A galaxy with a black hole smaller than expected

The galaxy RX J1140.1+0307 (Photo ESA/Hubble & NASA. Acknowledgement: Judy Schmidt)
The galaxy RX J1140.1+0307 (Photo ESA/Hubble & NASA. Acknowledgement: Judy Schmidt)

A photo taken by the Hubble Space Telescope shows the galaxy RX J1140.1+0307 and apparently it’s a spiral galaxy like there are a lot, including the Milky Way. However, normally these galaxies have at their center a supermassive black hole, instead RX J1140.1+0307 has a smaller, intermediate-mass black hole. This is an anomaly and astronomers are looking for an explanation.

Supermassive blacks holes have masses ranging from hundreds of thousands to billions of solar masses. The mechanisms that lead to their birth are not yet clear but an article published in the journal “Monthly Notices of the Royal Astronomical Society” in May 2016 describes the discovery of some candidate “seeds”. Their study will certainly help to understand how they form.

Researchers have been finding supermassive black holes at the center of most known galaxies but there are some exceptions. The galaxy RX J1140.1+0307 is among them because at its center there’s an intermediate-mass black hole, a category that includes black holes with masses ranging from one hundred to one million solar masses.

Intermediate-mass black hole were observed and then studied only in recent years. There are various hypotheses about their formation mechanisms, from the collisions of massive stars to normal black holes mergers to the possibility that they are primordial objects formed in the Big Bang. There are no answers and it’s possible that more than one of the hypotheses is correct.

A further problem in the study of the intermediate-mass black hole at the center of galaxy RX J1140.1+0307 is that it doesn’t respect the models for the emission coming from it. One hypothesis is that there are at work other interaction mechanisms between the inner and the outer parts of the accretion disk surrounding the black hole that generate the spectrum of the detected emissions.

In essence, there are many hypotheses about intermediate-mass black holes but at the moment no certainties. Some studies are focusing on ultraluminous X-ray sources (ULXs), another phenomenon of which the nature is unknown and one of the hypotheses is that those are intermediate-mass black holes.

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