The Swansong of a Cloud Approaching the Milky Way’s Supermassive Black Hole
Maunakea, Hawaiʻi – Two decades of monitoring from W. M. Keck Observatory on Maunakea in Hawaiʻi reveals a peculiar cloud being pulled apart as it accelerates toward the supermassive black hole at the center of our Milky Way galaxy.
Dubbed X7, astronomers from the UCLA Galactic Center Orbits Initiative (GCOI) and Keck Observatory have been tracking the evolution of this dusty gas filament since 2002; high-angular resolution near-infrared images captured with Keck Observatory’s powerful adaptive optics system show X7 has become so elongated, it now has a length of 3,000 times the distance between the Earth and Sun (or 3,000 astronomical units).
The study is published in today’s issue of The Astrophysical Journal.
“This is a unique chance at observing the effects of the black hole’s tidal forces at high-resolution, giving us insight into the physics of the Galactic Center’s extreme environment,” said Anna Ciurlo, a UCLA assistant researcher and lead author of the study.
Tidal forces are the gravitational pull that stretch an object approaching a black hole; the side of the object closest to the black hole is pulled much more strongly than the side farthest away.
“It’s exciting to see significant changes of X7’s shape and dynamics in such great detail over a relatively short time scale as the gravitational forces of the supermassive black hole at the center of the Milky Way influences this object,” said co-author Randy Campbell, science operations lead at Keck Observatory.
X7 has a mass of about 50 Earths and is on an orbital path around our galaxy’s black hole, called Sagittarius A* (or Sgr A*), that would take 170 years to complete.
“We anticipate the strong tidal forces exerted by the Galactic black hole will ultimately tear X7 apart before it completes even one orbit,” said co-author Mark Morris, UCLA professor of physics and astronomy.
Based on its trajectory, the team estimates X7 will make its closest approach to Sgr A* around the year 2036, then dissipate completely soon after. The gas and dust constituting X7 will eventually get dragged toward Sgr A* and may later cause some fireworks as it heats up and spirals into the black hole.
Artist’s rendering of what is anticipated to happen around the year 2036 when X7, an elongated filament of dust and gas, makes its closest approach to the Milky Way’s supermassive black hole. Credit: W. M. Keck Observatory/Adam Makarenko
These findings are the first estimate of X7’s mildly eccentric orbital path and most robust analysis to date of the remarkable changes to its appearance, shape, and behavior. To observe X7, the team used Keck Observatory’s OH-Suppressing Infrared Imaging Spectrograph (OSIRIS) and Near-Infrared Camera, second generation (NIRC2), in combination with the adaptive optics systems on the Keck I and Keck II telescopes.
X7 shows some of the same observational properties as the other strange dusty objects orbiting Sgr A* called G objects, which look like gas but behave like stars. However, X7’s shape and velocity structure has morphed more dramatically compared to the G objects. The stretched-out gas and dust filament moves rapidly, clocking in at speeds of up to 490 miles per second. Because of the extremely large mass of the black hole, everything in its vicinity moves much faster than we typically see anywhere else in our galaxy.
Though X7’s origin is still a secret waiting to be unlocked and confirmed, the research team does have some clues about its possible formation.
“One possibility is that X7’s gas and dust were ejected at the moment when two stars merged,” said Ciurlo. “In this process, the merged star is hidden inside a shell of dust and gas, which might fit the description of the G objects. And the ejected gas perhaps produced X7-like objects.”
The research team will continue to monitor the dramatic changes of X7 with Keck Observatory as the power of the black hole’s gravity yanks it apart.
“It’s a privilege to be able to study the extreme environment at the center of our galaxy,” said Campbell. “This study can only be done using Keck’s superb capabilities and performed at the revered Maunakea, with honor and respect for this special site.”
The Galactic Center Orbits Initiative (GCOI), is an Adaptive Optics (AO) study of our Galaxy’s supermassive black hole (SMBH) and its environs. This long-term program has been collecting AO data with the W.M. Keck Observatory for over 25 years. The GCOI has opened up a new approach to studying the physics and astrophysics of supermassive black hole through the measurement of stellar orbits. This unique dataset is enabling us to gain new insights into how gravity works near a supermassive black hole and unique astrophysical events.
ABOUT ADAPTIVE OPTICS
W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere. Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) and current systems now deliver images three to four times sharper than the Hubble Space Telescope at near-infrared wavelengths. AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors. Support for this technology was generously provided by the Bob and Renee Parsons Foundation, Change Happens Foundation, Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, NASA, NSF, and W. M. Keck Foundation.
The Near-Infrared Camera, second generation (NIRC2) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.
The OH-Suppressing Infrared Imaging Spectrograph (OSIRIS) is one of W. M. Keck Observatory’s “integral field spectrographs.” The instrument works behind the adaptive optics system, and uses an array of lenslets to sample a small rectangular patch of the sky at resolutions approaching the diffraction limit of the 10-meter Keck Telescope. OSIRIS records an infrared spectrum at each point within the patch in a single exposure, greatly enhancing its efficiency and precision when observing small objects such as distant galaxies. It is used to characterize the dynamics and composition of early stages of galaxy formation. Support for this technology was generously provided by the Heising-Simons Foundation and the National Science Foundation.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.