Black holes are the most powerful destructive forces in the universe. They can tear apart a star and scatter its ashes out of the galaxy at almost the speed of light. But these destruction machines can also pave the way for new stars to form, a new study in Nature shows.
The study explores the question of why some galaxies actively create stars while others do not. In the Milky Way, for example, an average of 1 to 2 new stars are created each year. Other galaxies, known as starburst galaxies, have extremely active star-producing regions. But in some galaxies there is almost no star production. These “erased” galaxies are often from smaller satellite galaxies to larger galaxies.
It is widely believed that satellite galaxies are often erased because they are more susceptible to intergalactic winds. This diffuse intergalactic gas can flow through these small galaxies, clearing them of gas and dust, thereby removing the material needed to form new stars.
Our galaxy has gas bubbles that can be seen in gamma ray light. Photo credit: NASA Goddard
Since these small galaxies often orbit larger ones, they would also be affected by the gas flow of active supermassive black holes within the main galaxy. The team found that satellite galaxies that pass through the flow area of the black hole are more extinguished than those outside the flow because the flow of a black hole should be more effective at removing matter from the satellite galaxy. However, when they looked at the star production of these galaxies, they found that it was slightly higher for regions in the current. It turns out that supermassive black holes actually help increase star production in satellite galaxies.
Based on computer simulations, the team believes they know why. An active supermassive black hole can remove gas and dust from a region, and this creates sparse regions or bubbles near its galaxy. These bubble regions are even less dense than the surrounding intergalactic regions. So when a satellite galaxy is in this bubble, less gas and dust is removed and it can make stars more easily.
This study is a great example of the complexity of galactic dynamics and how the behavior of one galaxy can affect the formation of stars in another. It’s also a great example of testing your hypotheses even if you think the answer is obvious. Sometimes the answer is completely unexpected and that’s when we can learn the most.
Reference: Martin-Navarro, Ignacio, et al. “Anisotropic satellite galaxy extinction, modulated by the activity of black holes.” Nature 594.7862 (2021): 187-190.