Early Galaxies Shed Light on the Rise of Monster Black Holes and Their Galactic Hosts

Kavli astrophysics institute researchers have leveraged the might of JWST to see the previously unobservable dim galaxies that host bright quasars

Rather like a heart beating inside a body, almost every galaxy known harbors a supermassive black hole in its core. And like animals, for whom heart and body sizes generally correlate—bigger animal, bigger heart—black holes and their host galaxies likewise proportionally match. Astrophysicists have long wondered about the origins of this shared scaling. Did galaxies and black holes start out small and grow larger together, or has the growth rate of one outpaced the other, and why?

The questions are not mere galactic evolutionary curiosities; comprehending the grander picture of the formation of all structure in the universe boils down to getting the story straight on how the earliest objects took their form as the cosmos cooled and expanded in the Big Bang's aftermath.

Key new insights into how this story unfolded are now being made. In a breakthrough study, researchers have reported seeing for the very first time host galaxies in the ultra-distant universe that harbor supermassive black holes. Star-filled galaxies like these have proved impossible to spot so far. That's because the comparatively dim galaxies have, counterintuitively, been lost in the glare of their hosted black holes. Monster black holes such as these actively devour matter, which blazes brightly before being swallowed up. The resultingly bright objects, dubbed quasars, have stood as the most distant objects humanity can discern. With distant quasars' host galaxies now also coming into view, researchers can newly hope to untangle the origins and evolutions of both supermassive black holes and starry galaxies.

Figure 1: JWST NIRCam 3.6 μm image of HSC J2236+0032. The zoom-out image, the quasar image, and the host galaxy image after subtracting the quasar light (from left to right). The image scale in light years is indicated in each panel. (Credit: Ding, Onoue, Silverman et al.)

"With these measurements, we achieve, for the first time, the comparison of the mass of the supermassive black hole to host stellar mass with two sources at an early stage of our universe," says Xuheng Ding, co-lead author of the study and a Project Researcher at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at The University of Tokyo. The measurements of these two galaxies, plus more expected soon, "will provide key information to understand the theory of the evolution of the galaxy and the growth of supermassive black holes," Ding adds.

"With this study, we are presenting the very first detection of starlight from distant quasars within the first billion years of the universe," says Masafusa Onoue, co-lead author of the study and an astrophysics fellow jointly affiliated with the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University and at Kavli IPMU.

Including Ding and Onoue, the new study—accepted by Nature and available as a preprint—has a total of five Kavli astrophysics institute members among its more than 40 international authors. These coauthors are John Silverman, a Professor at Kavli IPMU; Connor Hugh Bottrell, a project researcher also at Kavli IPMU; and Anna-Christina Eilers, a Pappalardo Fellow at the Massachusetts Institute of Technology (MIT) and a member of the MIT Kavli Institute for Astrophysics and Space Research (MKI).

JWST, the biggest telescope ever put into space and which launched in December 2022 after a quarter-century of development, made the new discoveries possible. Thanks to its 6.5-meter mirror, solar heat-blocking sunshield, and placement a million miles from Earth in an extremely cold orbital perch, the telescope can collect prodigious amounts of faint infrared light and see things humankind simply couldn't before. "JWST is currently the only instrument that can allow us to achieve the discoveries by the study," says Ding.

The two quasar-hosting galaxies captured by JWST have the designations J2255+0251 and J2236+0032, appear at the whopping distance of some 12.9 billion light years. In their bright, comparatively featureless quasar form, the objects were initially discovered in a deep optical survey by an instrument called the Hyper Suprime-Cam. The HSC, a high-tech camera installed on the Subaru Telescope in Hawaii, is a project with major Kavli IPMU involvement. For his part, Onoue has worked on HSC since he started his PhD while at the National Astronomical Observatory of Japan about 10 years ago.

"I feel very honored," says Onoue, about leading a "JWST project on HSC quasars and am very happy to have achieved the host starlight detection from the objects that we found with the Subaru telescope."

Unlike other ultra-distant quasar-hosting galaxies able to be studied to an extent so far, these two quasars are not extraordinarily bright. That's a good thing, because it means the quasars are not likely to be exceptions to the general principles governing galaxy and black hole evolution in their respective cosmic era. In many fields of science, researchers similarly seek to examine "ordinary" specimens to learn about the broad fundamentals in play for a given phenomenon, rather than only examining outliers, which via their extra-ordinariness may give skewed impressions.

Figuring out what is typical in the remote reaches of the early universe will be pivotal in answering mysteries these prior observations have already posed. Complicating any simple narrative of straightforward black hole and galaxy growth have been reports of ultra-bright quasars powered by tremendously big black holes in the very early universe, back when theorists expect such objects should still be fledgling. Researchers have accordingly been drawing up plausible ways for black holes to have grown up ultra-fast in the early universe, and perhaps not in proportion to their host galaxies.

The brand-new observations of the two early quasar-galaxy systems, however, suggest that there may be congruence in how each object matures. The bigger of the two black holes does "live" within the bigger of the two galaxies, the study found. Fortunately, JWST is expected to deliver many more early quasar-hosting galaxy observations to help settle the matter.

"Even with just the first two quasars, we may be seeing hints that black holes and their host galaxies are assembling in lockstep based on their masses as measured by JWST," says Silverman. "These results will place constraints on theoretical models of the early growth of supermassive black holes in our universe."

The two quasar-galaxy systems are but a mere sampling of the astrophysical treasures JWST is expected to ultimately deliver over its 10-plus-years lifetime. The ongoing research project with JWST that yielded the two systems has detected a total of 12 distant quasars, so more news is on the way. In addition, the researchers were recently awarded observing time on JWST in its second full year of operations to follow up on one of systems in detail. The research team looks forward to having more announcements to make in coming months about the far-flung reaches of the cosmos newly coming to light.