At its meeting in Paris today, the Science Program Committee of the European Space Agency (ESA) selected the “The Hot and Energetic Universe” as the theme for its next large (L-class) mission, which is expected to be launched in 2028. The theme was proposed by an international collaboration led by Kirpal Nandra, Director at the Max Planck Institute for Extraterrestrial Physics (MPE). Having made a compelling case for this exciting topic, the same team is now poised to propose a new mission concept to address some of the most pressing questions in modern astrophysics. The Advanced Telescope for High-energy Astrophysics (Athena) would provide the necessary angular and spectral resolution, throughput, detection sensitivity, and survey grasp to revolutionize our understanding of why the universe looks the way it does.
How did ordinary matter assemble into the large scale structures that we see today? How did black holes grow and shape the universe? These are some of the most important unanswered questions in modern astrophysics, and the next large ESA mission could provide critical answers.
“We are very pleased that ESA has decided that the ‘Hot and Energetic Universe’ will be one of its main mission targets,” says Nandra, spokesperson for the science theme and chair of the Athena collaboration, who prepared this science theme in a white paper. “We have a superb team of astrophysicists who made a compelling case for this exciting topic. But our job is not over — now we need to keep working to define the X-ray telescope that will provide us with the answers.”
Hot gas is actually the dominant form of ordinary matter in the universe, and is responsible for the largest coherent structures that we know today: clusters of galaxies. With temperatures of more than ten million degrees, the gas emits copiously at X-ray wavelengths. The key to understanding the formation and evolution of these structures is to build an X-ray space observatory that combines high sensitivity, i.e., large throughput and good angular resolution, high spectral resolution and a wide field of view. Athena was designed with exactly this goal in mind. With such a telescope, astronomers could obtain spectroscopic observations of distant galaxies and map the physical parameters of the largest bound objects — information that would dramatically advance our understanding of how hot gas structures started to assemble and form when the universe was in its infancy. Mapping the velocities, thermodynamics and chemical composition of the hot gas and tracking it through cosmic time would also allow the scientists to understand the complex astrophysical processes such as non-gravitational heating and turbulence which are crucial to understanding the development of ordinary matter structures.
With an X-ray telescope like Athena, the astronomers could also look even further back into the history of the universe to study its most energetic events and discover the first supermassive black holes, out to a time when the first galaxies were forming, less than one billion years after the Big Bang. Because of the extremely high temperatures and the huge energies deposited by matter as it falls into a black hole, X-ray emission is the most reliable and complete way of revealing such accreting monsters. Remarkably, processes originating close to the black hole seem able to influence galaxies and galaxy clusters on scales up to a billion times larger- this “cosmic feedback” is therefore an essential ingredient of galaxy evolution models, but it is not yet well understood.
Tracking the growth of supermassive black holes through cosmic time, in the earliest epoch of galaxy formation (at z redshift = 6-10) is impossible with current instrumentation. “We now have the X-ray optics technologies to provide the required leap in collecting area and angular resolution for wide field X-ray imaging,” says Nandra. “Over the past years at MPE we have been continuously developing our X-ray detectors for exactly this opportunity. Now there is the chance to use them to map the X-ray Universe with exquisite sensitivity over unparalleled sky areas. The earliest supermassive black holes are within our grasp.”
Now that the science theme has been accepted by ESA, the next step will be a call for an X-ray observatory able to achieve the science goals. As the proposers of the theme and with the required technologies in hand, the Athena team are confident their mission will make the grade. Once a mission concept has been selected there is expected to be a period of 3-4 years to consolidate the technology development. It will take another 10 years or so to build the observatory. In 2028, Athena should start to reveal the Hot and Energetic Universe in unprecedented detail, and to provide the answer to that most basic question — why does the universe look like it does today?
In addition to “the Hot and Energetic Universe,” ESA accepted “the Gravitational Universe” as the theme for its following L-class mission. The proposed evolved Laser Interferometer Space Antenna (eLISA), a space-based gravitational-wave observatory, is likely to be the mission proposed to address this theme.
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