My entire life researchers have been asking, are there blackholes in the hearts of dwarf galaxies and globular clusters? We know they lurk in the centers of large galaxies, and their sizes are proportional to the sizes and motions of the spheroids of stars they occupy. It has long been hoped that the relationship would extend down to smaller spheres of stars, but we just haven’t been able to find any evidence… until now.
As the story goes so far, astronomers have found binary star systems where only one star is visible and its motions indicate an second mass so small and dense that only a black hole can explain what we see. These stellar mass black holes range from 5 to 150 solar masses in size. On the other side of the size scale, the motions of stars and gas in the cores of galaxies indicate the presence of super massive black holes with masses larger than 100,000 solar masses.
It’s been assumed that black holes between 150 and 100,000 solar masses should exist, but … it’s easy to hide when you don’t emit or reflect light, and black holes… don’t emit or reflect light.
Our best guess for their hiding place has been inside dwarf galaxies or globular clusters, but studies to date haven’t been good enough to definitively rule in or rule out their existence.
But over time, the availability of new data can allow new discoveries.
Omega Centauri Cluster is weirdly large
For the past 20 years, the Hubble Space Telescope has been periodically, and for myriad purposes, taking images of the Omega Centauri Globular Cluster. This is a unique globular cluster for both its size and composition. It is 10 times more massive than a typical globular cluster, and its stars appear to have experienced multiple generations of star formation, with supernovae blasting gas out of the system in ways that have led to really weird ratios of different kinds of atoms in the different generations of stars.
Put another way, Omega Centauri is just plain weird, so people keep finding reasons to look at it with Hubble.
Over 20 years, Hubble has taken more than 500 images and a team led by Maximilian Häberle recently reviewed these images in detail, looking for the telltale signs of stellar motion that a significant mass would cause.
Looking at just the innermost 3” of the cluster, they were able to find 7 stars with just the kinds of motions that are expected.
An arcsec is the angular size of a piece of hair held at arms length. Essentially, your little finger at arm’s length is about 1 degree of arc across, and there are 60 minutes in a degree and 60 arc seconds in a minute, or 60 strands of hair.
These stars are moving across the sky a 2.41 milliarcsec a year. This is a ridiculously small amount, but with 20 years of data and 500 images to study, even these tiny movements can be seen.
These on the sky motions work out to physical velocities greater than 62 km/s. Under normal circumstances, based on what we see, these velocities would be greater than the clusters expected escape velocity, and these stars would be unbound from the system… but they’re not unbound. This indicates there must be a hidden mass that is gravitationally holding on to the stars and causing their motions.
According to calculations by Häberle’s team, a black hole with a mass around 8,200 solar masses would invisibly provide the needed gravitational pull.
And this is right in the middle of that intermediate mass black hole range.
An undergraduate working on this project, Matthew Whittaker, explains this discovery in a way that truly delights me. He says:
“There are black holes a little heavier than our sun that are like ants or spiders—they’re hard to spot, but kind of everywhere throughout the universe. Then you’ve got supermassive black holes that are like Godzilla in the centers of galaxies tearing things up, and we can see them easily. Then these intermediate-mass black holes are kind of on the level of Bigfoot. Spotting them is like finding the first evidence for Bigfoot—people are going to freak out.”
I’m not sure the joyous celebration of astronomers over this discovery is quite on par with what I’d expect if bigfoot were actually discovered, but … I this was, as collaborator Anil Seth put it, “A once-in-a-career kind of finding”.
I can now only hope that year by year, the data we’ve gathered of other, smaller systems, eventually allow us to identify other intermediate black holes.
This work appears in a new paper in Nature.
Is Dark Matter partly black holes?
Understanding the observed population of black holes is one of the more weird tasks astronomers find themselves faced with. The LIGO and VIRGO gravitational wave detectors are capable of measuring signals from the merger of all sizes of stellar mass black holes. In general, in our universe, smaller things greatly outnumber larger things, but in the gravitational wave data, there are significantly more detections of black holes with masses between 20 and 100 solar masses than lower mass systems between 5 and 20 solar masses.
In another new paper in Nature, researchers led by Przemek Mróz consider the weirdness of this mass distribution and they suggest perhaps we’re seeing the mergers of a population of primordial black holes that formed in the early universe that was predicted by Yakov Zeldovich. If this is the case, these 100 solar mass black holes could explain at least some of the dark matter known to exist in our universe. Essentially, these black holes that are invisibly haunting the outskirts of galaxies and spaces between star systems could be part of universes missing mass. They can’t be more than 3% of it or we’d see other well predicted effects, but… 3% is of dark matter is 3% we couldn’t explain before.
These particular black holes are cool because they are predicted to have formed in a way unlike other stellar mass black holes. Early in the universe, mass wasn’t evenly distributed, and while slightly more dense regions formed galaxies or even stars, some particularly chunky regions could have formed these black holes… and these black holes might have even formed before those stars and galaxies.
With reach new set of observations, it feels like we understand our universe a bit more… and our universe is just that much weirder.