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Peering Into Our Galaxy's Hidden Dark Heart

Every large galaxy in the visible Universe hides a mysterious dark heart. Even our vast, starlit spiral Milky Way Galaxy holds in its secretive center a gluttonous heart of darkness--a supermassive black hole that weighs millions of times more than our Sun. However, our Galaxy's dark-hearted resident is puny in comparison to some others of its bizarre kind. Indeed, some supermassive beasts that haunt the hidden hearts of their galactic hosts can weigh as much as billions of times solar-mass. Our Milky Way's supermassive black hole is named Sagittarius A* or Sgr A*, for short (pronounced Saj-A-Star), and it is a peaceful old black hole now, sleeping quietly most of the time--except for when a tasty morsel of some spaghettified star or cloud of doomed gas travels too close to its waiting maw. At that point, Sgr A* awakens for one brief shining moment to dine greedily and sloppily on this infalling banquet.

In astrophysics, the term spaghettification refers to the vertical stretching and horizontal compression of objects into long thin shapes in an extremely powerful and homogeneous gravitational field--giving these unfortunate objects a spaghetti-type appearance.

In May 2018, a team of astronomers announced that they have used a global array of telescopes, including the Atacama Pathfinder Experiment (APEX), in order to peer at the beast that lurks darkly in the heart of our Milky Way. This new study reveals the finest details collected so far on event horizon scales in the center of our Galaxy. The event horizon of a black hole is that dreaded point of no return from which nothing, nothing, nothing at all--not even light--can escape from the gravitational grip of the beast, and is doomed to be swallowed.

APEX is a radio telescope 5,100 meters above sea level at the Llano de Chajntor Observatory located in the Atacama desert in northern Chile. This 12 meter radio telescope has been outfitted with special equipment including broad bandwidth recorders and a stable hydrogen maser clock for the purpose of performing joint inteferometric observations with other telescopes at short wavelengths. The goal of these observations is to obtain the best-ever image of the shadow of the hidden black hole. The addition of APEX to the Event Horizon Telescope (EHT), which until recently was composed of antennas only in the northern hemisphere, was able to uncover in new and unprecedented detail the long-enshrouded structure of the secretive Sgr A*. The greatly improved angular resolution provided by the APEX telescope can now show long-hidden details of the asymmetric and not point-like source structure, as small as 36 million kilometers. This corresponds to dimensions that are three times larger than the still-hypothetical size of our Galaxy's resident dark-hearted supermassive beast.

The team of astronomers are seeking the holy grail that will ultimately prove Albert Einstein's Theory of General Relativity (1915)--which is to obtain a direct image of the shadow of a black hole. Their quest to find this elusive shadow is greatly aided by combining radio telescopes that are spread all over the Earth using a technique called Very Long Baseline Interferometry (VLBI). The telescopes participating in this search are located at high altitudes--in order to minimize the disturbance caused by our planet's atmosphere--and are also situated at remote locations with normally clear skies. This allows astronomers to observe the secretive compact radio source that reveals the mysterious presence of Sgr A* lurking in the dark heart of our Milky Way.

Supermassive Beasts

The supermassive black holes, that haunt the hearts of large galaxies, can weigh millions to billions of times more than our Sun. However, a black hole of any size can be described by only three properties: electric charge, spin (angular momentum), and mass. In addition to supermassive gravitational beasts, there are also black holes of "only" stellar mass, that are born when a particularly massive star has managed to burn all of its necessary nuclear-fusing fuel, and has reached the terrible end of that long stellar road when it contains a core of iron. At that point, the massive star collapses in the fiery fury of a supernova tantrum that results in the erstwhile star becoming a black hole of stellar mass. After a stellar mass black hole has formed from the wreckage of its massive progenitor star, it can continue to acquire more and more mass by "eating" ill-fated celestial objects that have wandered too close to its gravitational pull.

Black holes can be large or small, and these bizarre entities can be defined as an area of Spacetime where the tug of gravity has become so extreme that not even light can escape to freedom. The pull of gravity has grown intensely powerful because matter has been squeezed mercilessly into a very small space. Crush enough matter into a sufficiently small space, and a black hole will be born every time.

Most supermassive black holes, like Sgr A*, accrete matter somewhat lazily. This unfortunately makes it difficult for astronomers to distinguish them from the galactic dark hearts in which they lurk. For this reason, Sgr A* provides a valuable exception to this very frustrating rule. This is because astronomers are able to obtain a close view of its rather gentle X-ray emission.

Fortunately, astronomers have been able to learn quite a lot about Sgr A*. Our Galaxy's central supermassive beast weighs-in at about four million times that of our Sun--which, incredibly, makes it a relative runt, at least as far as supermassive black holes go. Sgr A* is encircled by a cluster of glittering baby stars, some of which have been unlucky enough to have wandered in to within only a few billion miles of where the gravitational beast lurks secretively in wait for its dinner. Sgr A* is quiet now, in its old age, but this was apparently not the case about a century ago when it messily feasted on an unfortunate blob of material that had traveled too close to where it lay hidden. This feast is responsible for creating a multicolored shimmering, glimmering explosive fireworks display that lit up our Galaxy's hungry dark heart.

Because Sgr A* is located relatively close to our own planet, it provides important information about the way that extreme gravity behaves, and thus helps shed new light on General Relativity.

The Strange Lair Of Sgr A*

In August 1931, the American physicist, Karl Jansky (1905-1950)--considered to be the father of radio astronomy--detected a mysterious radio signal coming from a location at the heart of our Milky Way. The strange signal originated in the direction of the constellation Sagittarius. Sgr A* itself was discovered on February 13 and 15, 1974, by astronomers Dr. Bruce Balick of the University of Washington and the late Dr. Robert Brown (1943-2014), using the baseline interferometer of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virgina. The name Sgr A* was first used by Brown in a 1982 research paper. This is because he believed that the mysterious radio source was "exciting"--and excited states of atoms are denoted with asterisks--hence, Sgr A*.

On October 16, 2002, an international team of astronomers, led by Dr. Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany, reported that, for more than a decade, they had been observing the movement of a star, dubbed S2, situated near Sgr A*. The team of astronomers proposed that the data they had obtained eliminated the possibility that Sgr A* harbors a cluster of dark stellar objects or a mass of degenerate fermions. Their proposal strengthened the evidence for the existence of a supermassive black hole lurking in the dark heart of our Milky Way.

Sgr A* itself is a very compact, bright radio source, located near the border of the constellations Sagittarius and Scorpius. It is a region located within a larger astronomical feature dubbed Sagittarius A.

Unfortunately, astronomers have not been able to observe Sgr A* in optical wavelengths. This is because it is enshrouded in a thick blanket of dust and gas that is situated between the source and our own planet. Several teams of astronomers have made the effort to image Sgr A* in the radio spectrum using very-long-baseline-interferometry (VLBI). At a distance of 26,000 light-years, the VLBI observations found that the mysterious radio source has a diameter of 44 million kilometers. By comparison, Earth is 150 kilometers from our Sun, and the innermost major planet Mercury is 46 million kilometers from our Sun when it is closest to it (perihelion).

As of April 2017, there have been direct radio images obtained of Sgr A* by astronomers using the Event Horizon Telescope (EHT). However, as of May 2018, this data is still being processed, and images have yet to be released. The EHT succeeded in combining images taken from widely spaced observatories at different locations on our planet. This was done in order for astronomers to obtain a higher picture resolution. It is hoped that the measurements will test Einstein's Theory of General Relativity more rigorously than earlier studies. If discrepancies exist between Einstein's theory and actual observations are found, it will suggests that scientists may have identified physical conditions under which the theory breaks down.

Current observations indicate that Sgr A*'s radio emissions are not being sent out into space by the black hole itself. Instead, the emissions seem to be originating from a bright region surrounding the black hole. This region is near the event horizon, possibly in the accretion disk. Alternatively, it could be a relativistic jet of material being hurled out from the disk. If the position of Sgr A* were precisely centered on the supermassive gravitational beast, it would be possible to observe it magnified larger than its actual size.This is because of the phenomenon of gravitational lensing. Gravitational lensing is a prediction of General Relativity proposing that the gravity of a foreground object can warp, bend, or magnify the light being emitted from a background object that it is aligned with. Thus, gravitational lensing is a natural gift, of sorts, to astronomers trying to observe remote objects that otherwise could not be seen--the foreground lens magnifies, or otherwise warps, the light emanating from the background object that is being lensed--thus making it visible to the prying eyes of curious astronomers. By using gravitational lensing as an observational tool, astronomers were able to determine that our Galaxy's resident supermassive black hole sports a mass of approximately 4 million times that of our Sun.

The Many Mysteries Of Our Galaxy's Heart Of Darkness

The research team of astronomers began their observations of Sgr A* in 2013, using the Very-Long-Baseline Interferometry (VLBI) telescopes located at four different sites. The telescopes that the researchers used include the APEX telescope, the Combined Array for Research in Millimeter Wave Astronomy (CARMA) array in California, the James Clerk Maxwell Telescope (JCMT) in Hawaii, the phased Submillimeter Array (SMA) in Hawaii, and the Submillimeter Telescope (SMT ) in Arizona. Sgr A* was spotted by all stations and the longest baseline length extended up to almost 10,000 kilometers. This suggests an ultra-compact and asymmetric (not point-like) source structure.

"The participation of the APEX telescope almost doubles the length of the longest baselines in comparison to earlier observations and leads to a spectacular resolution of 3 Schwarzschild radii only," commented Dr. Ru-Sen Lu in a May 24, 2018 Max Planck Institute for Radio Astronomy (MPIfR) Press Release. Dr. Lu is of of the MPIfR in Bonn, Germany, and is lead author of the paper describing the new research.

"It reveals details in the central radio source which are smaller than the expected size of the accretion disk," added Dr. Thomas Krichbaum in the same MPIfR Press Release. Dr. Krichbaum is the initiator of the mm-VLBI observations with APEX.

APEX's location in the southern hemisphere considerably improved the quality of the images for a source as far south in the sky as Sgr A*. Indeed, APEX has succeeded in "paving the way" for the inclusion of the large and very sensitive ALMA telescope into the EHT observations, which are currently being undertaken one time annually.

"We have worked hard at an altitude of more than 5000 meters to install the equipment to make the APEX telescope ready for VLBI observations at 1.3 mm wavelengths," explained Dr. Alan Roy in the MPIfR Press Release. Dr. Roy, who is also from the MPRfR, leads the VLBI team at APEX. "We are proud of the good performance of APEX in this experiment," he added.

The team of astronomers used a model-fitting procedure in order to study the event-horizon-scale-structure of Sgr A*. "We started to figure out what the horizon-scale-structure may look like, rather than just draw generic conclusions from the visibilities that we sampled. It is very encouraging to see that the fitting of a ring-like structure agrees very well with the data, though we cannot exclude other models, e.g., a composition of bright spots," Dr. R-Sen Lu explained in the same MPRfR Press Release. Observations in the future with still more telescopes added to the EHT will help sort out residual ambiguities in the imaging.

Our Galaxy's resident supermassive heart of darkness is embedded in a dense interstellar medium. This may affect the propagation of electromagnetic waves along the line of sight. "However, the interstellar scintillation, which in principle may lead to image distortions, is not a strongly dominating effect at 1.3 mm wavelength," noted Dr. Dimitrios Psaltis in the May 24, 2018 MPRfR Press Release. Dr. Psaltis is the EHT project scientist at the University of Arizona in Tucson.

Dr. Sheperd Doeleman, of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and director of the EHT project, noted in the same MPRfR Press Release that "The results are an important step to ongoing development of the Event Horizon Telescope. The analysis of new observations, which since 2017 also include ALMA, will bring us another step closer to imaging the black hole in the center of our Galaxy."