Milky Way's Second Most Massive Black Hole Found?
This artist's impression shows the swirling gases surrounding an "invisible object" at the center of CO-0.40-0.22.
Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets.
How to measure the spin of a black hole: This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.
Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM- Newton telescope, there were two competing models to explain why this peak might not appear to be sharp. The "rotation" model shown at top held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole. The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.
This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X- ray light, with energies between 3 and 70 keV.
This image taken by the ultraviolet-light monitoring camera on the European Space Agency's (ESA's) XMM-Newton telescope shows the beautiful spiral arms of the galaxy NGC1365. Copious high-energy X-ray emission is emitted by the host galaxy, and by many background sources. The large regions observed by previous satellites contain so much of this background emission that the radiation from the central black hole is mixed and diluted into it. NuSTAR, NASA's newest X-ray observatory, is able to isolate the emission from the black hole, allowing a far more precise analysis of its properties.
What XMM-Newton saw: The solid lines show two theoretical models that explain the low-energy X-ray emission seen from the galaxy NGC 1365 by the European Space Agency's XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the measurements from XMM-Newton, which are explained equally well by both models.
Two X-ray observatories are better than one: NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has helped to show, for the first time, that the spin rates of black holes can be measured conclusively. It did this, together with the European Space Agency's XMM-Newton, by ruling out the possibility that obscuring clouds were partially blocking X-ray right coming from black holes. The solid lines show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.
Astronomers have detected what could be the second most massive black hole in our galaxy and it may be the missing piece of a cosmic puzzle.
But radio astronomers didn’t directly detect the candidate black hole, rather they spied the whirling gases caught in its powerful gravitational grasp, potentially establishing a new method to track down elusive “intermediate-mass” black holes.
Using the Nobeyama 45-meter Radio Telescope, which is managed by the National Astronomical Observatory of Japan (NAOJ), the researchers found the object only 200 light-years from the Milky Way’s supermassive black hole Sagittarius A* (Sgr. A*). By tracking the emissions from a swirling gas cloud called “CO-0.40-0.22,” they found a “surprisingly wide velocity dispersion” — in other words, this cloud of gas is composed of material that is swirling at a wide range of speeds. There appears to be no supernova activity or any other energetic event in the region that could be driving this bizarre phenomenon.
Using computer models, the researchers were able to deduce that an extremely compact object — in other words, a black hole — lives in the “eye” of this interstellar storm and it must be massive. And by “massive” they mean in the order of 100,000 solar masses-massive. If confirmed, this would make the invisible object at the core of CO-0.40-0.22 a so-called “intermediate-mass” black hole, second in mass only to mighty Sgr. A* itself. Sgr. A* “weighs in” at a staggering 4 million solar masses.
“Considering the fact that no compact objects are seen in X-ray or infrared observations, as far as we know, the best candidate for the compact massive object is a black hole,” said Tomoharu Oka, of Keio University in Japan and lead author of a study published in the Astrophysical Journal.
Intermediate-mass black holes are truly mysterious creatures. They are the “missing” link of black hole evolution; we have stellar mass black holes (that form after the supernova death of a massive star) and we have supermassive black holes (that live in the cores of most galaxies), but if black holes start small and grow by merging with other black holes and consuming matter, they must go through a “medium” phase. Alas, astronomers have yet to confirm that black holes do indeed come in “medium” — they’ve only confirmed black holes in sizes “small” and “XXL.”
So that leaves us with a puzzle. Are intermediate-mass black holes simply hard to find? Or are they incredibly rare? The first question may be solved through improved detection techniques, but the second question poses a challenge to black hole evolution theories and could expose a huge flaw in our astrophysical thinking.
Some theories of galactic evolution suggest the Milky Way should contain 100 million black holes, but X-ray surveys have only turned up a tiny fraction of this number. This is where radio telescopes may fill a niche in seeking out “invisible” mid-sized black holes.
“Investigations of gas motion with radio telescopes may provide a complementary way to search for dark black holes” said Oka in a press release. “The on-going wide area survey observations of the Milky Way with the Nobeyama 45-m Telescope and high-resolution observations of nearby galaxies using the Atacama Large Millimeter/submillimeter Array (ALMA) have the potential to increase the number of black hole candidates dramatically.”
The location of CO-0.40-0.22 is also intriguing; should our black hole merger evolution model hold true for the growth of black holes on their way to becoming supermassive, there should be a concentration of massive black holes near galactic cores. As this candidate is only 200 light-years from Sgr. A*, it could indicate that, eventually, the object in CO-0.40-0.22 will eventually stray near Sgr. A* to add to its already impressive bulk.