Seeing Shadows of Ancient Galaxies

A new technique lets astronomers measure nearly invisible clouds of hydrogen gas from across the universe.

An artist's impression of a damped-lyman alpha system

An artist’s impression depicts how astronomers can now measure the size of gas clouds with a background galaxy (wider red cone) as opposed to a background quasar (narrow blue-white beam).
Adrian Malec (Swinburne University) / Marie Martig (Max Planck Institute for Astronomy, Heidelberg)

Most of the universe is hidden from view. The world’s best telescopes can’t pick up rocky asteroids that orbit the far reaches of the solar system, exoplanets that circle distant stars, or galaxies that formed in the early universe. Instead, science is often done in silhouette. Astronomers spot such evasive objects only by the shadow they cast when they pass, by chance, in front of a more distant light source.

A new study improves upon that method, bringing to light so-called damped lyman-alpha systems, giant clouds made mostly of hydrogen gas in the early universe. Jeff Cooke (Swinburne Institute of Technology, Australia) and John O’Meara (Saint Michael’s College) announced at the American Astronomical Society meeting in Kissimmee, Florida, that they’ve found one of these clouds that spans three times the width of the Milky Way.

Because neutral hydrogen emits no radiation, damped lyman-alpha systems are notoriously difficult to detect directly. Instead, astronomers use a simple trick: they catch the clouds’ shadows. The flood of light from distant quasars — a class of intensely bright galactic cores with supermassive black holes that are gobbling down gas and dust — casts a spotlight on otherwise invisible gas. Any intervening hydrogen atoms absorb a specific wavelength of the quasar’s light, leaving a dark absorption line in the spectrum that reaches Earth.

The shape and depth of that absorption line reveals some information about the intervening cloud. Astronomers now know, for example, that a similar total amount of hydrogen gas exists in these early clouds as in the interstellar medium of most galaxies today. As such, they’re a good proxy for the star-forming regions within galaxies in the early universe. Detailed measurements of these clouds could therefore shed light on how galaxies form and evolve.

How Big Are They?

To date, astronomers have used absorption lines within quasars’ light to study thousands of damped lyman-alpha systems. But there’s one big disadvantage. Because quasars themselves are very small objects, only a few light-years across, astronomers can only probe a tiny portion of these systems. Cooke compared this problem to illuminating a massive college campus with just the tip of a laser pointer. You’ll likely only spot a blade of grass or a sliver of a cement rooftop.

“For 40 years we've been using this technique to understand gas in early galaxies, but we've been missing out on two really important, fundamental pieces of information,” says O’Meara: the clouds’ sizes and masses. “That's kind of embarrassing.” As such, astronomers are unaware if these systems are just small clouds within a galaxy or massive clouds within the intergalactic medium that are ready to form a galaxy.

So Cooke (pronounced like the treat) and O’Meara decided to look for background galaxies instead. A larger background light source will illuminate far more — if not all — of an intervening cloud and reveal unprecedented detail. Distant galaxies are usually too faint to use as spotlights, but the technique becomes feasible with larger telescopes coming online in the near future.

Cartoon - Damped Lyman-alpha Systems

This simple cartoon shows what astronomers see when they probe damped lyman-alpha systems using quasars (panels a and c) and galaxies (panels b and d). The cloud's size can be found only when using background galaxies.
Jeff Cooke

Shadow of an Ancient Galaxy

As a proof of concept Cooke and O’Meara looked for damped lyman-alpha systems in the signatures of 54 bright galaxies using the Keck Observatory’s twin 10-meter telescopes and the Very Large Telescope’s four 8.2-meter dishes. One galaxy, known as VVDS 910298177 (located at a time when the universe was only 3 billion years old), clearly showed a damped lyman-alpha system. The absorption line drops all the way to zero all the way across the background galaxy, which means that the gigantic cloud in front of it must cover the whole thing. That makes the cloud at least three times the size of the Milky Way — much larger than astronomers could have guessed given a background quasar.

“So what we've done is increased our ability of understanding the size of things by about a factor of a hundred million, which is really kind of cool,” says O’Meara.

The finding might help solve the puzzle of where these early clouds reside. "The damped lyman-alpha systems we've found appear to be essentially 'stand alone' clouds in the intergalactic medium," says Cooke. They are so large that they "will likely form full mature galaxies like the present-day spiral galaxies like the Milky Way."

Dawn Erb (University of Wisconsin Milwaukee), who was not involved in the study, thinks the method will be a useful complement to the work she does. Erb studies galaxies at similar cosmological distances, but she does so by looking at their emitted light, rather than their absorbed light. By looking at these galaxies in both emission and absorption, she says, astronomers can gain a far better understanding of their overall morphologies.

The new technique holds a lot of promise in the era of 30-meter telescopes. “These monster-machines that are getting built are going to be able to use this technique by the thousands,” says O’Meara. And they’ll do so much more in 18 hours than Cooke and O’Meara were able to accomplish on the Very Large Telescope. There are far more galaxies than quasars in the first place, so soon enough astronomers will be able to “map out a three-dimensional tomography of all of the gas in the universe,” says Cooke.


Jeff Cooke and John O’Meara “A New Constraint on the Physical Nature of Damped Lyman Alpha Systems.” Astrophysical Journal Letters, 2015 October 15.

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