Astronomers Measure The Mass Of A Black Hole At The Center Of A Dwarf Galaxy

NASA's Spitzer Space Telescope captures galaxy NGC 4395 in infrared light. NGC 4395 is about 1,000 times smaller than the Milky Way. NASA

Astronomers believed that at the core of every galaxy there is a supermassive black hole, and the presence of a black hole has an impact on the galaxy. There is a relationship between the movement of the center of the galaxy and the mass of the black hole, but the origin of this relationship remains unclear.

It is also unclear if this applies to every galaxy (or just the ones the same size or bigger than the Milky Way). Sure, the larger ones show this relationship clearly, but what about small dwarf galaxies? As reported in Nature Astronomy, an international team of researchers has now measured the mass of the central black hole in the dwarf galaxy NGC 4395, located 14 million light-years away, and discovered it is smaller than they had thought.

The supermassive black hole is now estimated to weigh “only” 10,000 times the mass of the Sun, so more of an intermediate-mass black hole than a supermassive one.

For comparison, Sagittarius A*, the black hole at the center of the Milky Way, is 4.6 million times the mass of our Sun. But it seems that size doesn’t matter. Even the diminutive black hole of NGC 4395 respects the relation with its stars, known as the M-sigma relation – the observed correlation that the mass of the black hole is proportional to the range of velocity of stars around the galactic bulge.

"There's no reason why stars that live at orders of magnitude larger than the area where black hole gravity dominates should even know that there's a black hole in their galaxy, but somehow they do," co-author Dr Elena Gallo, from the University of Michigan, said in a statement. "Black holes somehow shape the galaxy they live in on very large scales, and because we don't know much about smaller galaxies with their smaller black holes, we don't know whether that's true all the way down. With this measurement, we can add more information to this relationship."

The measurements were possible because the massive black hole in NGC 4395 is feeding on material. It is surrounded by an accretion disk of hot plasma, which spins around it very fast. Researchers were able to measure a light echo between the radiation leaving the black hole and reaching a gas cloud that is also orbiting the black hole. This allowed them to estimate the mass with high precision.

Dwarf galaxies are unexplored territory when it comes to the M-sigma relation. Researchers are not even sure if all dwarf galaxies have a black hole. So this research begins to fill an important gap in understanding how smaller galaxies evolve. In turn, we might learn how the larger galaxies, like our own, might have looked like when they formed.

 

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