By Joshua Winn, Princeton University
The Milky Way is not currently an active galaxy. Our black hole, Sagittarius A*, is in between meals. But all big galaxies, including our own, go through phases of activity and inactivity, depending on the supply of gas to the central black hole. How? What is the relationship between them? Read on more to find out.
The Growth of Galaxies
There’s evidence that the Milky Way was last active a few million years ago. It’s based on a gamma-ray image of the entire sky made by the Fermi Space Telescope. There are two regions of excess gamma rays, above and below the center. They’re interpreted as the fading ‘bubbles’ of high-energy particles that were spewed out by the relativistic jets during an episode of accretion some time during the Pliocene Epoch here on the Earth.
Active galaxies might be part of the explanation for the connection between the mass of a black hole and the mass of the surrounding galaxy. When a galaxy grows by accreting a smaller galaxy, the central black hole feasts on the fresh supply of gas. And then the powerful jets from the newly activated accretion disk push back on the in-falling gas, expelling it, or at least preventing it from accreting. So, in this scenario, galaxy growth is self-regulating. If the galaxy starts growing too fast, the black hole gets out of control, producing out-flows that put the damper on any further growth. This type of negative feedback between the black hole and the surrounding galaxy could explain why their properties are linked so tightly.
We’ve seen that wherever we look, we see galaxies. The farther we look, the more we see. And yet, galaxies don’t have totally random locations. They’re clustered. If we start in one galaxy, we’re more likely to find another galaxy within a few megaparsecs, than if we’d started at a random point in the universe.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
Galaxy Clusters
One can see that in an all-sky map of the locations of all the brightest galaxies. If galaxies were sprinkled randomly, the map would look like static. But it doesn’t. Galaxies are clustered. Their distribution has a distinct texture, like a sponge, or a bundle of spider webs. The largest structures on the all-sky map encompass hundreds of thousands of galaxies. We call those superclusters, and the relatively empty regions are the voids.
On the other end of the spectrum, we have little groups of a few dozen galaxies, including ellipticals, disk galaxies, and merging galaxies. We also see clusters with hundreds or thousands of galaxies. In these big clusters, ellipticals tend to outnumber the disks, presumably because in dense clusters there’s a greater probability for the sort of galaxy collisions that produce ellipticals.
A galaxy cluster is a gravitationally bound system, with each galaxy following a complex orbit determined by the combined gravitational potential of all the galaxies in the cluster. We can use the virial theorem to estimate the mass of the entire cluster. This was first done by an astronomer named Fritz Zwicky, in the 1930s, in a study of the Coma cluster.
Galaxies and the Dark Matter Problem
Zwicky obtained spectra of individual galaxies, and saw that some were redshifted, and some were blueshifted, relative to the average. From the spread in these Doppler velocities, he measured the velocity dispersion to be 1000 kilometers per second. He also measured the cluster radius, so he could estimate the mass.
The answer he got was about 5 times 10 to the 13 solar masses. Which was very strange. As the Coma cluster has about a thousand galaxies, Zwicky’s calculation implied that the average galaxy mass is 5 times 10 to the 10 solar masses. But, based on the luminosities of the individual galaxies, his best estimate for the average mass was only a few times 10 to the 8. In other words, the mass of the cluster seemed to exceed the sum of the masses of all the stars within the galaxies, by a factor of 100.
This was the first clue that the overwhelming majority of the mass in the universe is not luminous. There’s something in the Coma cluster exerting gravitational forces on the galaxies, making them move fast. But we don’t know what. We do know it’s invisible, at all wavelengths. It’s dark. This is the famous dark matter problem.
And yet, the numbers have changed since Zwicky’s day. Today, we think the dark matter outweighs normal matter by a factor of 5 or 6. And, we have lots of other evidence for dark matter, too. Starting in the late 1970s, it became clear that dark matter pervades individual galaxies, too, not just the spaces between them.
Common Questions about Exploring the Relationship Between Active Galaxies and Black Holes
There’s evidence that the Milky Way was last active a few million years ago. It’s based on a gamma-ray image of the entire sky made by the Fermi Space Telescope.
Galaxies are clustered. Their distribution has a distinct texture, like a sponge, or a bundle of spider webs. The largest structures on the all-sky map encompass hundreds of thousands of galaxies.
A galaxy cluster is a gravitationally bound system, with each galaxy following a complex orbit determined by the combined gravitational potential of all the galaxies in the cluster.
