Astronomers with the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), have used data from the project to make the largest, most accurate 3D map yet of the light emitted by excited hydrogen in the early universe, 9 billion to 11 billion years ago. This specific form of light, called Lyman alpha, is emitted in large quantities when hydrogen atoms are exposed to a star's energy. That makes it a great tool for finding bright galaxies in this far-off time, which experienced a rash of star creation. However, the locations of fainter galaxies and gas, which also emit Lyman alpha, have remained largely unknown.
"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 from the Max Planck Institute for Astrophysics who led the development of the map. "But because they are far away, many objects in this time are faint and difficult to observe."
Using a technique called Line Intensity Mapping, the new map pulls these objects into view, adding shape and nuance to this formative era in our universe. Results were published on March 3 in The Astrophysical Journal .
All light can be broken apart into its various wavelengths. The result is called a spectrum. Astronomers examine spectra (the plural of "spectrum") for peaks and valleys which correspond to the presence of different elements. Line Intensity Mapping charts the distribution and concentration of specific elements across an entire region, rather than observing objects one-by-one.
"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 Muñoz, 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 Line Intensity Mapping isn't a new technique, this is the first time it's been used to chart Lyman alpha emissions in such a large set of data and with such high precision. Using the Hobby-Eberly Telescope at McDonald Observatory , HETDEX is charting the position of over one million bright galaxies in its quest to understand dark energy. The project is unique in gathering so much data – over 600 million spectra – for such a large swath of sky, measuring over 2,000 full Moons.
"However, we only use a small fraction of all the data we collect, around 5%," explained Karl Gebhardt, HETDEX principal investigator, chair of UT Austin's astronomy department, and co-author on the paper. "There's huge potential in using that remaining data for additional research."
"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," added Lujan Niemeyer. "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 create its map, the team wrote custom programming and used supercomputers at the Texas Advanced Computing Center to sift through roughly half a petabyte of HETDEX data. It then used the location of bright galaxies already identified by HETDEX to calculate the location of fainter galaxies and gas glowing nearby. Thanks to gravity's propensity for making matter clump together, where there is one bright galaxy, other objects are sure to be close.
"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 paper. The resulting map brings the regions around bright galaxies into greater focus and adds detail to the stretches in between.
"We have computer simulations of this period," continued Komatsu. "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."
Moving forward, the team hopes to compare their map with others that overlap the same region of the universe and focus on different elements. For example, a Line Intensity Map of carbon monoxide - which is associated with the dense, cold clouds where stars form - could add insight to the conditions surrounding the young stars emitting Lyman alpha wavelengths.
"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 Muñoz. "The Hobby-Eberly is a pioneering telescope. And with new, complementary instruments coming online, we're entering a golden age for mapping the cosmos."
