Solving The Mysteries Of Our Galaxy's Dark And Secretive Heart

Supermassive black holes are thought to hide in the dark and secretive hearts of every large galaxy in the Universe, and these hungry, hidden beasts can weigh millions to billions of times more than our Star, the Sun. In fact, our starlit, barred-spiral, Milky Way Galaxy harbors its own mysterious supermassive black hole. Our resident beast, lurking in sinister secret in the heart of our Galaxy, is named Sagittarius A * (pronounced Sagittarius- A-Star ) or Sgr A *, for short, and it is a relative light-weight, as far as supermassive black holes go, weighing-in at a “mere” 4 million solar-masses. Sgr A * has kept its many secrets well, but in October 2014, astronomers announced that they may be in the process of solving one of these myriad mysteries: is matter falling into the supermassive beast haunting the dark heart of our Milky Way or is it being ejected from it ? That is the question – and no one knows for sure, but a University of California at Santa Barbara (UCSB) astrophysicist is searching for an answer.

Dr. Carl Gwinn, a professor in UCSB's Department of Physics, and colleagues, have analyzed images obtained by the Russian spacecraft Radio Their findings are published in the September 30 2014 issue of The Astrophysical Journal Letters.

RadioAstron was launched into orbit from Baikonur, Kazakhstan, in July 2011, along with several other scientific missions, one of which was to study the scattering of pulsars by interstellar gas. Pulsars are the very dense cores of dead, massive stars that have perished in the raging, fiery fury of a supernova blast. What the team of astrophysicists discovered led them to study additional observations of Sgr A * –which is visible at radio, infrared and X-ray wavelengths.

Black Hearts

Over the past two decades, astronomers have managed to collect powerful evidence in support of the idea that our Galaxy does indeed host a supermassive black hole, hiding in sinister secret, waiting for its dinner – a doomed star, perhaps, or an unfortunate cloud of gas. Because this mysterious entity lurks relatively close to Earth, it provides valuable information about the weird, and bewildering, way that extreme gravity behaves – and it also sheds light on Albert Einstein's Theory of General Relativity (1915). Because black holes are so utterly black, astronomers must try to understand their bizarre properties by studying the light that is emitted from the searing-hot, glaring gas immediately circling them.

Despite their name, black holes are far from being empty space. Rather, they form when an extremely large amount of matter is crowded into a very small area! Supermassive black holes are unambiguously some of the most bizarre entities lurking in the Universe. These bewitching and strange objects gain weight by feasting on their surroundings, and they are insatiably hungry, greedily consuming gas, stars, and whatever else wanders in too close to their irresistible gravitational embrace! Black holes also have bad table manners, and are extremely sloppy as they greedily swallow their unfortunate dinners, seeking to bite of more than they can chew. Sgr A * is a calm, sluggish, elderly black hole now, but it was much more active in its voracious and brilliant blazing youth, billions of years ago, when our very ancient Galaxy was young.

Smaller black holes of “only” stellar mass also haunt the Cosmos. These relatively light-weight and comparatively petite gravitational monsters are born from the burned-out wreckage of a very heavy star that has perished in the incandescent violence of a glaringly bright and fiery supernova explosion – that blasted the former star into oblivion. The supernova fireworks signal the tragic end of a massive star's brilliant “life” as a main-sequence (hydrogen-burning) stellar denizen of the Cosmos. After a black hole has formed from the wreckage of what was once its star, it can continue to gain more and more weight by consuming whatever is unlucky enough to travel in too close to its greedy, grasping, snatching claws of merciless, pulling gravity.

Stars and gas swirl around and then down, down, down into the violently whirling vortex of enormous supermassive black holes, and this tumbling feast creates a huge disk, termed an accretion disk. This doomed banquet grows increasingly hotter and hotter, and then sends forth an enormous amount of radiation, as it arrives ever-closer to that hell-like point where it must abandon all hope – entering that infamous point of no return called the event horizon . The event horizon is at the innermost region of the accretion disk.

Albert Einstein's General Theory of Relativity predicts the existence of black holes, which he postulated to be entities possessing such deep gravitational wells that nothing –not even light – could escape to freedom. Anything unfortunate enough to travel in too close to one of these insatiably hungry black holes is doomed to be devoured. However, the real existence in the Universe of such gravitational monstrosities seemed so bizarre at the time, that even Einstein questioned his own prediction. However, eventually, he came to colorfully characterize them in this way: “Black holes are where God divided by zero”.

Black holes can be defined as a region in Spacetime where the grasp of gravity is so strong that literally nothing can escape from its pull. The incredible lure of gravity is intensely powerful because a large amount of matter has been squeezed into a very tiny space. Crowd enough matter into a small enough space, and you will get a black hole every time!

Most supermassive black holes, such as our Milky Way's own Sgr A *, accrete at a lazy rate, and are difficult to distinguish from the dark hearts of the galaxies in which they dwell. Sgr A * provides a precious and instructive exception to this general rule. This is because astronomers can get a closer view of its comparatively gentle X-ray emission. However, Sgr A * itself does not emit radiation, but is instead visible from the gas that swirls around it. The gas is being acted upon by Sgr A * 's extremely powerful gravitational field. The wavelengths that render Sgr A * visible are scattered by interstellar gas along the line of sight in a way that is comparable to how light is scattered by fog on our own planet.

Our Milky Way's Dark And Secretive Heart

Dr. Gwinn and his team discovered that the pictures taken by RadioAstron showed small spots. “I was quite surprised to find that the effect of scattering produced images with small lumps in the overall smooth image. We call these substructure. Some previous theories had predicted similar effects in the 1980s, and a quite controversial observation in the 1970s had hinted at their presence, “Dr. Gwinn explained in the October 13, 2014 UCSB Press Release.

In order to acquire a better understanding of the mysterious substructure, Dr. Michael Johnson, Dr. Gwinn's former graduate student who is currently at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, conducted theoretical research. He came to the realization that the strange anomalies could be used to determine the true size of the underlying source.

Still more observations were conducted using the Very Long Baseline Array , which is an interferometer made up of 10 identical antennas distributed across the United States. In addition, the 100-meter Green Bank Telescope located in West Virginia revealed the haunting presence of lumps in the image of Sgr A * . More recent upgrades have greatly enhanced the sensitivity of these telescopes but, even so, evidence of the lumps – or substructure – remain very faint.

“The theory and observations allow us to make statements about the interstellar gas responsible for the scattering, and about the emission region around the black hole. It turns out that the size of that emission region is only 20 times the diameter of the event horizon as it would be seen from Earth. With additional observations, we can begin to understand the behavior of this extreme environment, “Dr. Johnson said in the October 13, 2014 UCSB Press Release.

Even though no scientific team has yet been able to produce a complete image of Sgr A * 's emission, astronomers have drawn some inferences pertaining to scattering properties derived from observations conducted at longer wavelengths. “From these they can extrapolate those properties to 1 centimeter and use that to make a rough estimate of the size of the source. We seem to agree quite well with that estimate,” Dr. Gwinn said in the UCSB Press Release.

Dr. Gwinn and his team not only were able to directly confirm these indirect inferences about the size of Sgr A * , they were also able to provide new information regarding fluctuations in the interstellar gas that cause scattering. Their study demonstrates that the spectrum of interstellar turbulence is shallow.

“There are different ways of interpreting observations of the scattering, and we showed that one of them is right and the others are wrong. This will be important for future research on the gas near this black hole. This work is a good example of the synergy between different modern research infrastructures, technologies, and science ideas, “study co-investigator Dr. Yuri Kovalev explained in the UCSB Press Release. Dr. Kovalev is the RadioAstron project scientist.

There is a friendly international race currently going on to see who will be the first to finally image the black hole's emissions and thereby determine whether gas falls into the black hole or is being ejected in the form of a jet.

“The character of the substructure seems to be random, so we are keen to go back and confirm the statistics of our sample with more data. We're also interested in looking at shorter wavelengths where we think the emission region may be smaller and we can get closer to the black hole. We may be able to extract more information than just the size of the emission region. We might possibly be able to make a simple image of how matter falls into a black hole or is ejected from it. would be very exciting to produce such an image, “Dr. Gwinn commented in the UCSB Press Release.