Astronomers on Maunakea capture starlight images from 2 massive galaxies hosting growing black holes
An international team of scientists, including staff astronomer Chien-Hsiu Lee at W. M. Keck Observatory on Maunakea, has captured first time images of starlight from two massive galaxies hosting actively growing black holes, or quasars, from less than a billion years after the Big Bang.
The successful detection of these host galaxies represents the universe’s earliest period of time at which light from stars has been detected around a quasar.
These black holes have masses close to a billion times that of the Sun, and the ratio of the black hole mass to host galaxy mass is similar to those seen in the more recent universe.
Initially discovered in a deep survey program of the Subaru Telescope on Maunakea, the two quasars were then captured by the James Webb Space Telescope. This powerful combination of ground-based observations from the Subaru Telescope and space-based observations from the Webb telescope has paved a new path to study the distant universe.
The study — published in the June 28, 2023 online issue of the journal Nature — was led by project researcher Xuheng Ding with Kavli Institute for the Physics and Mathematics of the Universe, professor John Silverman with Peking University Kavli Institute for Astronomy and Astrophysics and Kavli astrophysics fellow Masafusa Onoue.
“This is the first time we’ve seen host galaxies from such an early age of the universe,” said Lee, co-author of the study. “It is only possible thanks to [James Webb Space Telescopeʻs] deep images, which enable us to model and subtract the light from the quasar to reveal the host galaxy. We’ve seen quasars from this age previously, but they were so bright it was impossible to subtract their light to reveal the host galaxy.”
Studying host galaxies and black holes in the early universe allows scientists to watch their formation and see how they are related to one another. Quasars are luminous while their host galaxies are faint, which has made it challenging for researchers to detect the dim light of the galaxy in the glare of the quasar, especially at great distances.
Before the Webb telescope, the Hubble Space Telescope was able to detect host galaxies of luminous quasars when the universe was just under 3 billion years old, but no younger.
The superb sensitivity and ultra-sharp images of the Webb telescope at infrared wavelengths has finally allowed researchers to push these studies to the time when quasars and galaxies first formed.
Just a few months after the Webb telescope started regular operations, the team observed two quasars, HSC J2236+0032 and HSC J2255+0251, at redshifts 6.40 and 6.34 when the universe was approximately 860 million years old. Both were discovered using Subaru Telescope’s deep survey program. The relatively low luminosities of these quasars made them prime targets for measuring the properties of their host galaxies.
The images of the two quasars were taken at infrared wavelengths of 3.56 and 1.50 microns with the Webb telescope’s NIRCam instrument. The host galaxies became apparent after carefully modeling and subtracting glare from the accreting black holes.
The stellar signature of the host galaxy also was seen in a spectrum taken by the Webb telescope’s NIRSPEC for J2236+0032, further supporting the detection of the host galaxy.
Photometric analyses found that these two quasar host galaxies are massive, one measuring 130 billion times and the other measuring 34 billion times the mass of the Sun. Measuring the speed of the turbulent gas in the vicinity of the quasars from the NIRSPEC spectra suggests the black holes that power them are also massive, measuring 1.4 and 0.2 billion times the mass of the Sun.
The ratio of the black hole to host galaxy mass is similar to those of galaxies in the more recent past, suggesting that the relationship between black holes and their hosts was already in place 860 million years after the Big Bang.
“I’m excited to see powerful ground-based and space-based telescopes working together to tackle these challenges,” Lee said. “Along with the Subaru Telescope, we will also be using Keck Observatory’s MOSFIRE instrument to identify similar targets that [the Webb Telescope] can observe and enlarge the sample of ancient galaxies hosting quasars in the early universe.”
The team of astronomers will continue this study using scheduled Cycle 1 Webb telescope observations, which will further inform models for the coevolution of black holes and their host galaxies.