(ORDO NEWS) — If, as astronomers believe, the death of large stars leaves behind black holes, then there must be hundreds of millions of them scattered throughout the Milky Way galaxy. The problem is that isolated black holes are invisible.
Now, a team of astronomers led by the University of California at Berkeley have discovered for the first time what could be a free-floating black hole by observing how the light from a more distant star is distorted by the object’s strong gravitational field something called gravitational microlensing.
A team led by graduate student Casey Lam and Jessica Lu, assistant professor of astronomy at the University of California at Berkeley, estimated that the mass of an invisible compact object is 1.6 to 4.4 times the mass of the Sun.
Because astronomers believe that the remnant of a dead star would have to be heavier than 2.2 solar masses to become a black hole, UC Berkeley researchers warn that the object may not be a black hole, but a neutron star.
Neutron stars are also dense, very compact objects, but their gravity is balanced by the internal pressure of the neutrons, which prevents further collapse into a black hole.
Whether it’s a black hole or a neutron star, the object is the first dark stellar remnant – a stellar “ghost” – discovered in a galaxy without a pair with another star.
The analysis by Lam, Lu and their international team includes four other microlensing events that the team believes were not caused by a black hole, although two of them were likely caused by a white dwarf or a neutron star.
The team also concluded that the probable population of black holes in the galaxy is 200 million – about as many as most theorists predicted.
Notably, a rival team at the Space Telescope Science Institute (STScI) in Baltimore has analyzed the same microlensing event and claims that the mass of the compact object is closer to 7.1 solar masses and is undeniably a black hole.
Both teams used the same data: photometric measurements of the brightness of a distant star as its light was distorted or “lensed” by an ultra-compact object, and astrometric measurements of the displacement of a distant star’s position in the sky as a result of gravitational distortion by a lensing object.
The photometric data were obtained from two microlensing studies: the OGLE experiment, which uses a 1.3-meter telescope in Chile, operated by the University of Warsaw, and the MOA experiment, which is installed on a 1.8-meter telescope in New Zealand, operated by the University of Osaka.
The astrometric data was obtained from NASA’s Hubble Space Telescope. The STScI manages the telescope’s science program and conducts its science operations.
Since the same object was captured in both microlensing studies, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462 for short.
Although such surveys annually detect about 2,000 stars bright from microlensing in the Milky Way galaxy, it was the addition of astrometric data that allowed the two teams to determine the mass of the compact object and its distance from Earth.
It is estimated by a team led by the University of California at Berkeley to be between 2,280 and 6,260 light-years (700-1920 parsecs) towards the center of the Milky Way galaxy and near the large bulge that surrounds the galaxy’s central massive black hole.
It is estimated by the STScI team to be about 5,153 light-years (1,580 parsecs) away. Analysis of the new data confirmed that OB110462 was likely a black hole or a neutron star.
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