Our best black hole image ever, inside and out

All throughout the Universe, black holes are abundant.

This view of about 0.15 square degrees of space reveals many regions with large numbers of galaxies clustered together in clumps and filaments, with large gaps, or voids, separating them. Each point of light is not a galaxy, but a supermassive black hole, revealing just how ubiquitous these cosmic objects are. By estimating the black hole mass function across cosmic time, researchers have a suggestive solution to the “seeds of supermassive black holes” question, suggesting that conventional astrophysics may have given rise to the objects we observe at all cosmic times.

Even light cannot escape from within these dense, gravitational regions.

Both inside and outside the event horizon of a Schwarzschild black hole, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape.

Many are formed from the core-collapse of massive stars.

The anatomy of a very massive star throughout its life, culminating in a Type II (core-collapse) Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive core-collapse supernovae typically result in the creation of black holes, while the less massive ones create only neutron stars.

Others arise from mergers of less massive objects.

The populations of black holes, only, as found through gravitational wave mergers (blue) and X-ray emissions (magenta). As you can see, there is no discernable gap or void anywhere above 20 solar masses, but below 5 solar masses, there’s a dearth of sources. This helps us understand that neutron star-black hole mergers are unlikely to generate the heaviest elements of all, but that neutron star-neutron star mergers can, and can also result in the formation of a black hole. The population of black holes and/or neutron stars between about 2 and 5 solar masses, at the lowest-end of the black hole mass range, is where the greatest uncertainties lie.

But the most massive ones of all reside at the centers of galaxies.

This map shows a 1-year view of the entire gamma-ray sky from NASA’s Fermi satellite. The growing-and-shrinking sources are active galaxies powered by supermassive black holes, but the transient “blips” that appear are the gamma-ray bursts that are so sought after, many of which are thought to also create black holes, albeit not the supermassive type. When the Moon enters the field-of-view of the telescope, it can temporarily become the brightest gamma-ray source in the entire sky.

Supermassive black holes grow via mergers and accretions, to millions or even billions of solar masses.

When two black holes merge, a significant portion of their mass can get converted into energy in one very short time interval. But for a much longer period of time, there’s an earlier stage where these black holes orbit with periods of 1-10 years, and pulsar timing can be sensitive to the cumulative effects of those systems throughout the cosmos. Supermassive black holes may primarily grow due to these types of mergers.

Even our own Milky Way has one: 4.3 million solar masses big.

This 20-year time-lapse of stars near the center of our galaxy comes from the ESO, published in 2018. Note how the resolution and sensitivity of the features sharpen and improve toward the end, all orbiting our galaxy’s (invisible) central supermassive black hole. Practically every large galaxy, even at early times, is thought to house a supermassive black hole, but only the one at the center of the Milky Way is close enough to see the motions of individual stars around it, and to thereby accurately determine the black hole’s mass. The actual number density of black holes in the Universe, and their number density as a function of mass, remains only poorly estimated, with large uncertainties remaining.

From our perspective on Earth, it’s the largest black hole in terms of angular size.

On September 14, 2013, astronomers caught the largest X-ray flare ever detected from the supermassive black hole at the center of the Milky Way, known as Sagittarius A*. In X-rays, no event horizon is visible at these resolutions; the “light” is purely disk-like. However, we can be certain that only matter remaining outside the event horizon generates light; matter passing within it gets added to the black hole’s mass, inevitably infalling into the black hole’s central singularity. Many types of transients are now known to exist across many different wavelengths of light.

But the second-largest has even more spectacular features: at the center of galaxy Messier 87.

Size comparison of the two black holes imaged by the Event Horizon Telescope (EHT) Collaboration: M87*, at the heart of the galaxy Messier 87, and Sagittarius A* (Sgr A*), at the center of the Milky Way. Although Messier 87’s black hole is easier to image because of the slow time variation, the one around the center of the Milky Way is the largest as viewed from Earth. The angular size of the event horizons, and how they appear to our telescopes due to relativistic effects, matches the predictions of Einstein’s general relativity exquisitely.

Messier 87 is the most massive galaxy within the Virgo Cluster: some 55 million light-years away.

Messier 87, best known as the supermassive galaxy whose black hole was first imaged by the Event Horizon Telescope, has its relativistic jets and the shockwaves created by their material imaged in the infrared by Spitzer, amidst the mass of shining stars (in blue). Messier 87 is the most massive (and second-brightest) galaxy within the entire Virgo cluster of galaxies, and it is the central black hole that generates these relativistic jets.

It emits a central jet of radiation extending for 5,000+ light-years.

Streaming out from the center of M87 like a cosmic searchlight is one of nature’s most amazing phenomena: a black-hole-powered jet of subatomic particles traveling at nearly the speed of light. In this Hubble image, the blue jet contrasts with the yellow glow from the combined light of billions of unresolved stars and the point-like clusters of stars that make up this galaxy. The jet itself extends for more than 5,000 light-years in space, and is visible in even optical wavelengths.

That jet is powered by a 6.5 billion solar mass supermassive black hole.

An illustration of an active black hole, one that accretes matter and accelerates a portion of it outward in two perpendicular jets. The normal matter undergoing an acceleration like this describes how quasars and active galaxies work extremely well. Flows of matter inside the accretion disk can lead to flares in a black hole’s emissions. All known, well-measured black holes have enormous rotation rates, and the laws of physics, particularly the conservation of angular momentum, all but ensure that this is mandatory.

We’ve now measured its extended X-ray emissions,

This image of the central region of galaxy Messier 87 comes in the X-ray, via NASA’s Chandra X-ray observatory. The central, supermassive black hole is blasting out energetic particles at 99%+ the speed of light, which produces X-rays visible for up to 18,000 light-years from the galactic center.

its extended radio lobes,

This three-panel image shows the extended radio emissions of Messier 87*, at top-left, the Hubble optical image of the jet, at top-right, and a radio image using very-long baseline technology of the region close to the black hole, clearly collimated by a magnetic field, with higher radio energies shown in red.

the accelerated matter arising from its accretion disk,

This image shows a map of the accretion disk around the event horizon of the black hole at the center of galaxy Messier 87, the extended radio jets being launched from that disk, and a reconstruction of the event horizon according to a likelihood estimator. The ring-like accretion structure, in theory, connects to the launched jet from the black hole, and that is seen here.

radio light at the event horizon itself,

The famous image of the first black hole ever directly observed, the one at the center of the galaxy Messier 87, changes over time. Observations from different days have different features, and taking an average causes us to lose the time-varying component of the data. With a light-travel time of about 1 day across the event horizon, larger differences are seen between the 2nd and 3rd images than either the 1st and 2nd or 3rd and 4th.

evolving over time,

Polarized view of the black hole in M87. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. Note how much “swirlier” this image appears than the original, which was more blob-like. It’s fully expected that all supermassive black holes will exhibit polarization signatures imprinted upon their radiation, a calculation that requires the interplay of General Relativity with electromagnetism to predict.

plus the polarization of that radio light.

By connecting the ring-like accretion structure at a black hole’s core with the observed jet from a variety of different observations, we have been able to piece together a continuous picture of how this jet is launched from right outside the event horizon of Messier 87* to several thousand light-years away. This makes Messier 87* the best-imaged black hole of all-time, from the inside out.

It’s the most clearly imaged black hole ever, from the event horizon to thousands of light-years away.

This zoom-illustration shows the full scale of the galaxy Messier 87 complete with its relativistic jet in optical light (main), a very-long baseline interferometry view of its central region with a ring-like accretion feature and launched jets (inset), and the polarized light view of the event horizon itself (second inset). From the inside out, it’s the most accurate view ever obtained of any black hole ever.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.