The space between galaxies is not empty. In a new map of the early universe, those \u201cblank\u201d stretches take on a faint, hydrogen-blue glow that had mostly escaped surveys until now.<\/p>\n
Astronomers working with the Hobby-Eberly Telescope Dark Energy Experiment<\/a>, better known as HETDEX, say they have built the largest and most accurate 3D map yet of a specific ultraviolet fingerprint from hydrogen called Lyman alpha. The map covers light emitted roughly 9 billion to 11 billion years ago, a period when the universe was busy making stars, and lighting up hydrogen atoms across enormous distances.<\/p>\n \u201cObserving the early universe gives us an idea of how galaxies evolved into their current form, and what role intergalactic gas played in this process,\u201d said Maja Lujan Niemeyer, a HETDEX scientist and recent graduate from the Max Planck Institute for Astrophysics<\/a> who led the map\u2019s development. \u201cBut because they are far away, many objects in this time are faint and difficult to observe.\u201d<\/p>\n IFU coordinates in the Fall (top panel), Spring (middle panel), and NEP (bottom panel) fields. Masked maps are shown as hatched regions. The backward-facing (forward-facing) diagonal hatchings represent boxes masked in the low-z (medium-z) bin, and the dotted hatchings indicate those masked in the high-z bin. (CREDIT: The Astrophysical Journal) A survey that usually ignores most of its own light<\/p>\n Lyman alpha is produced in large quantities when hydrogen atoms get energized by starlight. That makes it a strong beacon for finding bright, distant galaxies, and HETDEX already uses it to chart more than one million bright galaxies as part of its dark energy program. The problem is everything else.<\/p>\n Fainter galaxies<\/a> and diffuse gas also emit Lyman alpha, but their signals blend into noise when you hunt for objects one by one. HETDEX\u2019s standard approach, built to find clear, bright detections, leaves much of that dim background unclaimed.<\/p>\n \u201cHowever, we only use a small fraction of all the data we collect, around 5%,\u201d said Karl Gebhardt, HETDEX principal investigator, chair of The University of Texas at Austin\u2019s astronomy department, and a co-author on the paper. \u201cThere\u2019s huge potential in using that remaining data for additional research.\u201d<\/p>\n \u201cHETDEX 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,\u201d Lujan Niemeyer added. \u201cBut those galaxies are only the tip of the iceberg. There\u2019s a whole sea of light in the seemingly empty patches in between.\u201d<\/p>\n The new work tries to chart that sea.<\/p>\n Seeing the universe as a field, not a list of objects<\/p>\n The team relied on a technique called Line Intensity Mapping. Instead of building a catalog of individually detected galaxies, intensity mapping tracks the overall distribution of a chosen \u201cline\u201d in a spectrum across a wide region of space.<\/p>\n Left panel: suppression of the power spectrum monopole due to PCA cleaning. We show the mean power spectrum monopole obtained from the fiducial mocks in the Spring field in the medium-z bin. Right panel: the ratio of the power spectrum monopole with and without NPC PCs removed as a function of k and NPC. (CREDIT: The Astrophysical Journal) <\/p>\n All light can be separated into a spectrum, where specific wavelengths show up as peaks and valleys that correspond to different elements. In this case, the team focused on the Lyman alpha line from hydrogen. Intensity mapping does not demand that each patch of sky contain a clean, obvious galaxy. It uses noisy data to measure summary patterns in the light, so the combined contribution from faint galaxies and diffuse gas still counts.<\/p>\n Julian Mu\u00f1oz, a HETDEX scientist, assistant professor at UT Austin, and a co-author, offered a visual way to think about the difference.<\/p>\n \u201cImagine you’re in a plane looking down. The \u2018traditional\u2019 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,\u201d Mu\u00f1oz said. \u201cIntensity 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.\u201d<\/p>\n Intensity mapping itself is not new. The team says this is the first time it has been used to chart Lyman alpha emission<\/a> with this much precision across such a large dataset.<\/p>\n Half a petabyte, and a new way to use it<\/p>\n HETDEX is unusual for its scale. It has gathered more than 600 million spectra across a swath of sky larger than 2,000 full Moons.<\/p>\n To build the new map, the researchers wrote custom programming and used supercomputers at the Texas Advanced Computing Center to sift through roughly half a petabyte of HETDEX data. They then used the positions of bright galaxies already identified by HETDEX to help locate the faint stuff that does not show up as a clean detection.<\/p>\n Gravity plays a key role in that step. Matter clumps. Where you find a bright galaxy, you often find nearby fainter galaxies and gas as well, even if they hide below the threshold of a typical catalog.<\/p>\n Correlation matrices of the power spectrum monopoles in the same order. (CREDIT: The Astrophysical Journal) <\/p>\n \u201cSo, we can use the location of known galaxies as a signpost to identify the distance of the fainter objects,\u201d said Eiichiro Komatsu, a HETDEX scientist, scientific director at the Max Planck Institute for Astrophysics, and a co-author on the paper.<\/p>\n The end product is not a simple list of galaxies. It is a 3D picture of where hydrogen\u2019s Lyman alpha glow piles up, including around known bright galaxies<\/a> and in the less obvious stretches between them.<\/p>\n A check on simulations, and a path to other elements<\/p>\n The early universe in this era does not come with direct photographs. Astronomers often rely on computer simulations to explore how galaxies and gas should behave at these times, then test those simulations against whatever the sky allows them to measure.<\/p>\n \u201cWe have computer simulations of this period,\u201d Komatsu said. \u201cBut 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.\u201d<\/p>\n The team also described the map as a starting point. They want to compare their Lyman alpha intensity map with other maps that cover the same region but track different elements. One example they highlighted is carbon monoxide, which is associated with dense, cold clouds where stars form. A carbon monoxide intensity map could offer a different view of the conditions around the young stars<\/a> that drive Lyman alpha emission.<\/p>\n \u201cThis 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,\u201d Mu\u00f1oz said. \u201cThe Hobby-Eberly is a pioneering telescope. And with new, complementary instruments coming online, we’re entering a golden age for mapping the cosmos.\u201d<\/p>\n Practical implications of the research<\/p>\n