Researchers say they have new reason to hunt for pairs of black holes in the dark, outer reaches of spiral galaxies such as ours.
The US’s LIGO, the Laser Interferometer Gravitational Wave Observatory, made history in late 2015 when researchers directly detected gravitational waves for the first time ever.
In September of this year, Italy’s Virgo gravitational wave detector also observed the space-warping ripples of gravitational waves, and researchers concluded that the waves were created some 1.8 billion light years away when two black holes collided.
Super massive black holes have already been found at the center of almost all known large galaxies, including our own. But paired black holes of similar mass to those that created the gravitational waves detected in September are expected to be found in much smaller, dimmer galaxies.
Researchers are rethinking those assumptions, saying the outer rims of spiral galaxies, such as our own Milky Way, may harbor pairs of black holes on collision courses with one another.
Such a merge has never been directly observed. Technically, black holes can’t really be seen anyway. Neither visible light nor any other form of electromagnetic radiation can escape their monstrous gravitational pull. But there are ways of knowing that they’re there.
Could astronomers be on route to discovering new “monsters” in our own galaxy?
Previous work had suggested that pairs of black holes in this mass range are more likely to form in dim dwarf galaxies. But the new study shows that the quiet outer regions of larger, spiral-shaped galaxies — like our own Milky Way — may be better places to look.
“If our calculation is correct, the advantage is that if you’re trying to localize the signal, it’s a lot easier to find big galaxies, right? That’s pretty obvious,” said study lead author Sukanya Chakrabarti, a professor of physics and astronomy at the Rochester Institute of Technology (RIT).
Determining the precise location or home galaxy of these black hole pairs has multiple advantages for astrophysics. First, it would increase the odds of seeing light signals created by the merger of two black holes. While the black holes themselves are completely dark, nearby matter (such as a disk of gas and dust swirling around it) could radiate light. Studying this light could provide scientists with more information about these events.
Additionally, scientists want to use gravitational waves to make a measurement of the expansion rate of the universe — a value known as the Hubble Constant, named after the astronomer Edwin Hubble. Right now, there are two ways to measure this value, but they have produced slightly different values, and scientists don’t know why. Measurements from gravitational waves might solve the discrepancy.
“It’s the holy grail of gravitational wave cosmology,” Chakrabarti told Space.com.
Researchers believe the gravitational waves detected in Italy by the Virgo gravitational wave detector were created by fusing binary black holes, each with a mass about 20 to 50 times that of our sun.
When looking for other merging black holes this size, scientists say the bright regions of galaxies, such as galactic cores and spiraling arms don’t have the right building blocks.
Brighter regions of large galaxies can harbor Supermassive Black Holes, but probably not smaller, merging pairs like the ones Virgo scientists say hurled the gravitational waves they detected through space.
Instead, scientists think dim “outer disks” are the right place to look.
Black holes with 10 to 50 times the mass of the sun must form from stars with masses between about 40 and 100 solar masses, according to scientists with the Laser Interferometry Gravitational-Wave Observatory (LIGO) project, which spotted space-time ripples from the four black hole mergers (as well as those generated by a different kind of event — the collision of two neutron stars).
So where are these massive stars born?
To make a truly massive star requires a very simple starting mixture consisting almost exclusively of hydrogen and helium. “Heavier” elements (those with numbers higher than 2 on the periodic table) can dampen the formation of very massive stars. This happens because those elements give off more intense radiation compared to hydrogen; that radiation exerts an outward force that pushes gas and other material away, so the star doesn’t accumulate more matter and grow bigger.
The bright regions of large galaxies — like the beautiful, swirling arms of the Milky Way — are low on the list of possibilities, because they are so rich in heavy elements.
But nearly all spiral galaxies possess an outer disk consisting mainly of hydrogen, Chakrabarti said. These co-called “H1 regions” have very low overall star-formation rates, and are thus fairly dim compared to the bright central disks. In the case of the Milky Way, the H1 region is about as thick as the main disk is wide — about 30 kiloparsecs, or about 100,000 light-years across. In some galaxies examined in the paper, the H1 disk is 80 kiloparsecs across, Chakrabarti told Space.com — plenty of room for massive stars to be born.
How many monsters are out there?
Let us know what you think, and sound off in the comments below.