waited most of my career for this combination: a telescope and instrument powerful enough, and nature giving us a bit of lucky alignments to study not one but two DLAs in a rich new way,” says John O’Meara, chief scientist at the WM Keck Observatory. “It’s great to see science come to fruition.”

Each more than two-thirds the size of the Milky Way – and over three times larger than the average galaxy 13 billion years ago – the DLAs are huge at around 57,000 light years across.

“But to me, the most amazing thing about the DLAs we observed is that they aren’t unique – they seem to have similarities in structure, host galaxies were detected in both, and their masses indicate that they contain enough fuel for the next generation of star formation, ”Bordoloi says.

The scientists believe that their method will allow for more DLA detection and study. “With this new technology at our disposal, we are going to be able to dig deeper into how stars formed in the early universe,” Bordoloi adds.