David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration; NRAO/AUI/NSF
Dwarf galaxy NGC 4395, previously thought incapable of having a central massive black hole, is one of approximately 100 dwarf galaxies to have such black holes after all.
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.
Too often, the term “paradigm-shift” is used in science reporting for things that, really, aren’t much of a shift in our thinking at all. However, this week it was used to describe a new astronomical discovery, and for once I agree that this is a BIG DEAL.
The story is in the discovery of massive black holes in the centers of dwarf galaxies by Amy Reines of the National Radio Astronomy Observatory. Now, we are quite familiar with the story that all massive galaxies have supermassive black holes in their centers. In fact, the bigger the galaxy’s central bulge, the bigger the supermassive black hole, or SMBH. There seems to be a definite link between the evolution of galaxies and the evolution of their central black holes.
An outstanding question is: how do these supermassive black holes form? Many millions or billions of “regular” mass black holes would fit inside one SMBH, but its hard to imagine this ever happening realistically in the Universe. It is likely, however, that SMBHs started as simply Massive Black Holes (MBHs) during the early days of the Universe, either as gas clouds collapsed catastrophically or supermassive stars somehow formed massive seed black holes. These seed black holes are far too distant to observe with current capabilities though.
This is why astronomers are so excited to find an analog of these massive black holes in an unexpected place: dwarf galaxies. We already knew that all large galaxies, such as our Milky Way, harbor supermassive black holes in their centers, but the tiny dwarf galaxies that have no central bulge and tend to cluster around bigger galaxies didn’t appear to have such central black holes.
Then, a few years ago, Reines serendipitously discovered a massive black hole at the heart of dwarf galaxy Henize 2-10. This took her work in a whole new direction. Now, after having closely studying some 25,000 dwarf galaxies, she and her collaborators have found evidence for massive accreting black holes in 100 of them. These MBHs are not millions or billions of times the mass of the sun, but a few hundred thousand times the mass of the sun. These are very much like what the seeds of supermassive black holes may have been like in the early Universe, and they are close enough to study.
100 out of 25,000 doesn’t seem like a lot. However, these are only the ones that have material actively falling on to them. We know that about 10 percent of supermassive black holes are active, leaving the other 90 percent to be pretty quiet. We don’t know the percentage of active to quiet massive black holes in dwarf galaxies just yet, but it is likely that the 100 are just the tip of the iceberg. And so now we have many dwarf galaxies harboring central massive black holes as well as their larger counterparts, and that does indeed represent a paradigm-shift in what we know about black hole and galaxy evolution.
There is one more bonus to this story. The astronomers didn’t even have to take new data to make this amazing discovery. Instead, they analyzed spectra of galaxies that had already been taken with the huge Sloan Digital Sky Survey.
As astronomy moves into a era of more and more big telescopes and big surveys, it is crucial that these data be accessible so that any researcher can look back and find something we’d never thought we’d see without having to even use another telescope.
This work has been published in the Astrophysical Journal, and a preprint is available at arXiv.org.