Lyman-alpha Blobs: Gaseous Traces of the First Galaxies

Introduction to Lyman-alpha blobs

A phenomenon inherited from cosmic reionization

Lyman-alpha blobs are extended regions of neutral hydrogen gas ionized by the intense ultraviolet radiation emitted by the first galaxies formed in the young Universe. This phenomenon occurs during the era of cosmic reionization, a key phase between about 400 million and 1 billion years after the Big Bang, when the intergalactic gas passes from a neutral state to an ionized state. The observation and modeling of these blobs make it possible to study the nature of the first sources of light, the structuring of the primordial Universe, as well as the physical mechanisms related to the diffusion and absorption of Lyman-alpha radiation.

Physical properties of Lyman-alpha radiation

A resonant transition of neutral hydrogen

Lyman-alpha radiation corresponds to the electronic transition of a hydrogen atom when the electron falls from level 2 to the fundamental level 1, with a vacuum wavelength of \(\lambda_{Ly\alpha} = 121.567\) nm, i.e., in the near ultraviolet. The Lyman-alpha line is extremely resonant, which means that the emitted photons are very strongly scattered by the surrounding neutral hydrogen. This multiple scattering leads to a complex shape of the spectral profiles and to an extended spatial distribution of the observed signal, often in the form of blobs.

Formation and dynamics of Lyman-alpha blobs

Ionization, recombination, and expansion of blobs

The formation of a Lyman-alpha blob begins with the emission of ionizing UV photons from young massive stars or nascent supermassive black holes in the first galaxies. These photons ionize the surrounding neutral gas, forming a region of ionized hydrogen (HII). The recombination of protons and electrons in this region emits Lyman-alpha photons, which then escape into the surrounding Universe. The typical size of these blobs is on the order of several hundred kiloparsecs to a few megaparsecs, depending on the luminosity of the sources and the density of the medium.

Radiative transfer and numerical models

The temporal and spatial evolution of these blobs is governed by the radiative transfer equation coupled to hydrodynamic dynamics and hydrogen chemistry. Numerical models notably solve the Boltzmann equation for the distribution of Lyman-alpha photons: \( \frac{1}{c}\frac{\partial I_{\nu}}{\partial t} + \mathbf{n} \cdot \nabla I_{\nu} = -\kappa_{\nu} I_{\nu} + j_{\nu} \)

Observations and cosmological implications

High-redshift detection and constraints on reionization

Lyman-alpha blobs are detected mainly by their diffuse Lyman-alpha emission at high redshifts ($z \sim 6-10$), thanks to instruments sensitive to wavelengths shifted toward the near infrared. These observations provide constraints on:

• The star formation rate of the first galaxies,
• The temperature and density of the intergalactic medium,
• The chronology and morphology of cosmic reionization,
• The kinematic properties of the gas (velocity, turbulence).

Lyman-alpha blobs, through their size and distribution, trace the topology of reionization, a crucial parameter for validating cosmological models of structure formation.

Lyman-α blobs: luminous scars of the end of the Dark Ages

Fossil witnesses of the galactic dawn

Lyman-alpha blobs represent essential witnesses of the cosmic era when the first luminous structures transformed the Universe. They are fossil imprints of the galactic dawn. They make it possible to understand the end of the "Dark Ages" from ≈ 380,000 years (end of recombination after the Big Bang) to 500 million to 1 billion years after the Big Bang (beginning of reionization, triggered by the first stars and galaxies).