“Hubble simply doesn’t go far enough into the infrared” to see the hidden galaxies of the early universe, said Rogier Windhorst of Arizona State University, co-author of a new study using the near-infrared capabilities of NASA’s Hubble Space Telescope to probe known quasars, ‘quasi-stellar radio sources’, in hopes of spotting the surrounding glow of their host galaxies, without significant detections, suggesting that cocoons of dust that absorb visible light within the galaxies is obscuring the light of their stars.
Unlocking Secrets of the Infrared Universe
“We want to know what kind of galaxies these quasars live in. That can help us answer questions like: How can black holes grow so big so fast? Is there a relationship between the mass of the galaxy and the mass of the black hole, like we see in the nearby universe?” said lead author Madeline Marshall of the University of Melbourne in Australia, who conducted her work within the ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions.
The new study suggests that NASA’s James Webb Space Telescope, set to launch in 2021, will be able to reveal the hidden host galaxies of some distant quasars despite their small sizes and obscuring dust. using the Webb’s infrared detectors. The more distant a galaxy is, the more its light has been stretched to longer wavelengths by the expansion of the universe with ultraviolet light from the black hole’s accretion disk or the galaxy’s young stars shifted to infrared wavelengths. “Webb will open up the opportunity to observe these very distant host galaxies for the first time,” said Marshal.
Webb will primarily look at the Universe in the infrared, while Hubble studies it primarily at optical and ultraviolet wavelengths (though it has some infrared capability). Webb also has a much bigger mirror than Hubble, which means that Webb can peer farther back into time than Hubble is capable of doing. Hubble is in a very close orbit around the earth, while Webb will be 1.5 million kilometers distant at the second Lagrange (L2) point where the Webb’s solar shield will block the light from the Sun, Earth, and Moon.
These simulated images above show how a quasar and its host galaxy would appear to the James Webb Space Telescope (top) and Hubble Space Telescope (bottom) at infrared wavelengths of 1.5 and 1.6 microns, respectively. Webb’s larger mirror will provide more than four times the resolution, enabling astronomers to separate the galaxy’s light from the overwhelming light of the central quasar. The individual images span about 2 arcseconds on the sky, which represents a distance of 36,000 light-years at a redshift of seven. M. (Marshall/University of Melbourne).
Bluetides — First Billion Years of the Universe
To determine what Webb is expected to see, the team used a state-of-the-art computer simulation called BlueTides, developed by a team led by Tiziana Di Matteo at Carnegie Mellon University to study the formation and evolution of galaxies and quasars in the first billion years of the universe’s history
“Its large cosmic volume and high spatial resolution enables us to study those rare quasar hosts on a statistical basis,” said Yueying Ni of Carnegie Mellon University, who ran the BlueTides simulation. BlueTides provides good agreement with current observations and allows astronomers to predict what Webb should see.
The team found that the galaxies hosting quasars tended to be smaller than average, spanning only about 1/30 the diameter of the Milky Way despite containing almost as much mass as our galaxy. “The host galaxies are surprisingly tiny compared to the average galaxy at that point in time,” said Marshall.
“Like Precocious Children”
The galaxies in the simulation also tended to be forming stars rapidly, up to 600 times faster than the current star formation rate in the Milky Way. “We found that these systems grow very fast. They’re like precocious children – they do everything early on,” explained co-author Di Matteo.
The team then used these simulations to determine what Webb’s cameras would see if the observatory studied these distant systems. They found that distinguishing the host galaxy from the quasar would be possible, although still challenging due to the galaxy’s small size on the sky.
They also considered what Webb’s spectrographs could glean from these systems. Spectral studies, which split incoming light into its component colors or wavelengths, would be able to reveal the chemical composition of the dust in these systems. Learning how much heavy elements they contain could help astronomers understand their star formation histories, since most of the chemical elements are produced in stars.
New Insights into Extreme Systems
Webb also could determine whether the host galaxies are isolated or not. The Hubble study found that most of the quasars had detectable companion galaxies, but could not determine whether those galaxies were actually nearby or whether they are chance superpositions. Webb’s spectral capabilities will allow astronomers to measure the redshifts, and hence distances, of those apparent companion galaxies to determine if they are at the same distance as the quasar.
Ultimately, Webb’s observations should provide new insights into these extreme systems. Astronomers still struggle to understand how a black hole could grow to weigh a billion times as much as our Sun in just a billion years. “These big black holes shouldn’t exist so early because there hasn’t been enough time for them to grow so massive,” said co-author Stuart Wyithe of the University of Melbourne.
Future quasar studies will also be fueled by synergies among multiple upcoming observatories. Infrared surveys with the European Space Agency’s Euclid mission, as well as the ground-based Vera C. Rubin Observatory, a National Science Foundation/Department of Energy facility currently under construction on Cerro Pachón in Chile’s Atacama Desert. Both observatories will significantly increase the number of known distant quasars. Those newfound quasars will then be examined by Hubble and Webb to gain new understandings of the universe’s formative years.
The Daily Galaxy, Max Goldberg, via Arizona State University
Image credit top of page: Quasar image, Hubble Space Telescope.