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Webb Space Telescope Discovers Early Galaxy Growth Fueled by Dramatic Merger

An international research group led by the Italian National Institute for Astrophysics observed the dramatic interaction between a quasar inside the PJ308–21 system and two massive satellite galaxies in the distant universe using the Near-Infrared Spectrograph on board the James Webb Space Telescope. The findings were published in the journal Astronomy & Astrophysics.

Webb Space Telescope Discovers Early Galaxy Growth Fueled by Dramatic Merger
Map of the line emission of hydrogen (in red and blue) and oxygen (in green) in the PJ308-21 system, shown after masking the light from the central quasar (“QSO”). The different colors of the quasar's host galaxy and companion galaxies in this map reveal the physical properties of the gas within them. Image Credit: Decarli/INAF/A&A 2024

The September 2022 observations revealed breathtaking details never before seen, offering fresh perspectives on the development of galaxies in the early cosmos.

Observations of this quasar, previously detailed by the same authors in a study published last May, reveal sensational data quality. Studied with NIRSpec when the universe was less than a billion years old (redshift z = 6.2342), the instrument captured the quasar’s spectrum with an uncertainty of less than 1 % per pixel.

While one of the satellite galaxies displays low metallicity (defined as the abundance of chemical elements heavier than hydrogen and helium) and photoionization caused by star formation, the host galaxy of PJ308–21 displays high metallicity and photoionization conditions typical of an Active Galactic Nucleus (AGN); the second satellite galaxy, which is partially photoionized by the quasar, has a higher metallicity.

This finding has allowed astronomers to calculate the mass of the supermassive black hole at the center of the system, which is around 2 billion solar masses. It also verified that the quasar and the nearby galaxies are continuously growing, highly developed objects in terms of mass and metal enrichment.

This has significant implications for understanding cosmic history and galaxies' chemical evolution, underscoring the transformative impact of this discovery. 

Our study reveals that both the black holes at the center of high-redshift quasars and the galaxies that host them undergo extremely efficient and tumultuous growth already in the first billion years of cosmic history, aided by the rich galactic environment in which these sources form.

Roberto Decarli, Researcher and Study First Author, Italian National Institute for Astrophysics

The data were collected in September 2022 as part of Program 1554, one of the nine Italian-led programs of the JWST's first observation cycle. Decarli leads this program and observes the merging of two satellite galaxies of the galaxy hosting the quasar (PJ308-21) with the host galaxy.

The observations were performed in integral field spectroscopy mode, where the expansion of the cosmos is observed to have pushed the spectra of the entire optical band (in the source rest frame) toward the infrared for each image pixel. This makes it possible to investigate different gas tracers, or emission lines, in three dimensions.

Using this method, the INAF-led team could identify spatially extended emissions from various elements. These emissions were then used to investigate various aspects of the ionized interstellar medium, such as metallicity, dust obscuration, star formation rate, electron density and temperature, and the source and hardness of the photoionizing radiation field.

 In addition, the researchers found traces of starlight emission linked to companion sources.

Thanks to NIRSpec, for the first time we can study in the PJ308-21 system the optical band, rich in precious diagnostic data on properties of the gas near the black hole in the galaxy hosting the quasar and in the surrounding galaxies. We can see, for example, the emission of hydrogen atoms and compare it with the chemical elements produced by the stars to establish how rich the gas in galaxies is in metals.

Federica Loiacono, Astrophysicist, Italian National Institute for Astrophysics

Loiacono said, “The experience in reducing and calibrating these data, some of the first collected with NIRSpec in integral field spectroscopy mode, has ensured a strategic advantage for the Italian community in managing similar data from other programs.”

At the INAF JWST Support Center, Federica Loiacono is the Italian point of contact for NIRSpec data reduction.

Federica Loiacono adds, “Thanks to the sensitivity of the James Webb Space Telescope in the near and medium infrared, it was possible to study the spectrum of the quasar and companion galaxies with unprecedented precision in the distant universe. Only the excellent ‘view’ offered by JWST, with its unparalleled capabilities, can ensure these observations.”

Decarli continued, “The work represented a real ‘emotional rollercoaster,’ with the need to develop innovative solutions to overcome the initial difficulties in data reduction.”

This revolutionary effect of the James Webb Space Telescope's onboard equipment highlights the telescope's vital significance in furthering astrophysics research.

Until a couple of years ago, data on the enrichment of metals (essential for understanding the chemical evolution of galaxies) were almost beyond our reach, especially at these distances. Now, we can map them in detail with just a few hours of observation, even in galaxies observed when the universe was in its infancy.

Roberto Decarli, Researcher and Study First Author, Italian National Institute for Astrophysics

Journal Reference:

Decarli, R., et al. (2024) A quasar-galaxy merger at z~6.2: Rapid host growth via the accretion of two massive satellite galaxies. Astronomy & Astrophysics.

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