Three years after its initial test observations, the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is now training its full suite of instrumentation to reveal the nature and evolution of dark energy, the mysterious entity that is the primary constituent of the universe.
HETDEX, which is a large international consortium led by the University of Texas at Austin and involves approximately 100 scientists including Penn State researchers, plans to construct one of the largest maps of the cosmos ever made. The three-dimensional map of 2.5 million galaxies will help astronomers to better understand why the expansion of the universe is currently accelerating.
“Penn State is delighted to be a participant in this fundamental scientific investigation,” said Donald Schneider, a member of the Hobby-Eberly Telescope (HET) Board of Directors and distinguished professor and head of Penn State’s Department of Astronomy and Astrophysics. “The telescope’s innovative design was by two Penn State astronomers, Lawrence Ramsey and Daniel Weedman, and a number of Penn State astronomers are playing important roles in the observations and data analysis in HETDEX.”
The two black structures to the left and right of the Hobby-Eberly Telescope’s main mirror are nicknamed ‘saddlebags.’ They hold the dozens of spectrographs that make up the VIRUS instrument designed to undertake HETDEX, the Hobby-Eberly Telescope Dark Energy Experiment.
HETDEX is using the 10-meter HET, located at McDonald Observatory in western Texas, to obtain data from two large regions of the sky; one field is in the direction of the Big Dipper, the other is slightly southwest of the constellation of Orion. Each time the telescope is pointed at these regions, which typically last 20 minutes, HETDEX’s instrumentation records approximately 32,000 spectra, capturing the cosmic fingerprint of the light from every object within the 10-meter telescope’s field of view.
“HETDEX has arrived,” said University of Texas astronomer Karl Gebhardt, who is the survey’s project scientist. “We’re over a third of the way through our program now, and we have this fantastic dataset that we’re going to use to measure the dark energy evolution.”
HETDEX is a “blind” survey; rather than pointing at specific targets, it records light from all sources over a specific patch of sky. These spectra are recorded via 32,000 optical fibers that feed into more than 100 instruments working together as a single spectrograph. This assembly, the Visible Integral-field Replicable Unit Spectrograph (VIRUS), is a complex system consisting of dozens of copies of an instrument working together for efficiency. VIRUS was designed and built especially for HETDEX.
Building VIRUS “was quite a task to orchestrate,” said Gary Hill, a University of Texas astronomer and the designer of the instrument. “It’s the largest on many measures,” he said, noting that it has the most optical fibers, as well as having as much detector area as the largest astronomical cameras. VIRUS is also an extremely imposing instrument, claiming much of the volume inside the telescope dome.
This image shows the ‘focal surface’ of the Hobby-Eberly Telescope, where the optical fibers of VIRUS are arrayed. The circles each contain a square grid of 448 fibers. When the telescope is pointed and VIRUS takes an observation, each of the 32,000 fibers takes a spectrum simultaneously, recording a vast array of information on the speed, direction, and chemical makeup of every point inside the field of view, which is about the size of the full Moon.
The HETDEX team expects to complete their observations by December 2023. In total, the completed survey will include one billion spectra, “the largest ever spectral survey by far,” Gebhardt said.
“To investigate the properties of dark matter and its evolution, we must identify a few million galaxies of a specific type in the roughly one billion HETDEX spectra and create a map of their three-dimensional distribution,” explained Donghui Jeong, associate professor of astronomy and astrophysics at Penn State and leader of the HETDEX science group investigating large scale structures in the universe. “By examining the locations of these galaxies, we can compare the observations to models of dark energy and determine the influence of ordinary matter, dark matter, and dark energy at various points in the history of the universe.”
Other Penn State HETDEX participants include Professor of Astronomy and Astrophysics Robin Ciardullo, who is the observations manager for HETDEX; Research Professor Caryl Gronwall; and Associate Professor Derek Fox.
“The galaxies that will be studied in HETDEX are from the universe’s distant past,” said Gronwall. “The light we detect left these objects approximately ten billion years ago, when the universe was but a few billion years of age.”
This false-color image of the Pinwheel Galaxy (Messier 101) shows the power of the VIRUS instrument built for the HETDEX survey. The image is a mosaic made up of the central portion of 21 VIRUS pointings across a region of sky about half the size of the full Moon, with some small gaps in coverage. The colors show the contrast between young stars (blue/white) and older stars (red/orange). The breakout boxes show just four examples of the ‘cosmic fingerprint’ of objects in this view. Clockwise from top left: a white dwarf in our galaxy, an active galaxy 11 billion light-years away, a star-forming region in the Pinwheel Galaxy 20 million light-years away, and a star-forming galaxy 3 billion light-years away.
Taft Armandroff, director of The University of Texas at Austin’s McDonald Observatory and Chair of the HET Board of Directors, noted, “HETDEX represents the coming together of many astronomers and institutions to conduct the first major study of how dark energy changes over time.”
HETDEX is led by The University of Texas at Austin McDonald Observatory and Department of Astronomy with participation from Penn State; Ludwig Maximilians University, Munich; the Max Planck Institute for Extraterrestrial Physics; the Institute for Astrophysics, Gottingen; the Leibniz Institute for Astrophysics, Potsdam; Texas A&M University; The University of Oxford; the Max Planck Institute for Astrophysics; The University of Tokyo; and the Missouri University of Science and Technology.
In addition to Institutional support, HETDEX is funded by the National Science Foundation (grant AST-0926815), the State of Texas, the U.S. Air Force (AFRL FA9451-04-2-0355), and generous support from private individuals and foundations.