Messier 4 is a globular cluster full of ancient stars in the constellation of Scorpius, approximately 7,200 light-years away.
Few stellar systems in the Universe are as active and dynamically complex as globular star clusters.
Indeed, the interplay between stellar evolution and dynamical interactions allows globular clusters to serve as laboratories for a vast number of interesting astrophysical phenomena, such as formation of black hole mergers, gravitational waves, Type Ia supernovae, fast radio bursts, etc.
Finally, one of the potential outcomes of these dense environments is intermediate-mass black holes, with masses between 100 and 100,000 solar masses, thought to be the missing link of black hole evolution, with barely a few observed cases.
The unique capabilities of the NASA/ESA Hubble Space Telescope have now been used to zero in on the core of the closest globular cluster, Messier 4, to go black-hole hunting with higher precision than in previous searches.
“You can’t do this kind of science without Hubble,” said Dr. Eduardo Vitral, an astronomer at the Space Telescope Science Institute.
The suspected intermediate-mass black hole in Messier 4 can’t be seen, but its mass is calculated by studying the motion of stars caught in its gravitational field, like bees swarming around a hive.
Measuring their motion takes time, and a lot of precision. This is where Hubble accomplishes what no other present-day telescope can do.
Dr. Vitral and colleagues looked at 12 years’ worth of Messier 4 observations from Hubble and resolved pinpoint stars.
They estimate that the black hole candidate in the cluster could be as much as 800 times our Sun’s mass.
Hubble’s data tend to rule out alternative theories for this object, such as a compact central cluster of unresolved stellar remnants like neutron stars, or smaller black holes swirling around each other.
“We have good confidence that we have a very tiny region with a lot of concentrated mass,” Dr. Vitral said.
“It’s about three times smaller than the densest dark mass that we had found before in other globular clusters.”
“The region is more compact than what we can reproduce with numerical simulations when we take into account a collection of black holes, neutron stars, and white dwarfs segregated at the cluster’s center. They are not able to form such a compact concentration of mass.”
A grouping of close-knit objects would be dynamically unstable. If the object isn’t a single intermediate-mass black hole, it would require an estimated 40 smaller black holes crammed into a space only one-tenth of a light-year across to produce the observed stellar motions.
The consequences are that they would merge and/or be ejected in a game of interstellar pinball.
“We measure the motions of stars and their positions, and we apply physical models that try to reproduce these motions,” Dr. Vitral said.
“We end up with a measurement of a dark mass extension in the cluster’s center.”
“The closer to the central mass, more randomly the stars are moving. And, the greater the central mass, the faster these stellar velocities.”
The discovery is described in a paper published this week in the Monthly Notices of the Royal Astronomical Society.
Eduardo Vitral et al. 2023. An elusive dark central mass in the globular cluster M4. MNRAS 522 (4): 5740-5757; doi: 10.1093/mnras/stad1068