The galaxy Was 49 is the result of an unusual merger

The galaxy Was 49 (Image DCT/NRL)
The galaxy Was 49 (Image DCT/NRL)

A galaxy merger observed with NASA’s NuSTAR space telescope gave surprising results. The galaxy called Was 49 is being formed from the fusion of a large disk galaxy called Was 49a and a dwarf galaxy called Was 49b. The researchers were surprised when they realized that the supermassive black hole at the center of the dwarf galaxy was much bigger and more powerful than expected, going against current models regarding galactic mergers.

There’s a significant difference between the size of the two galaxies that are merging, so much that the galaxy Was 49a is basically absorbing the dwarf galaxy Was 49b, which is spinning around the inside of its companion’s disk at about 26,000 light years from its center.

In such a case, the astronomers expected that the supermassive black hole at the center of the larger galaxy was the one to become active swallowing the gas and dust pushed by the new gravitational interactions. Instead, it’s the other one that shows that kind of activities.

In the dwarf galaxy Was 49b there’s a quasar, a type of active galactic nucleus in which the central supermassive black hole is so active as to become very bright thanks to gas and dust that fall towards it. It has a mass that’s more than 2% of the entire dwarf galaxy, a percentage that may seem small but is actually much greater than the researchers expected in that kind of galaxy.

To better understand the characteristics of the supermassive black hole, the researchers also used NASA’s Chandra and Swift space telescopes obtaining additional information in particular on its X-ray emissions. The Discovery Channel Telescope in Arizona allowed to obtain observations at optical frequencies of the Was 39 system with colors optimized to separate the highly ionized gas emissions.

Distinguishing the light of the regions around the black hole from the light of the normal stars allowed to accurately determine the size of dwarf galaxy Was 49b. The intense ionizing radiation emitted by the active galactic nucleus are represented in pink while the starlight is represented in green. The light pink area is probably a part of the dwarf galaxy Was 49b.

One of the main questions about the supermassive black hole is whether its mass was already so high before the start of the galaxy merger or it grew in its initial phase. Finding the answer would help to understand the evolution of supermassive black holes in galactic mergers.

The observations carried out with the NuSTAR space telescope, launched in June 2012, are important because they provide details on the activities of the supermassive black hole at the center of the dwarf galaxy Was 49b. The researchers can take their time in their follow-up observations because the new galaxy and its two supermassive black holes will keep on evolving for a few more hundred million years.

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