"Observing the early universe gives us an idea of how galaxies evolved into their current form, and what role intergalactic gas played in this process," said Maja Lujan Niemeyer, a HETDEX scientist and recent graduate of the Max Planck Institute for Astrophysics who led development of the new map. "But because they are far away, many objects in this time are faint and difficult to observe." The new work uses a technique called Line Intensity Mapping to pull those elusive sources out of the background, adding shape and nuance to a formative era in cosmic history.
All light can be separated into its component wavelengths to form a spectrum, which astronomers inspect for peaks and valleys that mark the presence of specific elements. Line Intensity Mapping extends this approach by charting the distribution and concentration of a chosen spectral line across an entire region of sky, rather than focusing on individual, easily detected objects. "Imagine you're in a plane looking down. The 'traditional' way to do galaxy surveys is like mapping the brightest cities only: you learn where the big population centers are, but you miss everyone that lives in the suburbs and small towns," explained Julian Munoz, a HETDEX scientist, assistant professor at The University of Texas at Austin, and co author on the paper. "Intensity mapping is like viewing the same scene through a smudged plane window: you get a blurrier picture, but you capture all the light and not just the brightest spots."
Although astronomers have discussed and tested Line Intensity Mapping for some time, this study is the first to apply it to Lyman alpha emission over such a large and precise data set. HETDEX uses the Hobby-Eberly Telescope at McDonald Observatory to chart the positions of more than one million bright galaxies as part of its primary mission to probe dark energy. In the process, the project is amassing over 600 million individual spectra across a sky area equivalent to more than 2,000 full Moons, making it uniquely suited to intensity mapping.
"However, we only use a small fraction of all the data we collect, around 5%," noted Karl Gebhardt, HETDEX principal investigator, chair of the astronomy department at The University of Texas at Austin, and co author on the paper. "There's huge potential in using that remaining data for additional research." Niemeyer emphasized that the galaxies originally targeted by HETDEX are only the most conspicuous structures in a much richer scene. "HETDEX observes everything in a patch of sky, but only a tiny amount of that data is related to the galaxies that are bright enough for the project to use," she said. "But those galaxies are only the tip of the iceberg. There's a whole sea of light in the seemingly empty patches in between."
To build the new map, the collaboration wrote custom software and employed supercomputers at the Texas Advanced Computing Center to sift through roughly half a petabyte of raw HETDEX observations. The team then used the accurately measured positions of bright galaxies already cataloged by HETDEX as anchors to infer the locations of fainter galaxies and diffuse gas glowing nearby in Lyman alpha. Because gravity causes matter to clump, bright galaxies tend to sit in regions where other structures also gather, allowing the researchers to turn those luminous systems into signposts.
"So, we can use the location of known galaxies as a signpost to identify the distance of the fainter objects," said Eiichiro Komatsu, a HETDEX scientist, scientific director at the Max Planck Institute for Astrophysics, and co author on the study. The resulting line intensity map sharpens the view around the bright galaxies and, crucially, fills in details across the lower density regions that lie between them. With this new data set, astronomers have a way to check how closely state of the art numerical simulations of the young universe match the actual large scale distribution of gas and galaxies.
"We have computer simulations of this period," Komatsu said. "But those are just simulations, not the real universe. Now we have a foundation which can let us know if some of the astrophysics underpinning those simulations is correct." By comparing the observed Lyman alpha intensity patterns to theoretical models, researchers can test assumptions about how gas cools, collapses, and forms stars in the first few billion years after the Big Bang.
Looking ahead, the team plans to cross compare the new Lyman alpha map with other intensity maps that trace different elements in the same cosmic volume. One target is carbon monoxide, a molecule associated with the dense, cold clouds in which stars are born. A detailed carbon monoxide intensity map overlapping the HETDEX field would reveal how star forming gas, young stars, and the surrounding hydrogen rich environment connect in three dimensions during this key era.
"This study is a first detection, which is exciting on its own, and it opens the door to a new era of intensity mapping the universe," said Munoz. "The Hobby Eberly is a pioneering telescope. And with new, complementary instruments coming online, we're entering a golden age for mapping the cosmos." The research appears in The Astrophysical Journal.
Research Report:Lya Intensity Mapping in HETDEX: Galaxy-Lya Intensity Cross-Power Spectrum
Related Links
Hobby-Eberly Telescope Dark Energy Experiment
Understanding Time and Space
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