Monster Black Holes Can Kill Galactic Star Formation
Digitised Sky Survey/NASA Chandra/Very Large Array (Robert Dunn et al. 2010)
A powerful NASA telescope has found not one, but 10 supermassive black holes. And it did so by accident!
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.
Supermassive black holes could be quenching star formation in elliptical galaxies, forcing them to appear "red and dead," a new study reports.
Astronomers have long wondered why giant elliptical galaxies stop forming stars, becoming dominated over time with small, long-lived stars with a distinctive reddish tinge. Conventional wisdom had held that these galaxies lack the cold gas necessary for star birth.
But new observations suggest that a rethink is in order. Some big ellipticals do indeed harbor large amounts of cold gas — but these reservoirs likely get heated up or driven off by powerful jets of material blasted out by supermassive black holes, which lurk at the heart of most if not all galaxies. [The Strangest Black Holes in Space]
"These galaxies are red, but with the giant black holes pumping in their hearts, they are definitely not dead," study lead author Norbert Werner, of Stanford University, said in a statement.
Werner and his colleagues studied eight giant elliptical galaxies using the European Space Agency's Herschel Space Observatory. They found that six of the eight have lots of cold gas, which Herschel detected as far-infrared emissions from carbon ions and oxygen atoms.
The team then investigated the galaxies' stockpiles of hotter gas, looking at optical images as well as X-ray data gathered by NASA's Chandra X-ray Observatory.
They determined that, in the six galaxies with large quantities of cold gas, the hotter stuff is cooling down, as predicted by theory — but the cooling process has stopped for some reason. In the two galaxies without cold gas, the hot material seems not to be cooling down at all, researchers said.
"The contrasting behavior of these galaxies may have a common explanation: the central supermassive black hole," said co-author Raymond Oonk of ASTRON, the Netherlands Institute for Radio Astronomy.
The two ellipticals without cold gas have incredibly active central black holes at their cores, which are accreting material at a breakneck pace and launching energetic jets out into space, researchers said. These jets could be reheating the cold gas or clearing it away from the galaxies altogether.
This process may explain why star formation has halted in all eight elliptical galaxies, study team members said.
"Once again, Herschel has detected something that was never seen before: significant amounts of cold gas in nearby red-and-dead galaxies," said Göran Pilbratt, Herschel project scientist at ESA. "Nevertheless, these galaxies do not form stars, and the culprit seems to be the black hole."
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