There’s One In This Cluster, Disturbing Its Stars!

The past few weeks I’ve been working with the folks at Gemini Observatory on a press release (shared jointly with the Hubble Space Telescope folks) about some recent research pointing to the existence of a black hole at the center of the Omega Centauri globular cluster. I work as a consulting writer and editor with Gemini on a number of projects, including their twice-a-year publication GeminiFocus, which gives me a chance to see some cool science results and help shape the public stories astronomers tell to explain the results. This latest story has really piqued my imagination!

The image at left is Hubble’s view of this beautiful cluster. It was used to study the stars at the heart of the cluster, while data obtained using the Gemini Multi-Object Spectrograph at Gemini South in Chile, tracks their motions. Together, these two data sets show that there’s something massive in the center of Omega Centauri–massive enough to perturb the orbits of nearby stars.

The Gemini-related work was done by astronomer Eva Noyola while she was doing work for her Ph.D. at the University of Texas at Austin. We had a chance to swap some emails about this work, and one thing I came away with from these discussions was that black holes are not only NOT rare, but they’re becoming more and more the objects of choice when it comes to describing the evolution of galaxies. It’s pretty well known that supermassive galaxies have supermassive black holes at their hearts. So, what about smaller galaxies, the little ones that Eva terms “minuscule”? Well, they might look a lot like an errant globular cluster, which might actually once have been a dwarf galaxy that lost many of its outer stars in an ancient galaxy collision. And, those dwarf galaxies and globulars could have had black holes in sizes to match — the so-called intermediate-mass black holes that could be the seeds for even larger black holes. The trick was, as Eva told us, to find such a “baby” black hole. And, that’s what led her and her thesis advisor to look in the center of Omega Centauri.

So, why choose Omega Centauri? It’s interesting. And peculiar. It is way more massive than other globular clusters, and its star populations aren’t like other clusters. Omega Cen’s stars fall into several different populations of star types, all sorted by their metallicity. Metallicity is a sort of astronomy “shorthand” to indicate stars that have abundances of elements beyond hydrogen (i.e. they’re more “metallic). It turns out that Omega Cen’s stars have varying amounts of metals. This indicates that they were born at varying times and possibly even in different places. One appealing explanation for these differences is that Omega Centauri was once a dwarf galaxy. This ancient galaxy may have formed in two or more bursts of formation. That would explain the differing populations of stars. But, it raises another question: how did Omega Centauri go from being a dwarf galaxy to a globular cluster?

“This huge city of stars evidently passed through our galaxy, and a large percentage of its stars could have been stripped away in the process” said Noyola. “What remains is possibly the core of a former dwarf galaxy along with the black hole that once grew inside the galaxy’s nucleus .”

If you could see Eva’s baby black hole (it’s about 40,000 solar masses, and its event horizon is actually smaller than our solar system), it might look like the VERY exaggerated-scale artwork by space artist Lynnette Cook (right). We had her draw in some stellar orbits for reference, to give an idea of how a black hole could influence the nearby stars. Close to the black hole, star motions are faster than those farther away. Differential velocities of stars at different distances are one telltale signature of a black hole’s existence. Most of the clusters stars are cooler stars with a scattering of bluer hotter stars mixed in.

Now, the cool implication about all this is that intermediate-mass black holes (like Omega Cen’s “baby” black hole) could well be rare beasts in the cosmic zoo. They might only exist in former dwarf galaxies that have been stripped of their outer stars. Or, they could be more common than everybody thought, and now that we have the means to find them, we’ll see them in the centers of globular clusters as well.

As the HST web site points out, “A previous Hubble survey of supermassive black holes and their host galaxies showed a correlation between the mass of a black hole and that of its host. Astronomers estimate that the mass of the dwarf galaxy that may have been the precursor of Omega Centauri was roughly 10 million solar masses. If lower mass galaxies obey the same rule as more massive galaxies that host supermassive black holes, then the mass of Omega Centauri does match that of its black hole.”

You can read more about this black hole, and the current thinking about Omega Centauri and its place in galaxy evolution scenarios at the Gemini and HST websites linked above. You can see the GMOS data at the Gemini site, and the HST site has a cool little vodcast interview with Eva, talking about her new finding.

Finally, if you’re in a position observe Omega Centauri, there’s a little finder chart that I created for Gemini on their web page. Check it out!