Category: dwarf galaxies

The Smallest Galaxies Have Off-Kilter Black Holes, But Astronomers Know Why

“Over 100 dwarf galaxies are now known to possess these black holes, with the first verified one discovered in 2011. However, solely finding radio emissions isn’t enough: active black holes and star-formation bursts can create that signal. Researchers led by Dr. Amy Reines just conducted the first large-scale radio survey looking for black holes in dwarf galaxies. Using the Very Large Array, her team surveyed 111 dwarf galaxies, and found 13 of them that showed evidence for massive black holes. Remarkably, approximately half of the black holes were not located at the galaxy’s centers, but were significantly off-kilter.”

When we examine the supermassive black holes we find in the Universe, they’re pretty much always found at the centers of galaxies. However, these are for black holes of millions-to-billions of solar masses and galaxies comparable in mass (or even greater than that) to the Milky Way. But dwarf galaxies, the majority of galaxies in the Universe, are predicted to have much smaller black holes. The first large survey of these galaxies was just undertaken, revealing a population of dwarf galaxies with black holes. 

But half of them are located off-center, rather than at the center! Why is that? Astronomers know, and you can too!

Modified Gravity Could Soon Be Ruled Out, Says New Research On Dwarf Galaxies

“The fact that these two galaxies exhibit such different gravitational effects tell us that either something is very funny with one of them (something must be out-of-equilibrium), or that dark matter gets heated up by star formation and modified gravity cannot explain this. As always, more data, additional galaxies, and further research will be required to solve this mystery, but at long last, we’re looking at a viable way to prove modified gravity wrong on galaxy scales. Even without directly detecting a particle, dark matter might just achieve a knockout blow over its greatest competing alternative.”

If you have two galaxies in the Universe that look the same, you’d expect them to behave the same. After all, the laws that govern them ought to be identical, and so if their properties are identical, so should their behavior. The Draco and Carina dwarf galaxies are roughly the same mass, the same size, and have the same distribution of starlight. The only discernible difference is that one galaxy has only old stars, while the other has a mix of old and new stars. And yet, when we look at the gravitational effects of the mass on the stars, their behaviors are incredibly different. One seems to indicate a large unseen mass source in the center, and the other doesn’t.

In modified gravity, this makes no sense. But in dark matter theories, simple heating due to star formation could explain it all. Keep your ear to the ground, because this could lead to the death-knell for modified gravity!