UNIVERSITY PARK, Pa. — An international team of scientists, including Penn State astronomers, have used data from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) to construct the largest, most accurate 3D map yet of the light emitted by excited hydrogen in the early universe, a time span ranging from 9 to 11 billion years ago. This specific form of light, designated Lyman alpha, is emitted in large quantities when hydrogen atoms are exposed to a star’s energy.
“Lyman alpha radiation is an important characteristic of galaxies at this period in the universe’s history, an era of vigorous star formation,” said Robin Ciardullo, professor of astronomy and astrophysics in the Penn State Eberly College of Science, a member of the research team and the observing manager of HETDEX. “Previous to this study, the locations of fainter galaxies and gas, which also emit Lyman alpha radiation, have remained largely unknown.”
Using a technique called Line Intensity Mapping, the new map pulls these objects into view, adding shape and nuance to this formative era of the universe. The results were published today (March 3) in The Astrophysical Journal.
“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 in Munich, Germany, who led the development of the map. “But because they are far away, many objects in this time are faint and difficult to observe.”
All light can be broken apart into its various wavelengths, like how a prism can separate visible white light into the colors of the rainbow. The resulting catalogue of various wavelengths is called a spectrum. Astronomers examine spectra 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,” said 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 in Texas, HETDEX is charting the positions 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 and area equivalent to over 2,000 full moons.
“However, we only use a small fraction of all the data we collect, around 5%,” said 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.”
The team took advantage of this additional data to build its map of Lyman alpha radiation in the early universe.
“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,” Lujan Niemeyer 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.”