Galaxies light up hydrogen halos around neighbouring galaxies

October 01, 2022

Galaxies are embedded in large reservoirs of gas - mostly hydrogen and helium. This hydrogen gas has been found to glow faintly in a specific ultraviolet wavelength, or color, called Lyman-alpha. Scientists at the MPA have discovered that these Lyman-alpha halos are larger than previously thought, spanning several 100,000 light years. The inferred size and shape of the halos suggest that the light in the outer parts of the halos comes from surrounding galaxies or the gas in their environments rather than from the central galaxy itself.

Stars only account for 10% of the visible matter in the universe. The rest is mostly comprised of hydrogen and helium gas distributed in, around, and in between galaxies, called interstellar, circumgalactic and intergalactic media, respectively. The circumgalactic gas (CGM) is in close interaction with the galaxies. Cold gas streaming into galaxies enables star formation, while supernovae and actively accreting supermassive black holes heat up the gas and push it out of the galaxy. This complex interplay between the galaxies and the CGM is crucial for the evolution of the galaxies. Hence scientists are highly interested in the composition and dynamics of the CGM.

However, the gas outside of galaxies is dark and difficult to observe. Through long-exposure observations and statistical averaging, scientists have discovered a faint ultraviolet glow of the hydrogen gas around galaxies around the so-called cosmic noon, the epoch of peak star formation (roughly 11 billion years ago). More precisely, the gas is lit up in one specific wavelength, the so-called Lyman-alpha wavelength. These Lyman-alpha halos appear around different kinds of galaxies,  including both Lyman-alpha bright and Lyman-alpha faint galaxies.

Lyman-alpha halo

Rotation around a massive galaxy halo at redshift 3 in the IllustrisTNG50 simulation (Byrohl et al. 2020) showing the diffuse Lyman-alpha glow. Photons from star-forming regions inside the central galaxy, its satellites, and more distant galaxies have scattered in the surrounding gas.

So far, it is unclear how Lyman-alpha halos around star-forming galaxies are lit up. Lyman-alpha photons produced by star formation in the central galaxy can scatter off the hydrogen atoms in the CGM, illuminating it. Lyman-alpha photons could also be produced within the CGM through collisions of hydrogen atoms with free electrons or recombination of free protons and electrons to hydrogen atoms.

The Hobby-Eberly Telescope Dark Energy Experiment, short HETDEX, is currently observing about one million Lyman-alpha emitting galaxies around cosmic noon. From the galaxy positions, HETDEX will infer the expansion rate of the universe at that time, which provides important information to solve the Hubble constant discrepancy (see Monthly Highlight of August). HETDEX uses an instrument called VIRUS that maps the Lyman-alpha emission in huge three-dimensional volumes during each observation. 

Scientists at MPA took advantage of the large sample of galaxies in HETDEX to study their Lyman-alpha halos. They hand-selected a thousand galaxies with strong Lyman-alpha emission observed with HETDEX under the best observing conditions, removing those with active supermassive black holes (Lyman-alpha halos around these so-called quasars were studied in May’s Monthly Highlight). 

Because the selected halos are so faint, the researchers combined the noisy pictures of the individual Lyman-alpha halos into one average picture. More precisely, they measured the median radial profile, which is the Lyman-alpha surface brightness as a function of distance from the galaxy center. A comparison to a previous study of fainter Lyman-Alpha emitters (LAEs) at higher redshift showed that these Lyman-alpha halos have similar shapes, but are much larger than previously thought. The average size and shape of the Lyman-alpha halos agree with the prediction from a large cosmological Lyman-alpha radiative transfer simulation, which was run at MPA (see Monthly Highlight of November 2020). The simulation predicts that the inner parts of Lyman-alpha halos are mainly powered by scattering photons from the central galaxy, while the outer parts of Lyman-alpha halos are powered by scattering photons originating from other galaxies.

A follow-up study showed that Lyman-alpha halos are not limited to LAEs. Galaxies found through bright oxygen emission lines that are faint in Lyman-alpha also have large Lyman-alpha halos, similar to those around the LAEs. However, the inner parts of their halos are much fainter, as expected if these photons indeed originate from the central, Lyman-alpha faint galaxies. The similarity of the outer parts of the Lyman-alpha halos around the two types of galaxies strengthens the hypothesis that these photons do not originate from the central galaxies. The comparison of the total Lyman-alpha luminosity of the halo with the expected luminosity from their host galaxies, independently measured by their star-formation rates, supports this scenario. Thus, most photons in the outer halo parts are produced in the CGM itself or in other galaxies.

However, these findings cannot definitively determine the origin of Lyman-alpha halos. While the Lyman-alpha halos are too bright to be lit up solely by their central galaxies, they are consistent with originating from either cooling radiation, or scattered photons from other star-forming regions, or the so-called ultraviolet background. More detailed observations with long exposure times of individual Lyman-alpha halos are necessary to determine the origin of Lyman-alpha halos and interpret the signal in light of the content and dynamics of the CGM. Nevertheless, these HETDEX observations point to new exciting opportunities to directly image and study the gas surrounding and feeding galaxies more than 10 billion years ago.

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