Milky Way’s Mystery Objects Look Like Gas Clouds, But Behave Like Stars

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It has been said, with a great deal of truth, that the longer we know someone the weirder that person gets. The Universe is like that; keeping its weird secrets to itself, only revealing them reluctantly to those curious observers who find themselves obsessed with its many mysteries. The center of our Milky Way Galaxy is secretive in nature, housing an elderly supermassive black hole that lurks there quietly in its old age, showing only the vestiges of its once-youthful voracious, sinister appetite. In June 2018, a team of astronomers announced that they have discovered several bizarre objects haunting the lair of this massive gravitational beast, and these strange objects are hiding their true identity behind a mask of impenetrable dust. These bizarre objects, that participate in this exotic masquerade in our Galaxy’s heart of darkness, look like clouds of gas–but behave like stars.

On June 6, 2018, at the American Astronomical Society’s (AAS’s) summer meeting in Denver, Colorado, a team of astronomers presented their findings concerning the weird nature of these mysterious, masked objects. The team, led by University of California, Los Angeles (UCLA) Postdoctoral Scholar Dr. Anna Ciurlo, announced their results, which they obtained using 12 years of data taken from the W.M. Keck Observatory on Maunakea, Hawaii.

„These compact dusty stellar objects move extremely fast and close to our Galaxy’s supermassive black hole. It is fascinating to watch them move from year to year. How did they get there? And what will they become? They must have an interesting story to tell,“ Dr. Ciurlo told the press on June 6, 2018.

The astronomers made their discovery by obtaining spectroscopic measurements of our Milky Way’s Center gas dynamics using Keck Observatory’s OH-Suppressing Infrared Imaging Spectrograph (OSIRIS).

„We started this project thinking that if we looked carefully at the complicated structure of gas and dust near the supermassive black hole, we might detect some subtle changes to the shape and velocity. It was quite surprising to detect several objects that have very distinct movement and characteristics that place them in the G-object class, or dusty stellar objects,“ explained Dr. Randy Campbell to the press on June 6, 2018. Dr. Campbell is science operations lead at Keck Observatory.

The mysterious G-objects were first discovered lurking near our Galaxy’s supermassive beast over a decade ago; G1 was first spotted in 2004, and G2 was discovered in 2012. Both of these weird objects were originally thought to be clouds of gas–that is, until they made their closest approach to the supermassive black hole, dubbed Sagittarius A* (Sgr A*, for short, pronounced Saj-A-Star). At that deadly point, both G1 and G2 somehow managed to survive the black hole’s extremely powerful pull, which usually rips unfortunate gas clouds to shreds.

„If they were gas clouds, G1 and G2 would not have been able to stay intact. Our view of the G-objects is that they are bloated stars–stars that have become so large that the tidal forces exerted by the central black hole can pull matter off of their stellar atmospheres when the stars get close enough, but have a stellar core with enough mass to remain intact. The question is then, why are they so large?“ astronomer Dr. Mark Morris commented to the press at the AAS 2018 summer meeting. Dr. Morris is a co-principal investigator and fellow member of UCLA’s Galactic Center Orbits Initiative (GCOI).

Apparently a great deal of energy was dumped into the weird G-objects. This caused them to swell up and become much larger than normal stars.

GCOI proposes that these weird G-objects originate from stellar mergers–whereby a duo of stars in a binary system smash into one another as a result of the gravitational perturbations of a black hole. As time goes by, the black hole’s powerful gravity changes the orbits of the duo of binary stars until they collide with catastrophic results. The resulting newly combined stellar object that emerges from the mess could explain where the excess amount of energy originated.

„In the aftermath of such a merger, the resulting single object would be ‚puffed up‘, or distended, for a rather long period of time, perhaps a million years, before it settles down and appears like a normal-sized star,“ Dr. Morris continued to explain to the press at the AAS meeting.

„This is what I find most exciting. If these objects are indeed binary star systems that have been driven to merge through their interaction with the central supermassive black hole, this may provide us with insight into a process which may be responsible for the recently discovered stellar mass black hole mergers that have been detected through gravitational waves,“ Dr. Andrea Ghez explained to the press. Dr. Ghez is the director of GCOI, that she founded back in 1994. The GCOI studies the innermost regions of our Galaxy at the highest angular resolution possible. It does this in order to shine new light on the birth and evolution of galaxies and their accompanying supermassive black holes through the measurement of orbital motion. The GCOI depends on a data set that has been gathered for almost a quarter-century at the W.M. Keck Observatory. This valuable data set led to the important discovery of Sgr A* haunting the hidden heart of our Milky Way–as well as detecting an unexpected nearby population of stars. GCOI holds the promise of testing Albert Einstein’s General Theory of Relativity (1915) close to a supermassive black hole, a new and unexplored regime for this fundamental force of nature.

Hunting The Supermassive Beast

The American physicist Karl Jansky (1905-1950) was the first to encounter a weird radio signal originating from a region located at the center of our Galaxy. Jansky, the „father of radio astronomy“, discovered that the mysterious signal was coming from the direction of the constellation Sagittarius. However, the hidden gravitational beast itself was discovered decades later–on February 13 and 15, 1974, by the astronomer Dr. Bruce Galick of the University of Washington and the late Dr. Robert Brown (1945-2016). In order to make their discovery of Sgr A*, the two astronomers used the baseline interferometer of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia.

On October 16, 2002, an international team of astronomers, led by Dr. Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany, announced that they have been studying the motions of a star, dubbed S2, for more than ten years. S2 is located very close to Sgr A*, and the team of astronomers proposed that the data they had been collecting eliminated the possibility that Sgr A* played host to either a cluster of dark stars or to a mass of degenerate fermions. Fermions are subatomic particles, such as nucleons (protons and neutrons) that have half-integral spin. The astronomers‘ proposal strengthens the case for the existence of a supermassive heart of darkness lurking in the Galactic Center.

Unfortunately, astronomers are not able to study Sgr A* at optical wavelengths because it is heavily blanketed by a dense, thick shroud of gas and dust, that frustratingly resides between Sgr A* and Earth. Nevertheless, astronomers have managed to solve a number of our Galaxy’s resident supermassive black hole’s many mysteries. First, Sgr A* weighs-in at about four million times the mass of our Sun–which makes it rather puny, at least by supermassive black hole standards. Other galaxies have been found to host a central supermassive black hole that boasts an immense mass of billions of times solar. Second, astronomers have discovered that Sgr A* is encircled by a cluster of fiery neonatal stars. Alas, some of these stellar babies made a doomed dive to within only a few billion miles of where the supermassive beast waits for its dinner.

Even though our supermassive black hole slumbers peacefully in its old age, when it wakes up from its nap it can put on quite a fireworks display. This sort of colorful, noisy, and violent awakening occurred a century ago when Sgr A* dined hungrily on an unfortunate blob of material that had fatally traveled too close to its maw. This messy feast created a brilliant spectacle that lit up our Galaxy’s center.

On January 5, 2015, NASA announced the observation of an X-ray flare, originating from Sgr A*, that was 400 times more brilliant than normal. This fiery flare proved to be a record breaker when compared to the brightness of Sgr A*’s previously observed flares. This unusual event possibly resulted from either the fragmentation of a doomed asteroid tumbling into the black hole’s waiting maw, or by the entanglement of magnetic field lines within gas floating into Sgr A*.

Current data indicate that the Sgr A*’s radio emissions are not originating from the black hole itself, but instead seem to be coming from a bright spot in the region surrounding it, near the event horizon. The radio emissions are possibly originating in the accretion disk or a relativistic jet of material being shot out from the disk. If the apparent position of Sagittarius A* were precisely centered on the black hole itself, it would be possible for astronomers to observe it magnified beyond its actual size, as a result of gravitational lensing. Gravitational lensing provides astronomers with a useful natural tool. This phenomenon is a prediction of General Relativity whereby the gravity of a massive foreground object (the lens) can distort the light coming from an object directly behind it, sometimes magnifying the more distant light.

A Celestial Masquerade

What makes G-objects so weird is their „puffiness“. It is rare for a star to be enshrouded by a layer of gas and dust so thick that astronomers are unable to peer through it and observe the star directly. All that astronomers can see when observing the G-objects is a glowing envelope composed of dust. In order to see the objects through their obscuring dusty veils, Dr. Campbell developed a tool dubbed OSIRIS-Volume Display (OsrsVol).

Dr. Campbell developed a 3-D spectro-imaging data cube using OsrsVol. He developed this custom volume rendering tool in order to separate a trio of other G-objects in the Galactic Center–dubbed G3, G4, and G5–from background emission. Once the 3-D analysis was performed, the team of scientists were able to clearly distinguish the G-objects. This enabled them to follow their movement and observe their mysterious behavior around Sgr A*.

OsrsVol allowed us to isolate these G-objects from the background emission and analyze the spectral data in three diminsions: two spatial dimensions, and the wavelength dimension that provides velocity information. Once we were able to distinguish the objects in a 3-D data cube, we could then track their motion over time relative to the black hole,“ Dr. Campbell explained to the press at the AAS 2018 summer meeting.

Dr. Ciurlo then commented that the „Keck Observatory has been observing the Galactic Center every year for 20 years with some of the best instruments and technologies. This alone gives a very high quality and consistent data set, which allowed us to go deep into the analysis of data.“

G3, G4, and G5 are especially interesting because they share certain intriguing characteristics with G1 and G2. The team of astronomers plan to continue to follow the shape and size of the G-objects‘ orbits. This is because they could provide important clues explaining how they formed.

The astronomers are planning to pay especially close attention to these dusty stellar compact objects when they travel closest to Sgr A*. This will enable them to further observe their behavior and see whether the objects remain intact just as G1 and G2 did–or merely become a tragic snack for Sgr A*. Only at that point will they remove their masks and give away their true identity.

„We’ll have to wait a few decades for this to happen: about 20 years for G3, and decades longer for G4 and G5. In the meantime, we can learn more about these puffballs by following their dynamical evolution using OSIRIS,“ commented Dr. Morris to the press on June 6, 2018.

Dr. Ciurlo also explained to the press at the AAS meeting that „Understanding the G-objects can teach us a lot about the Galactic Center’s fascinating and still mysterious environment. There are so many things going on that every localized process can help to explain how this extreme, exotic environment works,“.

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Source by Judith E Braffman-Miller

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