‘Engine’ of luminous merging galaxies pinpointed for the first time

Roughly 500 million light-years away, near the constellation Delphinus, two galaxies are colliding. Known as merging galaxy IIZw096, this luminous phenomenon is obscured by cosmic dust, but researchers first identified a bright, energetic source of light 12 years ago. Now, with a more advanced telescope — the James Webb Space Telescope that started its observations in July 2022 — the team has pinpointed the precise location of what they have dubbed the “engine” of the merging galaxy.

They published their results on Nov. 15, 2022, in The Astrophysical Journal Letters.

“The James Webb Space Telescope has brought us completely new views of the universe thanks to it having the highest ever spatial resolution and sensitivity in the infrared,” said corresponding author Hanae Inami, assistant professor at Hiroshima University’s Hiroshima Astrophysical Science Center. “We wanted to find the ‘engine’ that powers this merging galaxy system. We knew that this source was deeply hidden by cosmic dust, so we could not use visible or ultraviolet light to find it. Only in the mid-infrared, observed with the James Webb Space Telescope, do we now see that this source outshines everything else in these merging galaxies.”

As galaxies merge, their stars, planets and other constituents can smash into one another, the debris serving as fodder for new celestial episodes. Most of these galactic collisions only emit infrared light, which has longer wavelengths than light visible to humans and is beyond the scope of human perception. In 2010, using the Spitzer Space Telescope, the same team found that the merging system was dominated by bright infrared emission. The researchers could measure the power of the engine — the source of such brightness — but could not identify its exact location due to the telescope’s limited resolution.

With the James Webb Space Telescope, they found that this engine is responsible for the bulk of the mid-infrared emission, which accounts for up to 70% of the total infrared emission of the system. They also found that the source has a radius no larger than 570 light years — a tiny fraction of the size of the merging system, which is about 65,000 light years across. This indicates that the energy is confined to a small space, according to co-author Thomas Bohn at Hiroshima University.

“It is intriguing that this compact source, far from the galactic centers, dominates the infrared luminosity of the system,” Bohn said.

According to Bohn, this source makes a significant contribution to the merging galaxies despite lying in the outskirts, like a speck of pepper on the white of a fried egg.

“We want to know what powers this source: is it a starburst or a massive black hole?” Inami asked. “We will use infrared spectra taken with the James Webb Space Telescope to investigate this. It is also unusual that the ‘engine’ lies outside of the main parts of the merging galaxies, so we will explore how this powerful source ended up there.”

Co-author Jason Surace of the California Institute of Technology said that the finding supports more recently developed understandings of the universe and how it changes.

“The last few decades, driven by new, mostly space-based observations in the infrared, have shown that the universe is a surprisingly dynamic and violently changing place,” Surace said. “In times past, it was thought that the galaxies — the largest things we knew of — simply spun essentially unchanging, like celestial temples in the heavens.”

In addition to identifying the location of the engine, the researchers found 12 “clumps” of light. While some of these were previously revealed by the near-infrared capabilities of the Hubble Space Telescope, five were newly detected with the James Webb Space Telescope. These, Inami said, are emitting mid-infrared colors that suggest they are forming stars.

“The James Webb Space Telescope mid-infrared imaging described in this paper revealed a hidden aspect of the merging galaxy IIZw096 and opened a door towards identifying heavily dust-obscured sources which cannot be found at shorter wavelengths,” Inami said. “Future planned spectroscopic observations of IIZw096 will provide additional information on the nature of the dust, ionized gas, and warm molecular gas in and around the disturbed region of this luminous merging galaxy.”

This work was conducted as a part of the JWST Early Release Science (ERS) Program of the Great Observatory All-sky LIRG Survey (GOALS) project, which targets four local merging galaxies, including IIZw096.

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Other contributors include Thomas Bohn and Shunshi Hoshioka, Hiroshima Astrophysical Science Center, Hiroshima; Jason Surace, Lee Armus, Joseph M. Mazzarella, Justin H. Howell, Philip Appleton and Thomas Lai, IPAC, California Institute of Technology; Aaron S. Evans, Loreto Barcos-Munoz, Yiqing Song, George C. Privon and Eric J. Murphy, National Radio Astronomy Observatory, United States; Kirsten L. Larson, AURA for the European Space Agency (ESA), Space Telescope Science Institute, Maryland; Sabrina Stierwalt, Physics Department, Occidental College, California; Sean T. Linden, Department of Astronomy, University of Massachusetts at Amherst; Christopher C. Hayward, Center for Computational Astrophysics, Flatiron Institute; Torsten Bånoker, ESA, Space Telescope Science Institute; Vivian U and Matthew A. Malkan, Department of Physics and Astronomy, University of California; Vassilis Charmandaris and Tanio Diaz-Santos, the Institute of Astrophysics, Foundation for Research and Technology-Hellas, in Greece and the School of Sciences, European University Cyprus, Cyprus; Anne M. Medling, Department of Physics & Astronomy and Ritter Astrophysical Research Center, University of Toledo, Ohio and the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D); Jeffrey A. Rich, The Observatories of the Carnegie Institute for Science, California; Susanne Aalto, Department of Space, Earth and Environment, Chalmers University of Technology in Sweden; Michael J. I. Brown, School of Physics & Astronomy, Monash University, Australia; Kazushi Iwasawa and Francisca Kemper, ICREA, Spain; David Law, Space Telescope Science Institute in Maryland; Jason Marshall, Glendale Community College, California; David Sanders, Institute for Astronomy, University of Hawaii; and Paul van der Werf, Leiden Observatory, Leiden University, Netherlands.

These authors have additional affiliations: Evans, Barcos-Munoz and Song with the Department of Astronomy, University of Virginia; Privon with the Department of Astronomy, University of Florida; Charmandaris with the Department of Physics, University of Crete, Greece; Iwasawa with the Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, Spain; and Kemper with the Institut de Ciències de l'Espai and the Institut d'Estudis Espacials, both in Spain.

The Japan Society for the Promotion of Science, the National Science Foundation, the Grote Reber Fellowship Program administered by the Associated Universities, Inc./National Radio Astronomy Observatory, the Simons Foundation, NASA, the European Research Council, the Swedish Research Council, the Knut and Alice Wallenberg Foundation and Spain’s Ministry for Science and Innovation supported this work.

About Hiroshima University

Since its foundation in 1949, Hiroshima University has striven to become one of the most prominent and comprehensive universities in Japan for the promotion and development of scholarship and education. Consisting of 12 schools for undergraduate level and 4 graduate schools, ranging from natural sciences to humanities and social sciences, the university has grown into one of the most distinguished comprehensive research universities in Japan. English website: https://www.hiroshima-u.ac.jp/en