Scientists have discovered how jets launched by supermassive black holes can determine how their host galaxies "die." The research answers the long-standing question of why galaxies don't have more stars.
After all, large galaxies, including the Milky Way, are surrounded by huge cocoons of gas and dust called the circumgalactic medium, and that material serves as the raw building blocks for stars. During intense star-forming phases, this gas moves into galaxies and fuels this process, but eventually something stops this material from cooling, clumping, and birthing stars, or from moving into galaxies at all, effectively "killing" them.
This research suggests that when feeding supermassive black holes blast jets of plasma from around them, this reaches far beyond the center of galaxies where these cosmic titans reside and beyond their outer limits, thus disrupting the gas around galaxies. This has a major effect on how galaxies form stars and thus how they grow and evolve.
"This is a pathbreaking result that solves the long-standing mystery of how black holes influence galaxies, their stars, and life as we know it," team co-leader Sanchayeeta Borthakur of Arizona State University said in a statement. "This work opens a new direction to explore further the intricacies of the connection between the supermassive black holes trillions of miles from where we are to how we came to be here."
Black holes maybe small (relatively) but they pack a punch
Ok, supermassive black holes can't exactly be called "small," packing a mass millions or billions of times that of the sun into a width about that of the solar system. But compared to galaxies, which can host 100 billion similarly sized star systems, even these cosmic titans are relatively small in stature.
Even though feeding black holes can pour vast amounts of energy back into their cosmic environments via jets and radiation, the effect they have is still somewhat perplexing.
"The surprising question is: How can something so small energetically impact something so enormous?", team co-leader Namrata Roy of the Raman Research Institute said.
Roy and Borthakur focused their investigation on feeding or active black holes, using observations of hundreds of galaxies with active jets, collected by the Dark Energy Spectroscopic Instrument (DESI) survey and radio jet measurements from the LOFAR Two-meter Sky Survey (LoTSS). They found points at which ionized gas glows brightly along their jets. This represents the plasma hitting and passing through clumps of surrounding gas, energizing it.
This indicates that black hole jets don't influence gas around galaxies in a uniform manner, but at preferred points. The greatest impact was seen close to the outer edges of galaxies, where plasma jets first hit the circumgalactic medium, and at the outer edge of the circumgalactic medium itself. That latter region is where the jets release the most energy.
The team's findings reveal clear fingerprints of the impact black hole jets have in disrupting potential star-forming material even right out to the circumgalactic medium. That's key in understanding how star formation is cut off and how galaxies are killed.
"What excites me most is the scale of the connection," Roy said. "A black hole is incredibly small compared to a galaxy, but its impact can reach hundreds of thousands of light-years, far into the galaxy’s outer reaches. The jet carries the energy outward, and the gas lights up along its path."
The findings also offer a new way for scientists to test the impact of black hole jets on galaxies.
The team's research was published in the Astrophysical Journal Letters.
