Messier 87, a massive elliptical galaxy in the Virgo cluster, is some 55 million light years from Earth, and even though the black hole at its center has a mass 6.5 billion times that of the Sun, it’s a relatively small object, about the size of our Solar System. Resolving an image of that black hole is, says the University of Arizona’s Dimitrios Psaltis, like “taking a picture of a doughnut placed on the surface of the moon.” But the M87 black hole is one of the largest we could see from Earth, making it a natural target for observations, in this case using radio telescopes working at a frequency of 230 GHz, corresponding to a wavelength of 1.3mm.
A decade ago, working with Avery Broderick, Harvard’s Avi Loeb highlighted the advantages of M87 as an observational target, finding it in many ways preferable to the black hole at the heart of our own Milky Way:
M87 provides a promising second target for the emerging millimeter and submillimeter VLBI capability. Its presence in the Northern sky simplifies its observation and results in better baseline coverage than available for Sgr A*. In addition, its large black hole mass, and correspondingly long dynamical timescale, makes possible the use of Earth aperture synthesis, even during periods of substantial variability.
That paper, “Imaging the Black Hole Silhouette of M87: Implications for Jet Formation and Black Hole Spin,” is worth revisiting (abstract), for those intrigued with how these observations get made and the kinds of things we can learn from them.
I was reminded, when I first saw the now famous image, of the nature of M87 itself. Elliptical galaxies, unlike our barred spiral Milky Way, show slow rates of star formation, their primary population being older stars, and as you would imagine, they contain little gas and dust, while also housing a large number of globular clusters. Back in 2012, I ran across a paper by Falguni Suthar and Christopher McKay (NASA Ames) assessing habitability in such galaxies. What an environment to set a science fiction story! Consider the image below before we cut to the black hole image that is now center stage in the news, because here’s the context:
Image: A composite of visible (or optical), radio, and X-ray data of the giant elliptical galaxy, M87. M87 lies at a distance of 55 million light years and is the largest galaxy in the Virgo cluster of galaxies. Bright jets moving at close to the speed of light are seen at all wavelengths coming from the massive black hole at the center of the galaxy. It has also been identified with the strong radio source, Virgo A, and is a powerful source of X-rays as it resides near the center of a hot, X-ray emitting cloud that extends over much of the Virgo cluster. The extended radio emission consists of plumes of fast-moving gas from the jets rising into the X-ray emitting cluster medium. Credit: X-ray: NASA/CXC/CfA/W. Forman et al.; Radio: NRAO/AUI/NSF/W. Cotton; Optical: NASA/ESA/Hubble Heritage Team (STScI/AURA), and R. Gendler.
Could life survive in environments like this? I bring this up again as background, but also because yesterday we looked at the question of hardy microorganisms and their ability to withstand high levels of X-ray and UV radiation. Here’s what McKay and Suthar said in 2012:
Complex life forms are sensitive to ionizing radiation and changes in atmospheric chemistry that might result. However, microbial life forms, e.g. Deinococcus radiodurans, can withstand high doses of radiation and are more ?exible in terms of atmospheric composition. Furthermore, microbial life in subsurface environments would be effectively shielded from space radiation. Thus, while a high level of radiation from nearby supernovae may be inimical to complex life, it would not extinguish microbial life.
It’s fascinating to me that we’ve begun studying such questions on a galactic scale. Fascinating too that we’re now peering into the heart of an active galaxy to reveal its powerhouse black hole. By now the image is familiar, but let’s see it again because it’s just extraordinary.
Image: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. Credit: Event Horizon Telescope Collaboration.
One thing I saw little attention given to in the coverage was that the Event Horizon Telescope, which produced the image, was supplemented by work from spacecraft. Remember that the EHT is comprised of telescopes located around the surface of our planet, to produce a planet-scale interferometer capable of making such an observation. But the Chandra X-ray spacecraft was also involved, as was the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Neil Gehrels Swift Observatory. All of these, working at X-ray wavelengths, observed the M87 black hole at the same time it was under study by the EHT in April of 2017.
I point to this because while the space assets could not image the black hole, data from them were used to measure the brightness of the M87 jet, particles driven by an enormous energy boost from the black hole itself and surging away from it at nearly the speed of light. The hope here is that X-rays can help us measure particle events near the event horizon to coordinate with the black hole images. Also involved in space was the Neutron star Interior Composition Explorer (NICER), a NASA experiment on the International Space Station that looked at the center of the Milky Way and the black hole known as Sgr A*. Part of the EHT’s mandate is to study the origin of jets like this one, so these extraordinary interactions now become visible.
As to the ground-based observatories of the EHT themselves, what an accomplishment! The international team involved totalled over 200 astronomers, whose work is presented in a special issue of Astrophysical Journal Letters. In the black hole work, the EHT used an array of eight radio telescopes with worldwide coverage, from the Antarctic to Spain, Chile and Hawaii, all located in high-altitude settings where conditions are ideal for observation.
Jonathan Weintroub (CfA) coordinates the EHT’s Instrument Development Group:
“The resolution of the EHT depends on the separation between the telescopes, termed the baseline, as well as the short millimeter radio wavelengths observed. The finest resolution in the EHT comes from the longest baseline, which for M87 stretches from Hawai’i to Spain. To optimize the long baseline sensitivity, making detections possible, we developed a specialized system which adds together the signals from all available SMA dishes on Maunakea. In this mode, the SMA acts as a single EHT station.”
Spectacular. The very long baseline interferometry creates a virtual dish that is planet-sized, able to resolve an object to 20 micro-arcseconds. Working with a conjunction of four nights that would produce clear seeing for all eight observatories, the telescopes took in massive amounts of data — 5,000 trillion bytes of data in all — saved on 1,000 storage disks. Transmitting all that information for subsequent processing was ruled out, for air transport from FedEx could take the hard disks onto which the data had been recorded to a single location much faster. These are signals that needed to be aligned within trillionths of a second to achieve a valid result.
The resulting imagery is the payoff. The central dark region is surrounded by a ring of light, as Einstein’s equations led scientists to expect. We can’t, of course, see the black hole itself, but plasma emitted from its accretion disk, where matter piles up as material falls into the black hole, is heated to billions of degrees and accelerated almost to lightspeed. We get an image of the black hole’s shadow’ that is about 2.5 times larger than the event horizon. M87’s event horizon is thought to be some 25 billion miles across, making it 3 times the size of Pluto’s orbit.
“Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well,“ said Luciano Rezzolla, professor for theoretical astrophysics at Goethe University and a researcher on the EHT. “This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.“
Image: This artist’s impression depicts the paths of photons in the vicinity of a black hole. The gravitational bending and capture of light by the event horizon is the cause of the shadow captured by the Event Horizon Telescope. Credit: Nicolle R. Fuller/NSF.
This is a black hole massive enough that a planet orbiting it could move around it within a week while traveling, says MIT’s Geoffrey Crew, close to the speed of light. Crew’s colleague Vincent Fish, also at MIT’s Haystack Observatory, amplifies on the point:
“People tend to view the sky as something static, that things don’t change in the heavens, or if they do, it’s on timescales that are longer than a human lifetime. But what we find for M87 is, at the very fine detail we have, objects change on the timescale of days. In the future, we can perhaps produce movies of these sources. Today we’re seeing the starting frames.”
Now that’s something worth waiting for, movies of the accretion disk caught in the tortured spacetime of a galaxy’s central black hole. M87 anchors a jet stretching tens of thousands of light years, so we’re talking about seeing the dynamics of the jet’s interactions with the black hole. Fine-tuning EHT methods and expanding its sites points in the direction of further breakthrough imagery.
But what an accomplishment we’ve already achieved via instruments all over the world — ALMA and APEX in Chile, the IRAM 30 meter telescope in Spain, the James Clerk Maxwell telescope and the Submillimeter Array (both in Hawaii), the Large Millimeter Telescope (LMT) in Mexico, the Submillimeter Telescope (SMT) in Arizona and the South Pole Telescope (SPT) in Antarctica.
The papers are The Event Horizon Telescope Collaboration et al., “First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole,” Astrophysical Journal Letters Vol. 875, No. 1 (10 April 2019) (abstract); and from the same issue: “First M87 Event Horizon Telescope Results. II. Array and Instrumentation” (abstract); “First M87 Event Horizon Telescope Results. III. Data Processing and Calibration” (abstract); “First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole” (abstract); “First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring” (abstract); and “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole” (abstract). The paper on M87 and galactic habitability is Suthar & McKay, “The Galactic Habitable Zone in Elliptical Galaxies,” International Journal of Astrobiology, published online 16 February 2012 (abstract).