Webb captures stunning quasar-galaxy merger in distant universe

An international research group led by the Italian National Institute of Astrophysics (INAF) and including 34 research institutes and universities from around the world used the Near Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST) to observe the spectacular interaction between a quasar inside the PJ308-21 system and two massive satellite galaxies in the distant Universe.

The observations, made in September 2022, revealed unprecedented and impressive details, offering new insights into the growth of galaxies in the early universe. The results, presented on July 5 at the European Astronomical Society meeting (EAS 2024) in Padua, Italy, will soon be published in Astronomy and astrophysics.

Observations of this quasar (already described by the same authors in another study published last May), one of the first studied with NIRSpec when the universe was less than a billion years old (redshift z = 6.2342), revealed data of sensational quality: the instrument “captured” the spectrum of the quasar with an uncertainty of less than 1% per pixel.

The host galaxy of PJ308–21 exhibits high metallicity and photoionization conditions typical of an active galactic nucleus (AGN), while one of the satellite galaxies exhibits low metallicity (which refers to the abundance of chemical elements heavier than hydrogen and helium) and photoionization induced by star formation; a higher metallicity characterizes the second satellite galaxy, which is partially photoionized by the quasar.

This discovery allowed astronomers to determine the mass of the supermassive black hole at the center of the system (about 2 billion solar masses). It also confirmed that the quasar and surrounding galaxies are highly evolved in terms of mass and metal enrichment, and constantly growing.

This has profound implications for our understanding of cosmic history and the chemical evolution of galaxies, underscoring the transformative impact of this research.

Roberto Decarli, a researcher at INAF in Bologna and first author of the article, explains: “Our study reveals that black holes at the center of high redshift quasars and the galaxies that host them undergo extremely efficient and tumultuous growth from the first billion years of cosmic history, aided by the rich galactic environment in which these sources form.”

The data were obtained in September 2022 as part of the 1554 program, one of the nine Italian projects of the first JWST observation cycle. Decarli leads this program to observe the merger between the galaxy hosting the quasar (PJ308-21) and two of its satellite galaxies.

The observations were carried out in full-field spectroscopy mode: for each pixel of the image, the spectrum of the entire optical band (in the rest frame of the source) can be observed, shifted towards the infrared by the expansion of the universe. This allows the study of different gas tracers (emission lines) using a 3D approach.

Using this technique, the INAF-led team detected spatially extended emissions from different elements, which were used to study the properties of the ionized interstellar medium, including the source and hardness of the photoionizing radiation field, metallicity, dust obscuration, electron density and temperature, and star formation rate.

Additionally, the researchers marginally detected starlight emission associated with companion sources.

Federica Loiacono, astrophysicist, researcher and postdoctoral fellow at INAF, says: “Thanks to NIRSpec, for the first time we can study in the PJ308-21 system the optical band, rich in valuable diagnostic data on the properties of the gas near the black hole in the galaxy hosting the quasar and in the surrounding galaxies.

“We can observe, for example, the emission of hydrogen atoms and compare it to the chemical elements produced by stars to establish how rich the gas in galaxies is in metals.

“The experience gained in the reduction and calibration of these data, among the first collected with NIRSpec in full-field spectroscopy mode, has ensured a strategic advantage to the Italian community in the management of similar data from other programs.” Loiacono is the Italian contact person for NIRSpec data reduction at the INAF JWST Support Center.

She adds: “Thanks to the sensitivity of the James Webb Space Telescope in the near and mid-infrared, it has been possible to study the spectra of quasars and companion galaxies with unprecedented precision in the distant universe. Only the excellent “view” offered by the JWST, with its unmatched capabilities, can guarantee these observations.”

The work represented a real “emotional roller coaster,” Decarli continues, “with the need to develop innovative solutions to overcome the initial difficulties of data reduction.”

The transformative impact of the instruments aboard the James Webb Space Telescope underscores its crucial role in advancing astrophysical research.

“A few years ago, data on metal enrichment (essential for understanding the chemical evolution of galaxies) were almost out of our reach, especially at these distances. Today, we can map them in detail in a few hours of observation, even in galaxies observed when the universe was still in its infancy,” concludes Decarli.