Fading Light, Fierce Winds: JWST’s Observations of a Low-Luminosity Quasar Unlocks Insight into the Evolution of Supermassive Black Holes at Cosmic Dawn
Steward Observatory Assistant Research Professor Jianwei Lyu and his team used JWST data to explore the properties of a low-luminosity quasar in a recent paper in Astrophysical Journal Letters.
Quasars—very high-energy sources powered by matter falling into supermassive black holes—are easiest to find when they are at their brightest, i.e. most luminous. This makes Steward Observatory Assistant Research Professor Jianwei Lyu’s recent paper in Astrophysical Journal Letters that much more unusual: Lyu, along with his team, has investigated a low-luminosity quasar less than a billion years after the Big Bang (redshift 6.25), when the Universe was only about 7% of its current age. This source contains a black hole sufficiently massive to make it a traditional quasar, but it is likely in a phase of its life long after the height of its brightness. His discovery adds nuance to what we stand to learn about the interplay between black holes and the galaxies that host them in the early Universe.
Ever since he was a graduate student at the Department of Astronomy at University of Arizona , Lyu has worked to discover active galactic nuclei—somewhat less luminous cousins of quasars—culminating with his use of the rich data gathered by the James Webb Space Telescope (JWST). In his comprehensive studies of the area previously most thoroughly explored for active galactic nuclei (AGN), Lyu has increased the number of known AGN by 34 percent. Many of these had been so deeply embedded in dust that they had not been found previously, but Lyu was able to make use of the capabilities of MIRI (the JWST Mid-Infrared Instrument) to more keenly identify new AGN than had ever been possible before. “This research established Lyu as a leading expert on all the manifestations of the AGN phenomenon,” said UA professor George Rieke, a co-author of the paper and the lead for the MIRI science team. Lyu’s recent paper, also co-authored by a number of other Steward Observatory astronomers, is a highlight of his long-term attention to AGNs.
The detection of a gaseous companion to quasar HSC J2239+0207 at a redshift of 6.2. The four stamps on the top left corner are the reconstructed images of the quasar field at different wavelengths. On the top right is an artist’s view of such a system. The bottom two panels show the comparison of the gas companion spectrum (green lines) and the quasar spectrum (middle, blue lines), and two high-z galaxy templates (bottom, magenta and dark yellow lines).
Compared to most quasars discovered previously, the quasar (HSC J2239+0207) described in Lyu’s paper is massive, metal-rich and highly evolved but has relatively low luminosity, indicating that the system is in a less active phase. The team also discovered a nearby blob of gas, possibly driven out by the AGN winds that happened sometime in the quasar’s past. “This offers fresh insights into how black holes and their host galaxies interacted in the early Universe,” said Lyu, “In particular, the detection of an isolated gas companion likely heated up by the quasar light shows the influence of the AGN activity during the mass assembly of the black hole.”
Studying this low-luminosity quasar in the faraway Universe will allow Lyu to explore what happens when quasars slow down and fade. “Eventually they become the quiescent centers of normal galaxies,” said Rieke, “and the fading gives us insights to how that happens.”
“What stands out to me about this project is how much about this quasar is completely impossible to glean without JWST,” said Meredith Stone, a co-author on the paper and a graduate student in the Department of Astronomy. The quasar had been discovered by the Subaru Telescope well before JWST's launch, so the properties of the supermassive black hole were relatively well-known. “But without JWST's infrared coverage, we knew next to nothing about the galaxy's stellar component,” said Stone. JWST is providing unprecedented views of high-redshift quasars like HSC J2239+0207, allowing astronomers to learn about these quasar’s histories, their futures, and the galaxies in which they live.
“And of course,” said Stone, “we're in an ideal position here at Arizona to make these discoveries, with all the JWST know-how concentrated at Steward Observatory!”
The paper is publicly accessible here.