Nearly all the sky is covered by Lyman-alpha emission around high redshift galaxies

Abstract: Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, fed by inflows from the intergalactic medium and by outflows due to galactic winds. Absorption-line measurements along the sightlines to bright and rare background quasars indicate that this circumgalactic medium pervades far beyond the extent of starlight in galaxies, but very little is known about the spatial distribution of this gas. A new window into circumgalactic environments was recently opened with the discovery of ubiquitous extended Lyman-alpha emission from hydrogen around high-redshift galaxies, facilitated by the extraordinary sensitivity of the MUSE instrument at the ESO Very Large Telescope. Due to the faintness of this emission, such measurements were previously limited to especially favourable systems or to massive statistical averaging. Here we demonstrate that low surface brightness Lyman-alpha emission surrounding faint galaxies at redshifts between 3 and 6 adds up to a projected sky coverage of nearly 100%. The corresponding rate of incidence (the mean number of Lyman-alpha emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high column density absorbers frequently detected in the spectra of distant quasars. This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected also in emission.

• The paper demonstrates that nearly 100% of the sky features Lyman-α emission from the circumgalactic medium, indicating ubiquitous high-redshift hydrogen.
• It employs MUSE's advanced spectroscopy to map extended Lyα halos around 270 galaxies, revealing the scale and structure of the CGM.
• The study introduces innovative azimuthal averaging and stacking methods to detect faint emissions, paving the way for refined models of galaxy evolution.

Overview of the Paper: Lyman-$\alpha$ Emission in High-Redshift Galaxies

This paper presents a significant advancement in our understanding of the circumgalactic medium (CGM) around high-redshift galaxies through the detection and analysis of Lyman-$\alpha$ (Ly$\alpha$) emissions. The research utilizes observations from the Multi-Unit Spectroscopic Explorer (MUSE) instrument at the ESO Very Large Telescope to explore the spatial distribution of atomic hydrogen gas around galaxies at redshifts between 3 and 6. The findings reveal a near-total coverage of the sky by Ly$\alpha$ emissions when viewed across these cosmological epochs, emphasizing the pervasiveness of the CGM in such environments.

Key Research Findings

The study builds upon the technological capabilities of MUSE, which allows for high-sensitivity observations of the faint Ly$\alpha$ glow previously unattainable. Key findings from this research include:

1. Sky Coverage and Incidence Rates: The Ly$\alpha$ emissions cover nearly 100% of the sky when observed in projection, indicating that the CGM is virtually ubiquitous around these high-redshift systems. The incidence rate, defined as the average number of Ly$\alpha$ emitters intersected by an arbitrary line of sight, was found to be above unity. This result is comparable to the incidence rate of high-column-density absorbers traditionally detected via quasar spectra, suggesting a comprehensive detection of circumgalactic atomic hydrogen.
2. Extended Halo Detection: MUSE's capabilities enabled the identification of extended Ly$\alpha$ halos around 270 galaxies, challenging previous limitations of statistical averaging or specialized observations. These halos can be traced to several arcseconds from the galaxy centers, representing the physical scale of the CGM.
3. Methodology for Emission Characterization: Through a combination of azimuthal averaging, image stacking, and spatial filtering, the research achieved detection of surface brightness levels significantly below those achievable for single sightlines. This methodological approach provides a framework for further studies seeking to characterize faint emissions in the universe.

Implications and Speculation

The detection of extensive Ly$\alpha$ emission has far-reaching implications for our understanding of galaxy formation and evolution during the critical epoch from redshift 3 to 6. The similarity of emission and absorption incidence rates suggests that we can now observe in emission much of the atomic hydrogen previously studied only through absorption lines. This transition from absorption to emission paradigms could reshape models of CGM interactions with galactic winds and inflows.

Practically, these findings enable a more complete mapping of matter distribution in the universe, presenting new opportunities to study the interplay between galaxies and their gaseous environments. Theoretically, they raise questions about the dynamics of Ly$\alpha$ halos, their association with star formation processes, and the role of CGM in the cosmic reionization era.

Future Prospects

This research opens up prospects for further exploratory and comparative studies of the CGM. Possible future developments could include:

• Enhanced spectral analysis of the Ly$\alpha$ line to gain insight into the physical processes, such as cooling flows and radiative transfer effects, occurring within these halos.
• Integration of higher-resolution instruments or upcoming observational facilities to refine our understanding of the spatial coherence and kinematics of the Ly$\alpha$ emitting regions.
• Cross-correlation studies with other wavelengths to examine the influence of Ly$\alpha$ emission on reionization and structural formation histories in the universe.

In conclusion, the paper's findings not only support advancements in observational astrophysics but also invite a re-evaluation of theoretical models concerning the CGM's role across cosmic timescales.