When Massive Black Holes Snuff Out Star Birth
As galaxies mature, they stop forming stars -- but why? Now astronomers are hot on the trail of finding the culprit. Continue reading →
As galaxies mature, they stop forming stars - but why? Now astronomers are hot on the trail of finding the culprit.
By now we know that the vast majority of galaxies have supermassive black holes in their cores. These galactic behemoths generate some energetic phenomena, especially when matter falls onto their accretion disks and event horizons. Often, the energy generated by active galactic nuclei (where these rambunctious black holes reside), will regulate the star formation processes in their host galaxy.
Now, in a new study published in the journal Monthly Notices of the Royal Astronomical Society, researchers believe that they've found the reason why maturing galaxies seem to "switch off" star formation all together.
"When you look into the past history of the universe, you see these galaxies building stars," said Tobias Marriage, of Johns Hopkins University and co-lead author of the study. "At some point, they stop forming stars and the question is: Why? Basically, these active black holes give a reason for why stars stop forming in the universe."
Marriage and his colleagues used an established method for studying large clusters of galaxies and applying it to single galaxies. By doing this, they discovered that supermassive black holes are driving "radio-frequency feedback," which is heating up the galaxies, preventing interstellar gases from cooling, clumping and forming new stars.
In short, massive black holes, at a certain age, act like a switch and are snuffing out star formation before it can even take hold.
Normally, the Sunyaev–Zel'dovich (SZ) effect signature is used to study how the primordial cosmic microwave background radiation (the ‘echo' of the Big Bang) interacts with the electrons inside interstellar gases locked in clusters of hundreds of galaxies. But for the first time, this method has been down-scaled to gauge the interstellar environment of single galaxies.
"The SZ is usually used to study clusters of hundreds of galaxies but the galaxies we're looking for are much smaller and have just a companion or two," said Megan Gralla, also of Johns Hopkins.
"What we're doing is asking a different question than what has been previously asked," Gralla said. "We're using a technique that's been around for some time and that researchers have been very successful with, and we're using it to answer a totally different question in a totally different subfield of astronomy."
So, while studying the SZ effect signature in galaxies, the researchers found that all the galaxies displaying radio-frequency feedback coincided with galaxies that also lacked signs of star formation. It just so happened that these particular galaxies were large and mature elliptical galaxies, where their heated interstellar gas was prevented from cooling down.
"If gas is kept hot, it can't collapse," said Marriage. If the gas cannot collapse, no new stars can form.
Radio-frequency feedback occurs when matter falls into the environment surrounding a black hole. Though the black hole will inevitably swallow some of this mass, through an as-yet to be understood mechanism, some of this matter will be accelerated to relativistic speeds and blasted from the black hole's poles. This highly energetic stream of plasma traveling close to the speed of light generates powerful radio emissions that go on to heat the gases throughout the host galaxy.
Although the exact mechanisms behind radio-frequency feedback is a matter of debate, the result is clear: Black holes are acting like a heating element on a stove, keeping interstellar gases hot and shutting down star formation in mature galaxies.
Source: Johns Hopkins University
Elliptical galaxy NGC 1132, as seen by NASA's Chandra X-Ray Observatory; the blue/purple in the image is the X-ray glow from hot, diffuse gas that is not forming into stars.
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.