The intergalactic medium (IGM) is the diffuse gas that exists in the vast spaces between galaxies, containing the majority of the Universe’s baryonic matter. Because this gas is too tenuous to be seen directly in emission, it is primarily studied through absorption line spectroscopy, using bright, distant quasars (QSOs) as background light sources. As quasar light travels toward Earth, it encounters intervening gas clouds that produce distinct absorption features at various redshifts.

Fig 1: Lyman alpha absorption mechanism.

The following are the primary types of absorption systems identified in the IGM:

The Lyman-$\alpha$ forest is a dense profusion of narrow absorption lines appearing on the blue side of a quasar’s $Ly\alpha$ emission line.
Physical Nature: It is caused by diffuse, highly ionized intergalactic gas that follows the “cosmic web” of dark matter filaments.
Composition: Although the forest is detected via neutral hydrogen, roughly 99.9% of the gas is actually ionized.
Cosmological Significance: These clouds are the most common type of absorption system and represent a significant reservoir of the Universe’s baryons. They also show traces of “missing metals”—pollutants from early star formation found even far from galaxies.

DLAs are the highest column density systems, defined by neutral hydrogen densities of $N(HI) \gtrsim 2 \times 10^{20} \text{ cm}^{-2}$.
Appearance: They are distinguished by broad absorption profiles with extended damping wings caused by the high optical depth of the gas.
Nature: These systems are dense enough to be self-shielding, meaning the gas remains mostly neutral rather than being ionized by background radiation. They are often interpreted as the progenitors of modern galactic disks or rotating gas clouds within galaxy halos.
Enrichment: DLAs show a gradual increase in metal abundance over cosmic time, though they typically remain metal-poor ($\sim 0.1 Z_\odot$).

Fig 2: Spectrum foa quasar at z=1.34 showing various Lyman absorptions.

These systems have column densities of $N(HI) \gtrsim 2 \times 10^{17} \text{ cm}^{-2}$.
Mechanism: At this density, the gas becomes opaque to photons with enough energy to ionize hydrogen (energies $> 13.6$ eV or wavelengths $< 912 \mathring{A}$).
Spectral Signature: This causes a sharp “break” in the quasar’s spectrum, where the flux drops nearly to zero at the Lyman limit of the absorber’s redshift.

These consist of narrow lines from heavy elements, typically found in groups at the same redshift.
Intervening Systems: Found at redshifts $0 < z_{abs} < z_{em}$, these systems are caused by gas in the extended halos of galaxies (stretching up to 100 kpc or more) that happen to lie along the line of sight. Common indicators include the MgII and CIV doublets.
Associated Systems: These systems have redshifts $z_{abs} \approx z_{em}$, suggesting the gas is physically associated with the quasar’s own host galaxy or environment.
OVI Systems: These trace very hot gas ($\sim 10^6$ K) and provide a rare probe into the warm-hot intergalactic medium (WHIM), where a large fraction of today’s baryons are thought to reside.

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Found in about 15% of quasars, these are not caused by the IGM but by material in the active galactic nucleus (AGN) itself.
Spectral Signature: They appear as broad troughs with widths indicating gas moving toward the observer at relativistic speeds (up to $0.1c$).
Origin: This is interpreted as a superwind or outflow of gas being violently ejected from the vicinity of the central supermassive black hole.

In the spectra of the most distant quasars ($z \gtrsim 5.8$), the $Ly\alpha$ forest becomes so dense that the individual lines blend into a continuous region of zero flux. This Gunn-Peterson trough indicates that the light is passing through diffuse neutral hydrogen that has not yet been fully ionized, marking the final stages of the epoch of reionization.

Fig 3: Gunn-Peterson trough.