Keck Observatory observations help constrain a long-predicted pathway for black hole formation
Maunakea, Hawaiʻi – Astronomers using W. M. Keck Observatory on Maunakea, Hawaiʻi Island contributed key observations to the identification of a rare stellar death in which a massive star appears to have collapsed directly into a black hole without first exploding as a supernova. The event occurred in the Andromeda galaxy and provides strong observational support for a long-theorized but rarely confirmed route to black hole formation.
The study, led by researchers at Columbia University, is published in today’s issue of the journal Science and combines archival space-based data with targeted ground-based follow-up observations from multiple observatories.
“This has probably been the most surprising discovery of my life,” said Kishalay De, professor of astronomy at Columbia University and lead author of the study. “The evidence of the disappearance of the star was lying in public archival data, and nobody noticed it for years until we picked it out.”
A quiet stellar death in Andromeda
The object, designated M31-2014-DS1, was a supergiant star located about 2.5 million light-years from Earth in the Andromeda galaxy. When it formed, the star was roughly 13 times the mass of the Sun. Over its lifetime, powerful stellar winds stripped away much of that mass, leaving it with about five times the mass of the Sun at the end of its life.
Archival observations from NASA’s NEOWISE mission revealed that the star gradually brightened in infrared light over several years before fading dramatically and disappearing from view. Unlike a typical supernova, the event showed no evidence of a powerful outward explosion. Instead, it left behind a shell of dust and a faint infrared glow.
“The dramatic and sustained fading of this star is very unusual, and suggests a supernova failed to occur, leading to the collapse of the star’s core directly into a black hole,” De said.
Critical follow up from the ground
To better constrain the nature of the event, the team conducted follow-up observations using the Near-Infrared Echellette Spectrograph (NIRES) on the Keck II Telescope, with observing time awarded via the NASA-Keck partnership.
Prior to the Keck observations, there were no ground-based infrared spectra of the source at sufficient sensitivity to test whether the star had truly faded at infrared wavelengths.
NIRES is optimized for studying extremely faint infrared sources and is particularly well suited to probing dusty stellar remnants. The data placed important limits on the temperature, composition, and evolution of the material left behind after the star disappeared, including faint emission from material expelled by the star, helping rule out alternative explanations such as an unusual supernova or intrinsic stellar variability.
“It was only with Keck’s sensitivity in the near infrared that we could confirm the star had truly faded at all wavelengths,” De said. “Even with NIRES, the source was barely detected, which allowed us to rule out normal hints of stellar variability or dust obscuration and strengthened the case that the star had genuinely disappeared.”
The Keck observations were analyzed alongside data from space-based telescopes and other ground-based facilities as part of a coordinated, multi-wavelength campaign.
Credit: Keith Miller, Caltech/IPAC – SELab
A Rare View into Direct Collapse
Astronomers have long known that black holes originate from massive stars, but direct observational evidence of that transformation has been scarce. While gravitational-wave detections have revealed black hole mergers across the universe, they do not show how those black holes initially formed.
Only one other candidate direct-collapse event has been reported previously, but it was significantly more distant and fainter, leaving its interpretation uncertain. The relative proximity of Andromeda and the quality of the available data make M31-2014-DS1 a particularly compelling case.
“We’ve known that black holes must come from stars,” said Morgan MacLeod, lecturer in astronomy at Harvard University and co-author of the study. “With events like this, we’re getting to watch it happen, and are learning a huge amount about how that process works along the way.”
Looking ahead
The findings suggest that direct collapse may be a more common outcome for massive stars than previously assumed. Future infrared surveys, combined with sensitive ground-based facilities like Keck Observatory, are expected to uncover additional examples and further clarify the physical conditions that determine how massive stars end their lives.
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ABOUT NIRES
The Near-Infrared Echellette Spectrograph (NIRES) is a prism cross-dispersed near-infrared spectrograph built at the California Institute of Technology by a team led by Chief Instrument Scientist Keith Matthews and Prof. Tom Soifer. Commissioned in 2018, NIRES covers a large wavelength range at moderate spectral resolution for use on the Keck II telescope and observes extremely faint red objects found with the Spitzer and WISE infrared space telescopes, as well as brown dwarfs, high-redshift galaxies, and quasars. Support for this technology was generously provided by the Mt. Cuba Astronomical 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 Hawaiʻi 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. For more information, visit: www.keckobservatory.org