Astronomers should search the smallest of galaxies for black holes weighing thousands of solar mass, according to a proposal developed by Marta Volonteri in 2008. She thought that the objects could teach us how the universe formed.
Big black holes weren't supposed to exist in small galaxies. The galaxies seemed to lack the strength needed to pack mass into a black hole. It sounded like Volonteri and her team were suggesting the same thing as finding a brontosaurus in a bathtub.
Volonteri said that black holes in dwarf galaxies were not possible.
Astronomers began to find the beasts. Powerful telescopes and innovative observational strategies have allowed researchers to more closely scrutinize dwarf galaxies. Black holes pop up when they do. The discoveries show how little astrophysicists know about black holes, and how difficult it is to explain where they all came from. The universe's earliest, and biggest, black holes may be explained by an increasingly accurate tally.
Ryan Hickox, an astronomer at Dartmouth College who recently helped locate one himself, said that people keep finding more of them.
Dwarf galaxies are new to astronomy. They are 10 to 100 times lighter than the Milky Way, and they don't have the moxie to pull themselves into the round shapes. They are patchy, dim and hard to study in detail.
The secrets of the stars are hidden by the tiny tangles. The researchers believe that the patchwork products of more than 10 billion years of mashups are the galaxies like the Milky Way. They are small because they haven't had many chances to collide yet, or because they have dodged encounters with other galaxies.
dwarf galaxies are similar to the earliest galaxies. The first stars and black holes were created in the first billion years of the universe. The black holes of those galaxies merged over time. The first black holes grew into the monsters boasting billions of solar mass that seem to sit at the heart of every large galaxy today through some mixture of mergers and gobbling up matter.
dwarf galaxies have less matter to feed their black holes. Volonteri and Natarajan reasoned that these unique conditions would be more likely to freeze black holes. If that is the case, dwarf galaxies have large black holes.
Black holes are fed by the jets of bright light they emit, which drown out the surrounding stars. At the time of Volonteri and Natarajan's proposal, researchers hadn't seen clear signs of such feeding frenzies. Many people questioned the existence of the objects.
Researchers discovered the mother lode in the year 2013).
Amy Reines, an astronomer at Montana State University, analyzed data from the Sloan Digital Sky Survey looking for patterns of visible light from black holes. There were signs of black holes in the dwarf galaxies that she scoured. They were unconfirmed candidates, but their number left little room for doubt.
Volonteri's proposal seemed less radical. She said that you make her feel like a reasonable person.
Astronomers have come up with new ways to find these giants.
The astronomer stumbled onto a new technique by chance. While trying to confirm one of the candidates from the radio survey, he noticed an orange glow. The iron atoms lost nine electrons in the radiation, which was a tell-tale sign. The researchers found the same trait in another candidate. It was hard to imagine what else could be doing such violence to iron atoms.
The code that was written by Molina was used to search through about 46,000 dwarf galaxies that were observed by the Digital Sky Survey. In dwarf galaxies, a giant black hole is heating the gas around it and blowing electrons from their atoms. They published their findings in a journal.
These black holes are beyond the reach of previous surveys, which struggled to pick out the black holes against the baby stars. The effort to look for a flashlight is similar to the effort to look for a spotlight. Large black holes pop out against the background, showing that smaller dwarfs with lots of stellar nurseries can also host giant black holes.
Volonteri, who was not involved in the research, said that he was glad to see that there were new ways to find black holes. That was the reason it was so bad.
Hidden black holes have been revealed by space telescopes. Hickox and his student reexamined some edge cases from the survey. They aimed NASA's space-based Chandra X-ray observatory at each dwarf galaxy for four hours. A speck at the center of one of the eight galaxies appeared brighter with high-energy X-rays than low-energy X-rays, suggesting that the radiation was hitting through a dense cloud of gas. They described their results at the American Astronomical Society conference.
The results raise the odds that many dwarf galaxies have black holes, but the fraction that possesses them is still a topic of debate. Astronomers would have said that fraction was zero in 2008. The true number is dependent on the percentage of black holes that are feeding and shining and the percentage of black holes that are enshrouded by gas and dust. Massive black holes could reside in the majority of dwarf galaxies if these percentages are similar to those observed in their supermassive siblings.
Hickox said that the argument that they are not that common may be starting to break down.
The answer to how common dwarf-galaxy black holes were in 2008 might help distinguish between two different explanations.
Billion-solar-mass black holes dating back to the universe's first billion years were seen a few years ago. They couldn't figure out how heavy they had gotten. It was like seeing a giant stand of trees on a volcanic island. Either they had shot up incredibly fast or they were already partially grown.