Revealing the nature of a diffuse Lyman-alpha glow around galaxies

November 01, 2020

Recently, astronomers discovered an extended glow of emission far beyond the stellar bodies of galaxies. While the emission is known to be associated with excited neutral hydrogen, the origin of this so called Lyman-alpha radiation is unknown. MPA researchers use new computational models to understand this emission, establishing that a large contribution is caused by light which originates from deep within galaxies but subsequently scatters at much larger distances.

Hydrogen, the most abundant element in the universe, emits ultraviolet radiation through the so-called Lyman-alpha spectral line, when sufficiently excited. In the 1970s it was theorized that this Lyman-alpha line should shine brightly, especially in young galaxies, thus allowing astronomers to observe distant galaxies ten billion light-years away. Since then, the Lyman-alpha line has indeed been verified as a powerful observational tool to study galaxy formation and cosmology.

Lyman-alpha halo

Rotation around a massive galaxy halo at redshift 3 in the IllustrisTNG50 simulation showing the diffuse Lyman-alpha glow. Photons have scattered from star-forming regions inside the central galaxy and its satellites into the surrounding gas.

Just in recent years, the sensitivity and spatial resolution of telescopes and satellites have become powerful enough to observe not just Lyman-alpha within the galaxies but also a faint extended Lyman-alpha glow surrounding them. This allows astronomers a glimpse of the gas that surrounds young galaxies that is of crucial importance for their future evolution. While more and more such observations become available, the source of this Lyman-alpha emission remains unknown.

There have been various hypotheses concerning the source of this glow. Generally speaking, there are two different types of possible mechanisms: On the one hand, the glow could stem from Lyman-alpha photons that are created in the star-forming regions within the galaxies and are subsequently scattered by the neutral hydrogen surrounding the galaxy. On the other hand, the diffuse Lyman-alpha emission could be created in the galaxy's surrounding gas. For example, gravitational cooling or small satellite galaxies could provide a significant energy source for such a diffuse glow.

Lyman-alpha surface brightness map for the entire TNG50 cosmological simulation at redshift z=3, highlighting the structure of the cosmic web as seen in Lyman-alpha emission. The inset panels show two individual Lyman-alpha halos, on the scale of the halo virial radii, for moderate mass objects of 50 and 120 billion solar masses (top and bottom, respectively). Lyman-alpha photons are predominantly emitted in the star-forming regions of the central galaxies, from where they resonantly scatter and illuminate the more extended gaseous halos, including filamentary inflows. The more massive halo (lower right) has a number of star-forming satellite galaxies, which also contribute Lyman-alpha emissivity and boost the local surface brightness.

Theoretical progress has been complicated by two factors: the Lyman-alpha line is resonant and in astrophysical settings, there is a high optical depth. This means that Lyman-alpha photons can scatter thousands or millions of times before a photon reaches us, making it impossible to know where the photon was originally emitted and what was its exact frequency. Given such physical complexity, numerical simulations of galaxy formation coupled to a radiative transfer code to account for photon scatterings are thus an important tool to study Lyman-alpha observations.

In the last years, various simulations have tried to explain the physical origin of Lyman-alpha emission around galaxies, also called Lyman-alpha halos. They performed explicit radiative transfer calculations that properly capture those effects such as scatterings and change in frequency. Detailed radiative transfer simulations can be run on top of cosmological hydrodynamical simulations, but previous studies could not do so on large scale. A statistically robust sample would require thousands of galaxies that are resolved down to just 100s of light-years, to match up with the manifold of observational data.

In a recent leap, researchers at MPA used the new high-resolution cosmological simulation TNG50 of the IllustrisTNG project and a new radiative transfer code called voroILTIS to determine the origin of the Lyman-alpha glow. The TNG50 simulation provides an unprecedented combination of volume and resolution, while the voroILTIS radiative transfer code includes models for the various Lyman-alpha emission sources mentioned above and explicitly follows virtual photons as they scatter their way towards an observer. This allows to statistically compare the simulation's predictions with existing Lyman-alpha halo observations, while also probing questions regarding the dominant origin and emission mechanism for Lyman-alpha halos.

Stacked radial profiles for simulated galaxies in five different stellar mass ranges from 0.1 billion to 30 billion solar masses at z=3. At fixed cosmic time, the central Lyman-alpha surface brightness increases monotonically with stellar mass. The overplotted dots are measured from 58 observed Lyman-alpha halos from the MUSE Ultra Deep Field (presented in Leclercq et al., 2017) at at redshifts between 2.9 and 3.5. Simulations and data show a good qualitative match.

Comparison of stacked and individual Lyman-alpha radial profiles from those simulations with observational data from the MUSE Ultra Deep Field reveals a promising level of agreement. Those simulated radial profiles also allow a decomposition into contributions from inside the galaxy, i.e. scattered photons from star-forming regions, and outside, such as recombination from the ultraviolet background and collisional excitations powered by gravitational cooling. This decomposition suggests that the Lyman-alpha glow is dominantly powered by emission from star-forming regions for the majority of galaxies.

Multiple shortcomings in the computational model, such as the lack of a description of clumping hydrogen and dust below the resolution scale and the ionizing flux from local sources, remain to be resolved in upcoming work. Nevertheless, these findings allow multiple exciting opportunities for astronomers in the future: With the nature of the Lyman-alpha glow established, future work can decipher the information contained in the Lyman-alpha glow to deepen our understanding of the underlying gas that surrounds galaxies and shapes their evolution. The recent theoretical findings at MPA also suggest that the larg-scale structure, the cosmic web, within which those galaxies reside, can be observed via such Lyman-alpha glow in the near future.

Other Interesting Articles

Go to Editor View