Black holes usually form from the violent death of a star. But that might not be the only way, a new study finds. It points to a remote galaxy that may harbor a black hole made instead from the collapse of a massive cloud of gas. This rarity might even explain how some galaxies built gargantuan black holes in the first billion years or so after the Big Bang.
The galaxy, called CR7, is unusual. It blasts out more ultraviolet light than other galaxies that lived at the same time. That was roughly 13 billion years ago, a mere 800 million years after the Big Bang. The gas in CR7 also appears to lack elements such as carbon and oxygen. These elements are forged within stars and then ejected into space.
One explanation for the large output of ultraviolet light and lack of heavier elements such as carbon in this galaxy: It is giving birth to first-generation stars. These stars would be similar to the first stars ever created in the universe. That means they would be made of only hydrogen, helium and very small amounts of lithium.
Another hypothesis is that CR7 harbors the first known black hole to form when a blob of interstellar gas collapsed under its own weight. This would have happened without first creating stars.
A black hole is the more likely explanation. At least, that’s what Aaron Smith and his colleagues now say. Smith is an astronomer at the University of Texas at Austin. In a new study, he was part of a team that developed a computer program to test that idea.
The computer simulates the conditions that likely would have been present. Then it lets gravity, energy and other forces in the universe predict — or model — how they should have played out over time.
That computer model focused on interstellar gas, which is the gas between stars. It predicted how the gas would have interacted with harsh radiation from primordial stars or a large black hole.
Light emitted from a collection of hot, young stars can’t explain one odd feature of CR7, this analysis found. There’s a parcel of gas racing away from the galaxy. How speedily? Try 580,000 kilometers per hour (some 360,000 miles per hour). That’s fast, although not record fast. Radiation from a superheated disk of debris, swirling around a black hole, can push the gas that fast, though. To do this, the researchers estimate the black hole must be roughly 100,000 times as massive as our sun!
They report their analyses in the August 11 Monthly Notices of the Royal Astronomical Society.
Nothing certain yet
There is no evidence yet that CR7 has a black hole. But if the galaxy does have one, it would be the first evidence of a black hole forming out of clouds that haven’t given birth to stars yet.
Such a discovery could help astronomers sort out a major issue about how some of the earliest supermassive black holes in the universe formed. For instance, how could they be created in about 1 billion years out of only smaller black holes merging together? “There’s just not enough time to do that,” Smith says. A black hole that formed from the direct collapse of gas, however, creates a massive seed all in one go. It could jump-start the growth of a behemoth black hole that would eventually weigh as much as several billion suns.
“This is definitely a good step forward,” says David Sobral. He is an astrophysicist at Lancaster University in England. He discovered CR7 last year. But it’s too early to say whether a black hole or a group of stars powers this black hole, he says. “I’ve tried to stay a bit away from it and argue that what we need is new observations instead of taking sides.”
With the data that are available, it’s hard to distinguish between stars or a black hole, Sobral says. That’s why he and his colleagues have reserved time with the Hubble Space Telescope in January. They are also awaiting new data from the Atacama Large Millimeter/submillimeter Array in Chile. Data from both observatories will help researchers look for traces of heavy elements in CR7.
To astronomers, a “heavy element” is any atom heavier than helium (which is number 2 on the periodic table). If the data still show no sign of heavy atoms such as carbon, then CR7 probably hosts a nest of first-generation stars. A black hole, on the other hand, probably would have formed long enough ago that there would be enough time for stars to form. Those stars, Sobral says, would would form heavier elements. And then a smidgen of such elements would pollute CR7.
A growing census of similar galaxies will help as well. “We’re now finding that CR7 is not alone,” Sobral says. He and colleagues have since found four other galaxies comparable to CR7 in the early universe. The team presented its results June 27 at the National Astronomy Meeting in Nottingham, England. “We don’t have to discuss one single thing,” he says. It’s now possible, he says, to “put [CR7] into a broader picture.”
(for more about Power Words, click here)
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
Big Bang The rapid expansion of dense matter that, according to current theory, marked the origin of the universe. It is supported by physics’ current understanding of the composition and structure of the universe.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
census An official count or survey of a population.
computer model or computer simulation A program that runs on a computer or network of computers that creates a model, or simulation, of a real-world feature, phenomenon or event.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that give them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
element (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
first-generation stars These are the first stars ever created in the universe. These stars were formed from pristine material left over from the Big Bang. That means they are composed of hydrogen, helium and very small amounts of lithium. Astronomers also refer to these stars as population III stars.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
helium An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -458 degrees Fahrenheit (-272 degrees Celsius).
hypothesis A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.
interstellar gas The gas between the stars.
lithium A soft, silvery metallic element. It’s the lightest of all metals and very reactive. It is used in batteries and ceramics.
observatory (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.
primordial An adjective that refers to something that goes back to the beginning of time or to the earliest existence of something.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
simulation To model, often using a computer, some conditions, functions or appearance of a physical system. A computer program does this by using mathematical operations that can describe the system and how it might vary in response to various situations or over time.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
ultraviolet light A type of electromagnetic radiation with a wavelength from 10 nanometers to 380 nanometers. The wavelengths are shorter than that of visible light but longer than X-rays.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
A. Smith, V. Bromm and A. Loeb. Evidence for a direct collapse black hole in the Lyman α source CR7. Monthly Notices of the Royal Astronomical Society. Vol. 460, August 11, 2016, p. 3143. doi: 10.1093/mnras/stw1129.