Earth’s location in the suburbs of the Milky Way galaxy provides us with a relatively quiet and safe haven, far from the hazards of all the high-energy phenomena that take place at the center of our galaxy, 26,000 light-years away, in the direction of the Sagittarius constellation. Despite our large distance from the Milky Way’s central regions, astronomers have been meticulously studying the latter’s surroundings for decades, in the hopes of gaining more insights about all the energetic goings-on that take place at the heart of our galaxy, while probing the physical processes which power the supermassive black hole that is believed to lie there. Now, by using the power of NASA’s Hubble and Chandra space telescopes, two independent teams of astronomers have recently managed to measure the velocities and chemical composition of the two giant bubbles of outflowing gas that have been found to extend above and below the galactic center, while also detecting X-ray flares of unprecedented intensity from the vicinity of its supermassive black hole.
Studying the Breeze of the Galactic Winds
Of all the objects in the Universe, black holes can be considered the most fascinating and mystifying: extremely massive objects of infinite density which occupy a zero volume and from where nothing, not even light, can escape. Due to their very nature, black holes can’t be observed directly, making their study a really painstaking process. Nevertheless, because of the immense gravitational forces that they exert on their surroundings, they are responsible for some of the most violent and energetic phenomena in the Cosmos, which can often be observed from billions of light-years away. A long list of observational evidence, which have been collected with ground- and space-based telescopes in recent decades, have indicated that indeed most galaxies in the Universe harbor such gargantuan powerhouses at their centers, containing between a few million to a billion times the mass of the Sun in an area of space no bigger than our own Solar System. The cores of many of these galaxies, which are better known as Active Galactic Nuclei, or AGNs, exhibit such high luminosities across the entire electromagnetic spectrum, outshining all the rest of the stars in the galaxy itself. The consensus among astronomers is that the mechanism behind these high-energy phenomena is the accretion of matter from the supermassive black holes that reside there, which is heated to many millions of degrees in the process and emits very high-energy X-rays before finally falling toward oblivion into the black hole itself.
The energy that is released from AGNs often results in the creation of powerful jets along the galaxies’ polar axes, which emit huge streams of matter that can stretch for many thousands of light-years away from the core into intergalactic space, like in the case of the nearby massive elliptical galaxy M87. In addition, the combined energy output of the jets and accretion disks, as well as the very high rates of star formation and supernova explosions in many active and starburst galaxies respectively, can result in the formation of diffuse, galaxy-wide outflows of material that can expel huge amounts of interstellar gas away from the galaxies themselves, at speeds of up to a few thousand kilometers each second, like in the case of spiral galaxies M82 and NGC 1068. These “galactic winds” are thought to play an important role in the evolution of galaxies, by affecting the distribution of their interstellar material and regulating their overall star formation rates, while also enriching the intergalactic medium with heavy elements, thus making the better understanding of the processes that drive them a major topic of research in astrophysics.
Despite their importance in the study of galactic dynamics, galactic winds had traditionally been poorly documented ever since they were first discovered more than 40 years ago, mainly due to the fact that their constituent gas becomes increasingly diffuse with distance, hindering astronomers’ ability to study it in distant galaxies. Nevertheless, a newer generation of ground- and space-based telescopes that have come online during the last 15 years have helped to revolutionize astrophysics, allowing scientists to make many important discoveries. One of these was the detection by NASA’s Fermi Gamma-ray Space Telescope in 2010 of two giant high-energy emitting bubbles of outflowing gas that extend approximately 25,000 light-years above and below the central regions of our own Milky Way galaxy. Even though the exact origin of these structures remains unknown, most astronomers believe that they are the result of galactic winds moving outward from the center of our galaxy, creating a bipolar distribution of material perpendicular to the disk of the Milky Way. Their exact source is still an object of debate, with the two most popular hypotheses being either the eruption of material from our galaxy’s supermassive black hole, or the result of outflowing winds from an intense star formation activity at the central regions of our galaxy. “In other galaxies, we see that starbursts can drive enormous gas outflows,” says David Spergel, a theoretical astrophysicist at Princeton University in New Jersey. “Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics.”
One way for astronomers to answer some of these questions would be to determine the chemical composition of the Fermi bubbles and to study their motion along our line of sight. To that end, a research team led by Dr. Andrew Fox, an astronomer at the Space Telescope Science Institute in Baltimore, Md., presented the results of just such observations, at the recent 225th Meeting of the American Astronomical Society, in Seattle, Wash. The researchers, whose study had been accepted for publication at The Astrophysical Journal Letters, utilised the superior capabilities of the Hubble Space Telescope, in order to conduct detailed spectroscopic observations of both the near and far side of the gas of the northern Fermi bubble, allowing them to probe its properties for the first time. “When you look at the centers of other galaxies, the outflows appear much smaller because the galaxies are farther away,” said Fox in a statement. “But the outflowing clouds we’re seeing are only 25,000 light-years away in our galaxy. We have a front-row seat. We can study the details of these structures. We can look at how big the bubbles are and can measure how much of the sky they are covering.”
Fox’s team used the Cosmic Origins Spectrograph, or COS, onboard Hubble in February 2014, in order to target the distant quasar PDS 456 in ultraviolet wavelengths, which was positioned just 15 degrees above the Milky Way center and whose light passed through the northern bubble along the researchers’ line of sight. By analysing the absorption lines that were imprinted in the quasar’s spectrum as its light traveled through the gas in the bubble, they were able to measure the latter’s speed relative to Earth, as well as its temperature and chemical composition. The astronomers’ results showed that the light from the gas at the bubble’s near side was shifted toward shorter wavelengths, indicating that it was moving toward Earth, while the one from the bubble’s far side was redshifted, indicating that it was moving away. Overall, Fox’s team was able to determine that the galactic winds from the Milky Way’s center, which were shaping the Fermi bubbles, were blowing at a speed of approximately 900 to 1,000 km/sec or 3 million km/h. “This is exactly the signature we knew we would get if this was a bipolar outflow,” explained Rongmon Bordoloi a post doctoral fellow at the Space Telescope Science Institute and co-author on the study. “This is the closest sight line we have to the galaxy’s center where we can see the bubble being blown outward and energized.”
“Once we have the speed [of the galactic wind] and we also know how far away from the center of the galaxy this gas has gone, we can determine its age – how long it has taken for the gas to get to that point,” explained Fox during the presentation of his team’s findings at the recent American Astronomical Society meeting. “And when we do that calculation, we find its between 2.5 and 4 million years old. What that means is, that a few million years ago, there was a very energetic event close to the galactic center, where we see a remnant of that event where the gas has been blown up into the [galactic] halo and is now venting into the Fermi bubbles.”
Complementary observations of PDS 456 with the Green Bank radio telescope in West Virginia in October 2014 revealed that the outflowing gas in the bubble had a temperature of approximately 10,000 degrees Celsius, which was much cooler than that of the superheated material of millions of degrees of the accretion disk around the Milky Way’s supermassive black hole. This could indicate, according to the researchers, that the much cooler interstellar gas is compressed and carried along by the bubbles’ outward motion into intergalactic space. “We are seeing cooler gas, perhaps interstellar gas in our galaxy’s disk, being swept up into that hot outflow,” says Fox. The elemental composition of the gas also hinted at its stellar origin, with astronomers identifying the chemical fingerprints of silicon, carbon, and aluminum ions, which are heavy elements that are routinely forged in stellar cores. “It looks like there’s a link between the amount of star formation and whether or not these outflows happen,” adds Fox. “Although the Milky Way overall currently produces a moderate one to two stars a year, there is a high concentration of star formation close to the core of the galaxy.”
The study of PDS 456 is part of an observing program by Fox’s team, to study the light of a total of 20 quasars with the Hubble Space Telescope that passes through the Fermi bubbles, allowing them to measure their velocities at various locations above the galactic disk. Such studies could allow researchers to put more constraints on theoretical predictions regarding the exact mechanisms that power the Milky Way’s galactic winds and possibly determine their exact cause. “What is driving the [galactic] wind causing it to be launched into the [galactic] halo, is still an ongoing mystery,” explains Fox. “The two main explanations for this are: you have a burst of star formation and supernovae close to the galactic center that drove the gas out, or alternatively it has something to do with the supermassive black hole. The latter can also accrete gas onto its surroundings and that gas can then be blown out into the Fermi bubbles. So, what we’re doing is that we’re enlarging our sample instead of just looking into a single line of sight, we’re gonna be looking at more directions and looking for trends to see if the gas is slowing down, or if it’s accelerating as it moves away from the core.”
Probing the Heart of Darkness
Yet, as fascinating as the observations by Fox’s team were, the heart of the Milky Way had nevertheless some more exciting offerings to give to astronomers recently.
Past studies of our galaxy’s central regions had indicated that, despite being relatively moderate and quiet compared to those of other galaxies, the black hole that is believed to lie there, called Sagittarius A*, or Sgr A* for short, is a truly monstrous object, with a mass of approximately 4 million times that of the Sun, occupying a volume of space with a radius of no more than 44 million km, slightly less than the aphelion of Mercury’s orbit around the Sun. More direct evidence for the presence of Sgr A* came in recent years, following a series of observations of the orbits of dozens of stars that seemed to be revolving very closely around an invisible and very massive point source, with orbital velocities that exceeded 5,000 km per second.
Among the many closely packed objects that astronomers had observed in the vicinity of Sgr A* during the last decade was a massive hydrogen gas cloud called G2, whose orbital period around the black hole was calculated to be about 300 years. Theoretical predictions had shown that the gas cloud was ultimately on a rendezvous with oblivion and was expected to fall onto the Milky Way’s black hole in late 2013 or early 2014. Astronomers lined up every major ground- and space-based telescope for the occasion, eagerly awaiting to be treated to a spectacular celestial fireworks display, courtesy of Sgr A*. But when the time came and went, all that telescopes recorded was, well … nothing. Subsequent observations revealed that the gas cloud had instead remained intact following its close gravitational embrace with Sgr A*, leaving astronomers perplexed as to the reason why.
One of the research teams monitoring the behavior of G2 during its closest approach to the black hole, with the help of NASA’s Chandra X-ray space Observatory and the Very Large Array at Socorro, N.M., was led by Dr. Daryl Haggard, an assistant professor of astronomy, at the Amherst College in Massachusetts. Even though Haggard’s team was disappointed by what initially seemed as a total lack of action from Sgr A*, they were nevertheless rewarded with some unexpected findings in the end. “Unfortunately, the G2 gas cloud didn’t produce the fireworks we were hoping for when it got close to Sgr A,” said Haggard in a statement during the unveiling of her team’s results at the recent American Astronomical Society meeting. “However, nature often surprises us and we saw something else that was really exciting.” What the astronomers discovered was a surprisingly bright X-ray flare coming from Sgr A, during mid- September 2013, with a luminosity that was 400 times brighter than the average X-ray output of the black hole itself, making it the largest such flare ever to be detected in its vicinity. Furthermore, this particular spike in X-rays from Sgr A* was almost three times greater than the previous one which had been recorded a year earlier and twice that of the following one that was observed in October 2014.
By pinpointing the exact position of the X-ray flares, Haggard’s team soon realised that they were unrelated to the G2 gas cloud, which was located many billions of kilometers away at the time. “We do not think that these flares are connected to G2,” explained Haggard, during the presentation of her team’s findings at the recent American Astronomical Society meeting. “This is not pieces of G2 being pulled off and the reason for that is that the time scales don’t quite match. The time scales for these flares is fairly rapid; thousands of seconds, something in the order of an hour or two, which is really characteristic for objects that are something like 1 Astronomical Unit [away from Sgr A], roughly something like the Sun-Earth distance. G2’s closest encounter [with Sgr A] is 150 AU. So the time scale doesn’t actually match up for G2, at least not at this particular point in time.” According to the researchers, two of the most probable reasons for this sudden, unexpected rise of X-ray emissions from Sgr A* are: either the black hole had devoured a large asteroid that had passed by too close, or the flares were magnetic in origin, caused by the breakup and reconnection of the black hole’s magnetic field lines which have been entangled together, releasing huge amounts of energy in the process. “If an asteroid was torn apart, it would go around the black hole for a couple of hours – like water circling an open drain – before falling in,” says Fred Baganoff, a research scientist at the Massachusetts Institute of Technology and co-author of the study. “That’s just how long we saw the brightest X-ray flare last, so that is an intriguing clue for us to consider.”
Theoretical hypotheses aside, the fact is that the exact nature of the X-ray flares of Sgr A* remains a complete mystery. “The skeptical among you should be looking at this and saying ‘you’ve got to be kidding me, your two models are magnetic reconnection like something that happens on the Sun, or asteroids being shredded apart, like, you can’t do better than that?'” commented Haggard. “And the answer is ‘not yet’. This is an unsolved mystery, we really don’t know.”
Despite all the unknowns, however, these latest findings at the heart of our galaxy represent a chance for scientists to gain more important insights regarding the supermassive black hole that lies there, possibly shedding more light into many fundamental questions of astrophysics and cosmology in the process.
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