Quasars are the brightest objects in the universe and among the most energetic. They outshine entire galaxies of billions of stars. A supermassive black hole lies at the heart of every quasar, but not every black hole is a quasar. Only the black holes that are feeding most voraciously can power a quasar. Material falling into the supermassive black hole heats up and causes a quasar to shine across the universe like a lighthouse beacon.
Although quasars are known to reside at the centers of galaxies, it’s been difficult to tell what those galaxies are like and how they compare to galaxies without quasars. The challenge is that the quasar’s glare makes it difficult or impossible to tease out the light of the surrounding host galaxy. It’s like looking directly into a car headlight and trying to figure out what kind of automobile it is attached to.
In a study recently published in The Astrophysical Journal, astronomers used the near-infrared capabilities of NASA’s Hubble Space Telescope to study known quasars during the first billion years in hopes of spotting the surrounding glow of their host galaxies, without significant detections. This suggests that dust within the galaxies is obscuring the light of their stars and that NASA’s James Webb Space Telescope, set to launch in 2021, will be able to peer through the dust and uncover the hidden galaxies.
Madeline Marshall of the University of Melbourne in Australia is the lead author of this study, with co-authors including Arizona State University scientists Rogier Windhorst, Seth Cohen, Rolf Jansen, Jenna Robinson, Evan Scannapieco and Brent Smith of the School of Earth and Space Exploration. Additional co-authors affiliated with ASU include former graduate student Mira Mechtley, former Hubble Fellow Linhua Jiang and former undergraduate student Victoria Jones.
“Hubble simply doesn’t go far enough into the infrared to see the host galaxies. This is where the James Webb Space Telescope will really excel,” said Windhorst.
Lead author Marshall’s latest research, recently published by the Royal Astronomical Society, added a state-of-the-art computer simulation called BlueTides, developed by a team led by Tiziana Di Matteo at Carnegie Mellon University in Pittsburgh, to determine what Webb is expected to see.
“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 Marshall.
Answering these questions is challenging for a number of reasons. In particular, the more distant a galaxy is, the more its light has been stretched to longer wavelengths by the expansion of the universe. As a result, ultraviolet light from the black hole’s accretion disk or the galaxy’s young stars gets shifted to infrared wavelengths.
“BlueTides is designed 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.
This video zooms into a highly detailed simulation of the universe called BlueTides. Much like the iconic Powers of Ten video, each step covers a distance 10 times smaller than the previous one. The first frame spans about 200 million light-years while the fourth and final frame spans only 200,000 light-years and contains two galaxies. Researchers used this simulation to investigate the properties of galaxies that contain quasars — bright galactic cores powered by accreting supermassive black holes. Credit: Y. Ni (Carnegie Mellon University) and L. Hustak (STScI)
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.
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 in the sky.
“Webb will open up the opportunity to observe these very distant host galaxies for the first time,” said Marshall, who with co-author Stuart Wyithe of the University of Melbourne are a part of Windhorst's James Webb Space Telescope team and will use some of their Guaranteed Observing Time on Webb to observe two of these quasars in the first billion years.
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.
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 Wyithe.
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 BlueTides simulation (project PI: Tiziana Di Matteo at Carnegie Mellon University) was run at the Blue Waters sustained-petascale computing facility, which is supported by the National Science Foundation.
The James Webb Space Telescope will be the world's premier space science observatory when it launches in 2021. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, the European Space Agency and the Canadian Space Agency.
This article was written by Christine Pulliam of the Space Telescope Science Institute in Baltimore with contributions by Karin Valentine of ASU’s School of Earth and Space Exploration.
Top image: This artist’s illustration portrays two galaxies that existed in the first billion years of the universe. The larger galaxy at left hosts a brilliant quasar at its center, whose glow is powered by hot matter surrounding a supermassive black hole. Scientists calculate that the resolution and infrared sensitivity of NASA’s upcoming James Webb Space Telescope will allow it to detect a dusty host galaxy like this despite the quasar’s searchlight beam. Credit: J. Olmsted (STScI)