Inside the Black Hole Picture 1


The Event Horizon Telescope network has obtained the first image of a black hole — at the centre of the galaxy M87. Credit: EHT Collaboration

You will have no doubt seen the above Black Hole picture and observed rather a lot of people getting excited about it. You look, and all you see is a fuzzy picture, and so you begin to wonder why they are all getting so excited. Let’s dig into this a bit.

What you can see is the Event Horizon of a black hole with light orbiting it. The folks behind it are also poets and describe it as follows …

“We have seen the gates of hell at the end of space and time,” said astrophysicist Heino Falcke of Radboud University in Nijmegen, the Netherlands, at a press conference in Brussels. “What you’re looking at is a ring of fire created by the deformation of space-time. Light goes around, and looks like a circle.”

Nature, the journal, asked the researchers what this breakthrough means for them and for science, and here is what they said …

The First image of a black hole: A three minute guide

Nature also have the following primer on it all that explains exactly why this fuzzy image is truly revolutionary …

It’s not just about a Fuzzy Picture

Another piece of significance regarding the fuzzy picture is that it is not the end-game, this is just the beginning. The Event Horizon network has opened up a new window that will enable us to see what is going on and so we can begin to seriously tackle the question of trying to understand …

“I was so delighted,” says Andrea Ghez, an astronomer at the University of California, Los Angeles. The images provide “clear evidence” of a ‘photon ring’ around a black hole, she says.

The team observed two supermassive black holes — M87’s and Sagittarius A*, the void at the Milky Way’s centre — over five nights in April 2017. They mustered enough resolution to capture the distant objects by linking up eight radio observatories across the globe — from Hawaii to the South Pole — and each collected more data than the Large Hadron Collider does in a year (see ‘Global effort’). The data set is likely to be the largest ever collected by a science experiment, and it took two years of work to produce the pictures.

After combining the observatories’ data, the team started analysis in mid-2018. They quickly realized that they could get a first, clean picture from M87. “We focused all our attention on M87 when we saw our first results because we saw this is going to be awesome,” says Falcke.

What comes next?

Astrophysicist and collaboration member Monika Moscibrodzka, also at Radboud, said that the measurements so far are not precise enough to measure how fast the M87 hole spins — a crucial feature for a black hole. But it indicates the direction in which it’s spinning, which is clockwise in the sky, she said. Further studies could also help researchers understand how the black hole produces its gigantic jets.

The teams will also now turn their attention to the Sagittarius A* data. Because Sagittarius A* is nearly 1,000 times smaller than the M87 black hole, matter orbited it many times during each observing session, producing a rapidly changing signal rather than a steady one, says Luciano Rezzolla, a theoretical astrophysicist at the Goethe University Frankfurt in Germany and a member of the EHT team. That makes the data more complicated to interpret, but also potentially richer in information.

What exactly are we looking at?

Try and grasp this – you are seeing light in orbit.

The event horizon is a distortion of space-time. The black hole is so strong that light rays bend so much that they in effect are in orbit. Another effect is that it looks a lot larger than it actually is, and again this is because of this actual distortion of space-time. It is akin to putting a spoon into a glass of water and seeing what appears to be a far larger spoon than it actually is.

A planet sized telescope

The team that has done this has created a collaboration that ties together radio telescopes all around the planet, and so this gives them an instrument that is effectively the size of Earth. It really needs to be that big to see that far.

They still have gaps, and so they have plans to improve their network of dishes by putting instruments in places such as Africa and also Greenland.

Deeper Details: The published papers

There is not one, but instead a related set of five scientific papers that go through it all in great detail. We shall take a very quick pass at each. They are all available within “The Astrophysical Journal Letters” and were published on 10th April 2019.

(Side note: These papers all have a vast number of contributors, and they are long, very long, very detailed, and quite technical).

First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole

When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. … Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5 ± 0.7) × 109 M. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

First M87 Event Horizon Telescope Results. II. Array and Instrumentation

The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ~1.3 mm, EHT angular resolution (λ/D) is ~25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s−1, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

First M87 Event Horizon Telescope Results. III. Data Processing and Calibration

We present the calibration and reduction of Event Horizon Telescope (EHT) 1.3 mm radio wavelength observations of the supermassive black hole candidate at the center of the radio galaxy M87 and the quasar 3C 279, taken during the 2017 April 5–11 observing campaign. These global very long baseline interferometric observations include for the first time the highly sensitive Atacama Large Millimeter/submillimeter Array (ALMA); reaching an angular resolution of 25 μas, with characteristic sensitivity limits of ~1 mJy on baselines to ALMA and ~10 mJy on other baselines. The observations present challenges for existing data processing tools, arising from the rapid atmospheric phase fluctuations, wide recording bandwidth, and highly heterogeneous array. In response, we developed three independent pipelines for phase calibration and fringe detection, each tailored to the specific needs of the EHT. The final data products include calibrated total intensity amplitude and phase information. They are validated through a series of quality assurance tests that show consistency across pipelines and set limits on baseline systematic errors of 2% in amplitude and 1° in phase. The M87 data reveal the presence of two nulls in correlated flux density at ~3.4 and ~8.3 Gλand temporal evolution in closure quantities, indicating intrinsic variability of compact structure on a timescale of days, or several light-crossing times for a few billion solar-mass black hole. These measurements provide the first opportunity to image horizon-scale structure in M87.

First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole

We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of ~40 μas, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others’ work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions.

First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring

The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87’s large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.


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One thought on “Inside the Black Hole Picture

  • Donald Sauter

    How can something collapse to infinite density in a finite time period? How would it change things if the unseen thing in the middle were not infinitely dense, but only very dense, such as pea-sized, or baseball-, moon-, earth-, sun-, or solar-system-sized?