Over three days this week, we’ll be celebrating this year’s astronomical awardees for the Nobel Prize in Physics: Roger Penrose, Reinhard Genzel, and Andrea Ghez.
On October 6th, the 2020 Nobel Prize in Physics was announced, awarding Roger Penrose, Reinhard Genzel, and Andrea Ghez for their work on the universe’s notorious mystery objects: black holes. Penrose predicted that black holes exist based on relativity, and Genzel and Ghez discovered the supermassive black hole (SMBH) at the center of our Milky Way. Genzel and Ghez are sharing the observational half of this prize since they both worked concurrently on this discovery, independently confirming each others’ findings in a friendly competition that pushed science forward.
This Astrobite focuses on Professor Andrea Ghez and the UCLA Galactic Center Group, following the arc of their work on the center of our Milky Way. (We also have bites focusing on Genzel and Penrose!) Professor Ghez earned her B.S. in Physics from MIT, followed by her PhD in Physics from Caltech and a postdoctoral position at the University of Arizona, before becoming faculty at UCLA in the early 1990s where she leads the UCLA Galactic Center Group. She is known as not only a prolific scientist, but also a mentor to many students, an engaging teacher, an inspiring public speaker, and even a mother. Ghez is now also the 4th woman to ever win the Nobel Prize in Physics, following Marie Curie (1903), Maria Goeppert Mayer (1963), and Donna Strickland (2018), and the first female astronomer to win the prize. Let’s now take a closer look at a few papers from Ghez and her group on the center of the galaxy and how they discovered and measured properties of our galaxy’s SMBH.
First Clues from the Stars’ Motions
Astronomers have had an idea that something strange sits at the center of our galaxy since the 1970s, when a compact radio source, referred to then as Sgr A* (the asterisk was because its discoverer thought it was exciting!), was discovered. Other active galaxies were known to host black holes at their centers, but what about a quieter galaxy like ours? In 1998, Ghez and her collaborators found evidence that this radio source is, in fact, a supermassive black hole.
This 1998 work looked at the proper motions of stars in the center of the Milky Way, using the Keck Telescope in infrared. They studied the movements of these stars over two years using short exposures and data processing techniques to avoid blurring from the turbulence of the atmosphere, leading to an “unprecedented resolution” of 1/20th of an arcsecond. They created a “census” of stars in the galactic center, without considering the position of the Sgr A* radio source to remain unbiased. These stars clearly showed clustering around some central mass (as shown in Figure 1) with incredible velocities; the highest velocity star was observed moving at 1400 km/s, a whopping 0.5% the speed of light, and eleven others had velocities over 500 km/s. (For comparison, New Horizons—one of our fastest spacecraft—was launched at ~45 km/s relative to the Sun.) They also compared their measurements to stars tracked in a few earlier works, by none other than Genzel.
To analyze the orbits, they fit a power law to the velocity dispersions and radii of these stars. The power law fit an exponent of -0.53, which is exceptionally close to the -0.5 expected from Keplerian orbits, implying there was a central mass smaller than 0.015 pc controlling these orbits through gravity. (You might be wondering—why does this r-0.5 power law mean anything? We can get Keplerian orbital velocity from combining the centripetal force and gravitational force…which gives us an orbital velocity proportional to r-0.5!) Their measurements of stellar velocities determined that this central mass weighed in at over 106 solar masses, with a density of >1012 solar masses per parsec. This extreme mass density led to the conclusion that there must be a black hole in the center of the Milky Way!
Living on the Edge: S02 and Sgr A*
Knowing that we have a supermassive black hole in our neighborhood, of course we’d want to study it and find out more about its properties! By observing the galactic center for a few more years to watch more of the stars’ orbits, the collaboration measured both velocities AND accelerations in 2000. This extra detail showed that the minimum mass density in the galactic center was a whole order of magnitude higher than previously estimated, and they even more precisely located the center of these orbits to the position of the Sgr A* radio source. They also interestingly identified one star (known as S0-2 by Ghez’s group, shown in Figure 2) that could have a period as short as 15 years, meaning that there’s a chance they could observe its entire orbit around Sgr A* (which would be a fantastic test of relativity near black holes, and help further constrain the black hole’s characteristics).
Seeing as S0-2 is an interesting subject, passing so close to our galaxy’s black hole, Ghez’s team did a bit more investigating. In 2003, using the newly available technology of adaptive optics, the team measured the absorption line spectrum of S0-2 (the first observed spectrum of any of these high-velocity galactic center stars!). Spectral typing showed S0-2 is a high mass young star (O or B type), which leads to questions about how a big star like that recently formed in such large tidal forces (or did it migrate in somehow?). Using radial velocity measurements from S0-2’s spectrum, they also narrowed down its inclination, which showed that S0-2 would actually pass behind the black hole during its closest approach.
Better Technology, Better Measurements
Ghez and the Galactic Center Group kept on taking measurements of the galactic center, slowly tracing the orbits of these stars around Sgr A* as shown in Figure 3. With each year of knowing the stars’ positions, the constraints on the black hole’s properties grew tighter and tighter.
The growing use and refinement of adaptive optics systems was crucial to these precision measurements, too; adaptive optics uses deformable mirrors that correct the incoming light waves for turbulence in the atmosphere, leading to a sharper, clearer image (and thus better astrometric measurements!) as shown in Figure 4.
Close Call with a Black Hole
In 2018, S0-2 finally made its predicted closest passage to Sgr A*—a great test of relativity. General relativity predicts that a star passing that close to a black hole should show a “relativistic redshift.” That is, its light will be shifted towards the red side of the spectrum since the waves are “stretched out” by a combination of the relativistic Doppler effect and gravitational redshifting. By combining their legacy data on the galactic center with some new spectroscopic measurements from multiple telescopes on Mauna Kea, the UCLA Galactic Center Group observed S0-2’s relativistic redshift, providing evidence for GR even in the extreme environment near a black hole. (Note: the GRAVITY collaboration, which includes our other Nobel awardee Genzel, also published a detection of S0-2’s relativistic redshift!)
What comes next?
After over a decade of observing the galactic center with Keck, the UCLA Galactic Center group have traced the orbits of multiple stars near Sgr A* as shown in Figure 5, including a full orbit of S0-2.
Although scientists have made a lot of progress in understanding the galactic center, there is still much to learn. An Astro2020 white paper from Ghez, Dr. Tuan Do (now also a UCLA professor in the Galactic Center Group!), and their collaborators outlines some of the most exciting questions for the next ten years of science in the galactic center, such as:
- How do stars (or other things, like the source known as G2) interact with the central black hole Sgr A*?
- Does general relativity work to describe extremely strong gravitational fields, like those found near supermassive black holes?
- What is the distribution of dark matter like at the galactic center?
- How do stars form in the extreme environment of the galactic center?
- Can we more precisely measure Sgr A*’s mass and the distance to the galactic center?
Just as advances in adaptive optics enhanced observations, ELTs (extremely large telescopes) will be the next step up for observing the galactic center (Figure 6), enabling detailed observations of many more stars (and enabling investigation into these questions!).
Ghez is well deserving of the Nobel Prize, as her contributions to astronomy are clearly impactful. The discovery of the black hole in the center of our Milky Way, known as Sgr A*, has led to numerous studies about the nature of galaxies, black holes, relativity, and more. In her group at UCLA, Ghez and her collaborators are keeping the momentum going; training the next generation of astronomers, continuing their now decades long observations of stars like S0-2, and working on unraveling new and exciting mysteries at the galactic center.
Astrobite edited by: Huei Sears and Jessica May Hislop
Featured image credit: Royal Swedish Academy of Sciences / Niklas Elmehed
Thank you to Abhimat Gautam and Kelly Kosmo O’Neil (UCLA Galactic Center group) for sharing their expertise for this article!
