Why Is This 'Naked' Black Hole on a Crash Diet?
Astronomers have discovered a rather odd discrepancy in the heart of a distant galaxy -- it contains two supermassive black holes, but one of those black holes is 'naked', with few stars surrounding it.
Astronomers have discovered a rather odd discrepancy in the heart of a distant galaxy - it contains two supermassive black holes, but one of those black holes is "naked", with few stars surrounding it. In stark contrast, its black hole sibling is buzzing with stars.
This discovery, using observations from the Hubble Space Telescope and NASA's Chandra X-ray Observatory, has gotten astronomers thinking: why do some black holes possess a population of stars surrounding them whereas others do not? Also, how does this contrast influence black hole evolution and, indeed, how does it impact the evolution of galaxies?
As reported by Discovery News earlier this week, the first clue to the odd disparity in SDSS J1126+2944 is that this particular galaxy is the result of a galactic merger. Mergers occur when two (or more) galaxies collide and get trapped in their mutual gravitational well. Some stars are scattered in the dramatic upheaval, but for the most part, the stars mingle and then settle. (It is thought that massive galaxies like the Milky Way are in fact the cannibalized remains of many smaller galaxies.)
Eventually the supermassive black holes, which are thought to be hiding in the cores of most galaxies in the universe, may themselves merge, creating a super-supermassive black hole. This is an inevitable part of black hole growth in galactic evolution.
But in the case of SDSS K1126+2944, the two black holes are still some distance apart and in new research presented this week at the American Astronomical Society (AAS) meeting at Kissimmee, Fla. and published in the Astrophysical Journal, researchers have taken a stab at understanding why one of the black holes is lacking a population of stars, a feature that will undoubtedly impact its supply of in-falling matter.
"One black hole is starved of stars, and has 500 times fewer stars associated with it than the other black hole," said lead investigator Julie Comerford, of Colorado University, Boulder. "The question is why there's such a discrepancy."
According to Comerford, there are two possibilities. The first is that, during the merging of the two galaxies, tidal and gravitational forces ripped through the black hole's neighborhood, scattering its surrounding stars. But there's another and rather curious explanation that could fill the gap in our knowledge about how black holes grow.
In black hole astrophysics, we know of "stellar mass" black holes - basically black holes of 5 to 100 solar masses that remain behind after a massive star goes supernova - and we are familiar with "supermassive" black holes at the cores of galaxies that have masses of between a couple of hundred thousand to millions (or even billions) of solar masses. You may have noticed that there's a huge mass gap between these two types of black holes. If black holes start small and grow more massive over time, what kind of black hole forms the bridge between the stellar mass black holes and the supermassive monsters?
Astronomers have been on the lookout for "intermediate mass" black holes (IMBHs) to fill this gap, and knowing the preponderance of stellar mass and supermassive black holes, there should be a lot of IMBHs out there - basically medium-sized black holes on their way to becoming supermassive. But there are only a handful of candidates, which is just weird. If our theories of black hole evolution are correct, we shouldn't be having such a hard time tracking down intermediate black holes, which would fall in the mass range of 100 to a million solar masses. Are their emissions simply very hard for us to detect? Or is the lack of observational evidence a clue to their rarity?
Intermediate mass black holes are too massive to have been formed by exploding massive stars, so they would likely be objects that have slowly formed through black hole mergers and mass accretion processes over billions of years. Some observations of low-luminosity active galactic nuclei hint of their existence and detections of ultra-luminous X-ray sources (ULXs) in nearby galaxies have also provided clues. In addition, it is thought dwarf galaxies contain intermediate mass black holes in their cores. Dwarf galaxies have a lower density of stars than more massive galaxies.
Could it be that the "naked" black hole in SDSS J1126+2944 is in fact an intermediate mass black hole that originated inside a cannibalized dwarf galaxy?
"Theory predicts that intermediate black holes should exist, but they are difficult to pinpoint because we don't know exactly where to look," said co-author Scott Barrows, also from the University of Colorado, Boulder. "This unusual galaxy may provide a rare glimpse of one of these intermediate mass black holes."
Should this IMBH candidate have originated from a merged dwarf galaxy, more work is needed to confirm it as such, but it is certainly a tantalizing clue as to the origins of this mysterious class of "medium-sized" black hole.
This is an image of the galaxy SDSS J1126+2944 taken with the Hubble Space Telescope and the Chandra X-ray Observatory. The arrow points to the black hole that is lacking a population of 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.