Astronomy
Using Quasar Absorption Spectra to Probe the Intergalactic Medium
Quick fact
When the light of a bright quasar passes through intergalactic clouds of neutral hydrogen, the atoms absorb specific ultraviolet wavelengths, leaving behind telltale absorption lines in the quasar's spectrum that reveal the gas's presence and properties.
Why this is interesting
What if you could see the invisible gas that fills the space between galaxies? Quasar absorption spectra let astronomers read the hidden story of the universe's structure.
Read the full explanation
Understanding Using Quasar Absorption Spectra to Probe the Intergalactic Medium
Quasars are extremely luminous galactic centers, like cosmic lighthouses, shining across billions of light-years. Their light travels through space, passing through thin clouds of gas—the intergalactic medium. This gas is mostly hydrogen, the simplest element. Each hydrogen atom can absorb a specific wavelength of light, the Lyman-alpha line, which normally sits in the ultraviolet part of the spectrum. As the universe expands, it stretches the wavelength of light traveling through it, so when this absorption happens far away, the line shifts to longer (redder) wavelengths. By spreading the quasar's light into a spectrum, astronomers see a series of absorption lines at different wavelengths, each one marking a gas cloud at a specific distance. The positions and strengths of these lines reveal the number of clouds, their density, and their ionization state along the line of sight.
A deeper explanation
The underlying principle is that atoms absorb light at discrete, fixed energies, and the universe's expansion shifts those energies via the Doppler effect. Cosmological redshift, z, measures the wavelength shift: the observed wavelength is (1+z) times the rest wavelength. Therefore, each absorption feature in a quasar spectrum corresponds to a specific redshift, indicating that the absorber lies at a certain distance and epoch. The strength (optical depth) of the Lyman-alpha absorption depends on the column density of neutral hydrogen along the line of sight, allowing astronomers to estimate the gas density. Because the gas is distributed in a web of filaments and clumps, the spectrum shows a dense 'forest' of lines due to numerous intervening clouds. This 'Lyman-alpha forest' is a powerful probe of the early universe, revealing the distribution of matter and the onset of the reionization era when the first stars and galaxies ionized the cosmic gas. By studying these absorption features, astronomers map the cosmic web, measure the intergalactic medium's temperature and density, and trace the evolution of structure over cosmic time.