What Lyman-alpha tells astronomers

The hydrogen fingerprint

Neutral hydrogen — one proton, one electron — interacts with light very strongly at one far-ultraviolet wavelength, 1216 ångström (121.6 nanometres). That is the exact energy of the jump between the electron’s lowest orbit and the next one up. Feed the atom a photon of that energy and the electron climbs; let it fall back down and a photon of that energy comes out. That one transition, taken in either direction, is the Lyman-alpha line — and the two directions are the whole story below: emission when the light comes out, absorption when it is taken away.

Lyman-alpha emission

Wherever hydrogen is ionized — its electron torn away, as happens around hot young stars or the fierce core of an active galaxy — and then recombines, the returning electron cascades back down its orbits and frequently passes through the Lyman-alpha step, releasing a 1216 ångström photon. Enough such gas glows visibly at that wavelength. In a distant galaxy busily forming stars, Lyman-alpha can be the single brightest line in the spectrum, which makes it a workhorse for finding and confirming galaxies in the early universe: catch that one bright line, measure how far the expanding universe has stretched it, and you have the galaxy’s redshift — and with it a rough distance and its place in cosmic time.

The emission also traces the gas it comes from. Lyman-alpha photons scatter very easily off neutral hydrogen, bouncing many times before they get out, so they leak away slowly and light up a diffuse halo around the galaxy. The size and shape of that halo, and the exact profile of the line, are sensitive to how much neutral gas surrounds the galaxy and how readily ionizing radiation can escape it — which is exactly the question asked of the galaxies thought to have helped reionize the early universe.

Lyman-alpha forest

Now turn the geometry around. Point a telescope at a bright, distant quasar and spread its light into a spectrum. Just short of the Lyman-alpha wavelength the smooth glow breaks into a dense thicket of dark absorption lines — dozens or hundreds of them, packed together. That thicket is the Lyman-alpha forest, and every line in it is the shadow of a cloud of hydrogen the quasar’s light crossed on its way to us.

A single cloud would leave a single line. But the light from a distant quasar crosses many separate clumps and filaments of gas, each at a different distance — and so at a different redshift. Each one absorbs at 1216 ångström in its own frame, yet by the time that shadow reaches us the expanding universe has stretched it to a longer wavelength, by an amount that depends on how far away the cloud sits. The result is not one line but a whole comb of them spread across the spectrum — a forest, each tree marking a cloud at its own distance.

What the forest traces

The gas making these lines is the intergalactic medium: the thin hydrogen that fills the space between galaxies, drawn along the same cosmic web of filaments and sheets that galaxies trace. Where the web is denser, the absorption is deeper; where it is sparse, more light comes through. So the forest is a map of ordinary matter in the early universe, read not from what glows but from what is silhouetted against a background lamp. It is richest at high redshift — roughly redshift 2 and beyond — where the stretched Lyman-alpha line falls into the visible band and the intervening gas is thick enough to leave a dense forest.

A ruler at the edge of the map

This is why surveys like DESI care about it. Galaxy surveys eventually run out of galaxies bright enough to measure at the greatest distances, but bright quasars are visible much further, and their Lyman-alpha forests carry the same baryon-acoustic ruler imprinted on all matter. By correlating the absorption along many quasar sightlines — and cross-correlating it with the quasars themselves — DESI recovers the acoustic scale at redshifts above 2, extending its map of cosmic expansion into an era that galaxies alone cannot reach.

A probe of the ionizing universe

The forest is also a sensitive gauge of how ionized the universe is. Most intergalactic hydrogen is not neutral at all: it is kept ionized by ultraviolet light from galaxies and quasars, and only the small fraction that stays neutral makes Lyman-alpha absorption. So the depth and texture of the forest depend on the strength of that ionizing background — which in turn depends on how readily ionizing photons escape the galaxies that make them. This closes the loop with Lyman-alpha emission above: the escaping ionizing radiation that astronomers try to catch around individual galaxies is what powers that background — so the haloes of single galaxies and the forest between them become two ways of reading the same physics. Study which galaxies leak ionizing light and you are studying what shapes the forest; read the forest carefully and you constrain the ionizing budget of the whole cosmos.

In one sentence

The Lyman-alpha line — hydrogen’s 1216 ångström transition — reaches us two ways: as emission, lighting up distant galaxies and the gas haloes around them, and as the absorption forest that a quasar’s light collects on its way to us; the first helps astronomers find galaxies and gauge how their radiation escapes, the second maps the gas between galaxies and measures cosmic expansion out to where galaxies run out.