According to observations and cosmological simulations, dwarf galaxies are the most abundant type of galaxies in the early Universe and the galaxy merger rate is dominated by dwarfs. However, these mergers are generally too distant to be directly observed, and dwarf-dwarf galaxy merger-related pairs of supermassive black holes are notoriously hard to find. In a new paper in the Astrophysical Journal, astronomers from the University of Alabama at Tuscaloosa present the first results of large-scale search for this elusive type of object and the first two candidates for dual supermassive black holes in dwarf-dwarf mergers. The dwarf galaxies detected by the researchers are on collision courses and are found in the galaxy clusters Abell 133 and Abell 1758S.
Two dwarf-dwarf galaxy mergers in galaxy clusters Abell 133 (left) and Abell 1758S (right) 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). Image credit: NASA / CXC / University of Alabama / Micic et al. / International Gemini Observatory / NOIRLab / NSF / AURA.
Galaxies are made up of billions of stars, interstellar gas and dust, and large amounts of dark matter.
Every (or almost every) galaxy is expected to host a supermassive black hole in its center.
Galaxies and their central black holes grow and evolve together predominantly through mergers; smaller objects merge to create larger ones over time. However, the earliest stages of galaxy evolution involving the mergers of the first galaxies are poorly understood.
It is unclear how the first mergers affected the morphology of ancient galaxies and their star formation.
Astronomers also do not know how massive the first black holes were that inhabited the first galaxies, nor how the first mergers influenced their ability to accrete material.
It is challenging to answer these important questions because the first mergers are too distant and faint to be directly observed.
One way to overcome this issue is to look for local analogs. In other words, scientists need to find pairs of small, dwarf galaxies that have had very quiet lives, with almost no mergers, that have only recently met and started interacting.
Such galaxies have experienced little to no evolution so they are analogs of distant, ancient galaxies, and observations of their mergers would represent the local case study that illustrates the hierarchical growth of structures in the early Universe.
Their central black holes are also expected not to have grown much and preserve information about primordial seeds, potentially holding the key to resolving the outstanding problem of the origin of supermassive black holes.
“Astronomers have found many examples of black holes on collision courses in large galaxies that are relatively close by,” said Dr. Marko Micic, an astronomer at the University of Alabama at Tuscaloosa.
“But searches for them in dwarf galaxies are much more challenging and until now had failed.”
Dr. Micic and colleagues overcame the challenges by implementing a systematic survey of deep Chandra X-ray observations and comparing them with infrared data from NASA’s Wide Infrared Survey Explorer (WISE) and optical data from the Canada-France-Hawaii Telescope (CFHT).
They searched for pairs of bright X-ray sources in nearby pairs of colliding dwarf galaxies as evidence of two black holes and discovered two examples.
One pair of dwarf galaxies (left image) is in Abell 133, a galaxy cluster located 760 million light-years away from Earth.
This pair appears to be in the late stages of a merger, and shows a long tail caused by tidal effects from the collision.
The authors have nicknamed it ‘Mirabilis’ after an endangered 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 other pair (right image) was discovered in Abell 1758S, a galaxy cluster about 3.2 billion light-years away.
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.
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.
“Most of the dwarf galaxies and black holes in the early Universe are likely to have grown much larger by now, thanks to repeated mergers,” said Dr. Brenna Wells, also from the University of Alabama at Tuscaloosa.
“In some ways, dwarf galaxies are our galactic ancestors, which have evolved over billions of years to produce large galaxies like our own Milky Way.”
“Follow-up observations of these two systems will allow us to study processes that are crucial for understanding galaxies and their black holes as infants,” added Dr. Jimmy Irwin, also from the University of Alabama at Tuscaloosa.
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Marko Mićić et al. 2023. Two Candidates for Dual AGN in Dwarf-Dwarf Galaxy Mergers. ApJ, in press; arXiv: 2211.04609
