Despite the remarkable maps that astronomers have created to explore the distant universe, many cosmic phenomena have remained elusive. While the brightest galaxies are relatively straightforward to catalog, the faint hydrogen glow surrounding them, along with numerous smaller galaxies, has largely gone unnoticed.
A groundbreaking study published in The Astrophysical Journal addresses this gap. Researchers utilized data from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) to construct a three-dimensional map of Lyman-alpha light--a specific ultraviolet glow emitted by excited hydrogen--dating back 9 to 11 billion years.
This innovative approach reveals a more comprehensive view of galactic environments.
Beyond the Brightest Beacons
Conventional galaxy surveys excel at identifying prominent cosmic structures but often overlook the faint emissions that exist between and around these bright entities. In contrast, line intensity mapping adopts a different strategy: it measures the cumulative light from extensive regions of space instead of focusing solely on individual galaxies. This method sacrifices some detail but offers a broader perspective.
Julian Muñoz, a HETDEX scientist at the University of Texas at Austin, likened traditional galaxy surveys to mapping only the brightest cities, which leads to missing the smaller towns and suburbs. He explained, "Intensity mapping is akin to viewing that scene through a smudged window: while the picture may be blurrier, it captures all the light, not just the brightest spots."
Although HETDEX primarily aims to study dark energy by mapping over a million bright galaxies, it has inadvertently collected a wealth of additional data. The survey has recorded more than 600 million spectra, of which only a fraction--around 5%--has been utilized thus far.
"There's immense potential in leveraging the remaining data for further research," stated Dr. Karl Gebhardt, HETDEX principal investigator.
In this study, researchers tapped into the underutilized data to recover faint Lyman-alpha emissions that lie beyond cataloged galaxies.
Turning Noise Into a Map
The task of extracting this signal was complex. The team meticulously processed approximately half a petabyte of HETDEX data, filtering out interference from Earth's atmosphere, foreground objects, and instrumental noise. They employed the positions of known bright galaxies as reference points to identify emissions from fainter galaxies and surrounding gas.
Dr. Eiichiro Komatsu from the Max-Planck Institute for Astrophysics, a co-author of the study, noted, "By using the locations of known galaxies, we can pinpoint the distances to these fainter objects."
The study tracked the hydrogen glow across three slices of the early universe. Many sources were too faint to be included in the galaxy catalog independently, but collectively, they generated a measurable signal.
This new map provides astronomers with a valuable tool to validate their galaxy formation theories against the actual appearance of the universe. Furthermore, when combined with other surveys of the same areas, it may enhance our understanding of how galaxies evolved and how star-forming gas was distributed around them.