|Illustration Credit: FECYT, IAC|
Astronomers believe there could be as many as 100 million black holes among the stars in the Milky Way galaxy alone. Despite the many theorized black holes, it’s been a struggle for scientists to identify an isolated black hole. Finally, after six years of ‘meticulous observations,’ NASA’s Hubble Space Telescope has ‘provided direct evidence for a lone black hole drifting through interstellar space by a precise mass measurement of the phantom object.‘ This is a major milestone, as previously, all black hole masses have only been inferred statistically or ‘through interactions in binary systems or in the cores of galaxies.’ Stellar-mass black holes are typically found with companion stars, making the observed isolated black hole a special case.
The detected black hole is about 5,000 light-years away, wandering in the Carina-Sagittarius spiral arm of the Milky Way galaxy. The discovery of this distant black hole allows astronomers to estimate that there may be an isolated stellar-mass black hole much nearer to Earth, perhaps only 80 light-years away. For reference, the nearest star to our solar system, Proxima Centauri, is slightly more than 4 light-years away.
Black holes scattered throughout our galaxy are formed when ‘rare, monstrous’ stars at least 20 times more massive than our Sun explode into supernovae, leaving behind a remnant core crushed by gravity into a black hole. Less than one-thousandth of the galaxy’s stars are large enough for this to occur. Self-detonation isn’t perfectly symmetrical, so a black hole could be shot off into a different direction, ‘careening through our galaxy like a blasted cannonball.’
We know that there are potentially tens of millions of black holes in our galaxy, but they are challenging to observe. The primary reason is that they don’t emit any light. However, for the same reason a black hole doesn’t emit light – its extremely powerful gravitational field doesn’t let light escape – we can see the evidence of a black hole by how it warps space and deflects and amplifies starlight from any star that lines up exactly behind it relative to the point of observation. Simply put, scientists can detect black holes because of the effect the black hole has on the matter around it.
Ground-based telescopes are constantly scouring the night sky, monitoring the brightness of millions of stars toward the central bulge of our Milky Way. When an observation shows a sudden brightening of a star when a massive object passes between the telescope and the star, Hubble follows up for further investigation.
Two teams used Hubble data for their investigations, including one led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, MD. The other team is led by Casey Lam of the University of California, Berkeley. The two teams came to slightly different results, but each team’s research suggests the presence of ‘a compact object.’
NASA writes, ‘The warping of space due to the gravity of a foreground object passing in front of a star located far behind it will momentarily bend and amplify the light of the background star as it passes in front of it. Astronomers use the phenomenon, called gravitational microlensing, to study stars and exoplanets in the approximately 30,000 events seen so far inside our galaxy.’
A foreground black hole is distinct from other microlensing events because the gravity of the black hole can stretch out the duration of a lensing event for over 200 days. Further, if the cause of the microlensing event is a star, it would cause a color change – a black hole event has no such transient color change.
‘Next, Hubble was used to measure the amount of deflection of the background star’s image by the black hole.’ Hubble is remarkably precise and detected an offset of about a milliarcsecond. That’s like being in New York and measuring the diameter of a quarter coin in Los Angeles.
The astrometric microlensing technique provided information about the black hole’s mass, distance and velocity. Sahu’s team used the amount of deflection caused by the black hole’s warping of space to determine that the black hole weighs about seven solar masses. Lam’s team reports a slightly lower mass range of about 1.6 to 4.4 times the mass of the Sun, meaning that the invisible object could be a neutron star, rather than a black hole. If the mass is at the higher end of the range, it’s a black hole. Otherwise, it’s a neutron star.
‘As much as we would like to say it is definitively a black hole, we must report all allowed solutions. This includes both lower mass black holes and possibly even a neutron star,’ said Jessica Lu of the Berkeley team. Lam added, ‘Whatever it is, the object is the first dark stellar remnant discovered wandering through the galaxy, unaccompanied by another star.’
There’s a bright, unrelated star close in angular resolution to the source star, making the measurements very challenging. ‘So it’s like trying to measure the tiny motion of a firefly next to a bright light bulb,’ said Sahu. ‘We had to meticulously subtract the light from the nearby bright star to precisely measure the deflection of the faint source.’ Sahu’s team estimates that the isolated black hole travels at about 160,000 kph (100,000 mph) across the galaxy. If so, it could travel from Earth to the Moon in less than three hours. If the speed measurement is accurate, the black hole is traveling faster than neighboring stars in that region of the galaxy.
‘Astrometric microlensing is conceptually simple but observationally very tough,’ said Sahu. ‘Microlensing is the only technique available for identifying isolated black holes.’ The microlensing event lasted about 270 days, but several more years of Hubble observations were required to follow how the background star’s position ‘appeared to be deflected by the bending of light by the foreground black hole.’
Other measured stellar-mass black holes have been measured within binary star systems by measuring X-rays. NASA writes, ‘Gas from the companion star falls into the black hole and is heated to such high temperatures that it emits X-rays. About two dozen black holes have had their masses measured in X-ray binaries through their gravitational effect on their companions. Mass estimates range from 5 to 20 solar masses. Black holes detected in other galaxies by gravitational waves from mergers between black holes and companion objects have been as high as 90 solar masses.’
‘Detections of isolated black holes will provide new insights into the population of these objects in our Milky Way,’ said Sahu. However, it’s believed only one in a few hundred microlensing events are the result of isolated black holes, meaning that it’s a challenging search, to say the least. NASA’s upcoming Nancy Grace Roman Space Telescope will observe ‘several thousand microlensing events,’ out of which many are expected to be due to black holes. The telescope will be able to measure deflections with extreme precision.