Hello Space Fans and welcome to another edition of Space Fan News. This week astronomers find convincing evidence of a new class of black holes that fall between stellar-sized and supermassive-sized; astronomers find a bridge of stars between the large and small Magellanic Clouds; and we now have a new method for detecting some of the faintest galaxies in the early universe.
Last week I told you about a stray black hole that was found wandering around the galaxy that was seen as it blasted through a gas cloud and I told you then that seeing these things is hard unless they are actively feeding on material falling in and we can see the radiation given off as gas and stuff are accelerated as they fall in.
So yes, they are hard to see, and because of that, there is a lot we don’t know about them. For example, up to now, we’ve only been able to detect two basic types of black hole: small, stellar-sized one (these are up to about 100 solar masses), and larger - waaaay larger black holes, which are about 100 million to billions of solar masses.
That last kind are in the centers of most galaxies and are called supermassive black holes.
So we’ve seen the tiny ones, and we can see the really big ones. And not much in between.
And astronomers think that is strange. I mean, how do these supermassive black holes get so big? Do they jump from being stellar-sized to supermassive-sized? Galaxy collisions don’t explain it because that would just be the merging of two, super-sized black holes.
So the big question is, How do black holes get so big? If there is an evolution of them, which seems reasonable (like we see with galaxies), then we should be able to see black holes in the middle, what astronomers are calling Intermediate-sized black holes or IMBH’s.
IMBH’s are black holes that range in mass from 100 to 10,000 solar masses, and astronomers expect that they should be there. So where are they?
47 Tucanae is a 12-billion-year-old star cluster located 13,000 light-years from Earth in the southern constellation of Tucana the Toucan. It contains hundreds of thousands of stars crammed in a ball only about 120 light-years in diameter. It also holds about two dozen pulsars that were important targets of this investigation.
IMBH’s are important because they are the missing link between stellar-mass and supermassive black holes. They may be the primordial seeds that grew into the monsters we see in the centers of galaxies today.
Astronomers have looked for a black hole at the center of this globular cluster before with no success. Most of the time, in the centers of galaxies for example, astronomers can detect the black hole by looking at the motions of stars as they are affected by the black hole’s gravitational pull.
But the stars in 47 Tucanae are too packed together to see their motions at the center, and because globular clusters are so old, they have no gas for the black hole to feed on either. So anything that might be there was invisible.
That is, until now.
Astronomers detected the presence of the IMBH at the center of this cluster by using two different methods.
The first one involved looking at the overall motions of stars throughout the cluster. A globular cluster is so dense that the heavier stars tend to sink to the center of the cluster. If there were an IMBH at the cluster's center, it would act like a cosmic "spoon" and stir the pot, causing those stars to slingshot to higher speeds and greater distances. This imparts a subtle signal that astronomers can measure.
By employing computer simulations of stellar motions and distances, and comparing them with visible-light observations, the team found evidence for just this sort of gravitational stirring.
The second line of evidence came from pulsars, these are compact remnants of dead stars whose radio signals are easily detectable. These objects also get flung about by the gravity of the central IMBH, causing them to be found at greater distances from the cluster's center than would be expected if no black hole existed.
Taken together, these two lines of evidence suggests the presence of an IMBH of about 2,200 solar masses within 47 Tucanae.
Astronomers are hoping that these techniques will help them find similar IMBH’s hiding in other globular clusters. Locating them will require similar data on the positions and motions of both the stars and any pulsars within the clusters.
So as I was writing the script for this story, something kept bugging me that I’ve reported something like this before. Sure enough, I did some checking and hard core space fans will remember that way back in SFN 55 I reported on the discovery using Hubble data of a 20,000 solar mass black hole at the center of a star cluster in another galaxy and modeling nearby star cluster motions.
So we’ve doubled our census of intermediate-sized black holes to two, but I expect to hear a lot more about others in the future. I’m also happy to report that I’ve at least doubled the sound quality for SFN episodes. Don’t be a hater though, back then all I had was a cheapo video recorder.
You can thank SFN Patreon Patrons for the improved sound quality. Go check that video out. Now wait, don’t go check it out, let’s let that one die a quiet death.
Next, you guys may not realize this, but our galaxy has two satellite galaxies in orbit around it. They are called the large and small Magellanic Clouds and can be easily seen with the naked eye in the southern hemisphere.
They are classified as dwarf galaxies, and they’ve been up in our night sky and puzzled humanity for centuries. Even to this day, not much is known about these galaxies because we haven’t had the proper tools to study them.
That’s starting to change now that we’re in the Golden Age of Astronomy. Astronomers using the Gaia Space Telescope, which I’ve told you about many times, were looking at RR Lyrae variable stars and are very old and have been around since the beginning of the galaxy’s existence.
What’s great about Gaia is that it has the same resolution as the Hubble Space Telescope but with a very wide field of view. It was designed to measure stars very accurately and unlike typical telescopes, Gaia does not just point and stare: it constantly spins around its axis, sweeping the entire sky in less than a month.
What this means is that it not only measures the instantaneous properties of the stars, but also tracks their changes over time. This provides a perfect opportunity for finding a variety of objects, for example stars that pulsate or explode - even if this is not what the satellite was primarily designed for.
So using GAIA, and tracing these RR Lyrae variables, astronomers found a stream of stars spanning the distance between the small and large Magellanic Clouds. What they think happened is that some of the stars were stripped from the small by the large Magellanic cloud some two hundred million years ago.
The researchers are hoping the RR Lyrae bridge will help to clarify the history of the interaction between the clouds and our galaxy.
Finally, astronomers using Hubble have perfected a technique for observing some of the earliest galaxies in the universe which shed some light (no pun intended) on an important period known as the era of reionization.
Not soon after the universe formed, these faint, early galaxies gave rise to the Epoch of Reionization, a time when the energetic radiation they gave off bombarded all the gas between all galaxies in the universe. This caused the atoms in this diffuse gas to lose their electrons (that is, become ionized).
Astronomers think that it was early galaxies that caused this reionization but for that to work, there had to be galaxies a hundred times fainter than we could see with Hubble and they had to be really, really common.
The problem was, until JWST gets finished and launched, we can’t really see these early galaxies to know if they are common or not. But that didn’t stop people using Hubble. Astronomers boosted the power of Hubble by looking at these far away galaxies as they lay behind huge galaxy clusters that were between us and the distant galaxy.
Those clusters acted like a lens, making Hubble more powerful, but only for those galaxies that lay behind the cluster. The program is called the Frontier Fields and they’ve looked at six galaxy clusters.
Frontier Fields have allowed us to see galaxies that Hubble wasn’t powerful enough to see on it’s own and astronomers have seen many of the faint galaxies they were looking for. This problem was though that the big galaxy cluster in the middle was in the way. It was this big bright thing in between us and the really dim thing we were trying to see.
Again, this didn’t stop these guys. Using a cool new computer model, the Frontier Fields team announced that they found a way to remove the lensing galaxy cluster so they could see all the galaxies behind it.
The Frontier Fields team have used the new method on two of the six galaxy clusters: Abell 2744 and MACS 0416. It has enabled them to identify faint galaxies seen when the universe was about a billion years old, galaxies 100 times fainter than those found in the Hubble Ultra Deep Field, for instance, which is the deepest image of the night sky yet obtained.
Their observations showed that these faint galaxies are extremely numerous, consistent with the idea that large numbers of extremely faint galaxies were the main power source behind reionization.
So there you go, one less thing to discover about the universe.
Well, that’s it for this week Space Fans. Thanks to all SFN Patreon Patrons, our contest will be announced next week! Thanks to all of you for watching and as always, Keep Looking Up!