NASA tracks supermassive black holes in their collision cycle

• Milky Way, likely formed larger galaxies through collisions in the early Universe.
• These newly-discovered merging dwarf galaxies can be used as analogs for more distant ones that are too faint to observe.
• The dwarf galaxies are on collision courses and are found in the galaxy clusters Abell 133 and Abell 1758S.

Evidence for two pairs of supermassive black holes in dwarf galaxies on collision courses has been found with Chandra. The two pairs are shown in X-rays from Chandra and optical light from the Canada-France-Hawaii telescope. The merger on the left is in a late stage and was given the single name of Mirabilis. The other merger is in the early stages and the two dwarf galaxies are named Elstir (bottom) and Vinteuil (top). Astronomers think that dwarf galaxies – those about 20 times less massive than the Milky Way – grow through mergers with others. This is an important process for galaxy growth in the early Universe and this discovery provides examples for scientists to study in greater detail. Credit: X-ray: NASA/CXC/Univ. of Alabama/M. Micic et al.; Optical: International Gemini Observatory/NOIRLab/NSF/AURA

A new study using NASA’s Chandra X-ray Observatory has tracked two pairs of supermassive black holes in dwarf galaxies on collision courses. This is the first evidence for such an impending encounter, providing scientists with important information about the growth of black holes in the early Universe.

By definition, dwarf galaxies contain stars with a total mass less than 3 billion Suns — or about 20 times less than the Milky Way. Astronomers have long suspected that dwarf galaxies merge, particularly in the relatively early Universe, in order to grow into the larger galaxies seen today. However, current technology cannot observe the first generation of dwarf galaxy mergers because they are extraordinarily faint at their great distances. Another tactic — looking for dwarf galaxy mergers closer by — had not been successful to date.

The new study overcame these challenges by carrying out a systematic survey of Chandra’s deep X-ray observations and comparing them with infrared data from NASA’s Wide Infrared Survey Explorer (WISE) and optical data from the Canada, France and Hawaii Telescope (CFHT).

Chandra was particularly valuable in this study because the material surrounding black holes can be heated to millions of degrees, giving off large amounts of X-rays. The team looked for pairs of bright X-ray sources in colliding dwarf galaxies as evidence of two black holes, and discovered two examples.

Evidence for a pair of supermassive black holes has been found in dwarf galaxies in collision cycles with Chandra. The pair is seen in X-rays from Chandra and optical light from the Canada-France-Hawaii Telescope. The merger on the left is at a late stage and has been given the individual name of Mirabilis. The other merger is in its early stages and the two dwarf galaxies have been named Elstir (bottom) and Vinteuil (top). Astronomers believe that dwarf galaxies – those 20 times smaller than the Milky Way – grow through mergers with others. This is an important process for the growth of galaxies in the early universe and this discovery provides examples for scientists to study in more detail. Credit: X-rays: NASA/CXC/Univ. Alabama / m. Micic et al.; Optical: Gemini Observatory International/NOIRLab/NSF/AURA

One pair is in the Abell 133 galaxy cluster located 760 million light-years from Earth, seen in the composite image at left. Chandra X-ray data in pink and optical data from CFHT in blue. This pair of dwarf galaxies appears to be in the late stages of a merger, exhibiting long tails caused by tidal effects from the collision. The authors of the new study named it Mirabilis, after it has become critically endangered[{” attribute=””>species of hummingbird known for their exceptionally long tails. Only one name was chosen because the merger of two galaxies into one is almost complete. The two Chandra sources show X-rays from material around the black holes in each galaxy.

X-ray and optical composite of Mirabilis. Credit: X-ray: NASA/CXC/Univ. of Alabama/M. Micic et al.; Optical: International Gemini Observatory/NOIRLab/NSF/AURA

The other pair was discovered in Abell 1758S, a galaxy cluster about 3.2 billion light-years away. The composite image from Chandra and CFHT is on the right, using the same colors as for Mirabilis. The researchers nicknamed the merging dwarf galaxies “Elstir” and “Vinteuil,” after fictional artists from Marcel Proust’s “In Search of Lost Time”. Vinteuil is the galaxy on the top and Elstir is the galaxy on the bottom. Both have Chandra sources associated with them, again from X-rays from material around the black holes in each galaxy. The researchers think these two have been caught in the early stages of a merger, causing a bridge of stars and gas to connect the two colliding galaxies from their gravitational interaction.

X-ray and optical composite of Elstir & Vinteuil. Credit: X-ray: NASA/CXC/Univ. of Alabama/M. Micic et al.; Optical: International Gemini Observatory/NOIRLab/NSF/AURA

The details of merging black holes and dwarf galaxies may provide insight to our Milky Way’s own past. Scientists think nearly all galaxies began as dwarf or other types of small galaxies and grew over billions of years through mergers. Follow-up observations of these two systems will allow astronomers to study processes that are crucial for understanding galaxies and their black holes in the earliest stages of the Universe.

A paper describing these findings was published in a recent issue of the[{” attribute=””>Astrophysical Journal.

Reference: “Two Candidates for Dual AGN in Dwarf-Dwarf Galaxy Mergers” by Marko Mićić, Olivia J. Holmes, Brenna N. Wells and Jimmy A. Irwin, 22 February 2023, The Astrophysical Journal.
DOI: 10.3847/1538-4357/aca1bb

The authors of the study are Marko Micic, Olivia Holmes, Brenna Wells, and Jimmy Irwin, all from the University of Alabama at Tuscaloosa.

NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.