Galactic Center Group
Right to left: Professors Shoko Sakai, Tuan Do, Eric Becklin, Andrea Ghez, Mark Morris. Photo by Annette Buhl.
A major highlight in December 2022 for the Galactic Center Group (GCG) was Nobel Week, celebrating Andrea Ghez’s 2020 Nobel prize. Several senior group members, along with a number of donors, attended this week-long event in Stockholm, Sweden.
One of the group’s exciting research results, headed by Anna Ciurlo and Mark Morris, has been the publication of a thorough study of a dusty, gaseous filament that is orbiting closely around the Galaxy’s central black hole. Known as X7, this object’s orbit has been carrying it closer and closer to the black hole.In the process, the tidal force exerted by the black hole has been stretching X7 into a long streamer. Only 50 times the mass of the Earth, X7 does not have enough self-gravity to resist the tidal stretching, so it is doomed to continue being pulled apart. Although X7 is not presently orbiting directly toward the black hole, the accretion flow of gas onto the black hole from the surrounding vicinity might ultimately drag the dispersed material from X7 toward the black hole, leading to a period of enhanced accretion activity in coming decades.
Work on the binary star systems by postdocs Abhimat Gautam and Devin Chu has also revealed surprising results. Using over 17 years of images taken of the half-parsec region surrounding the supermassive black hole at the Galactic center, Gautam led the analyses searching for young stars that are in binary star systems, where two stars orbit around each other. We detected 3 binary star systems, including one that we discovered as a binary star. We found that most young stars in the central half-parsec region at the Galactic center are, in fact, binary stars, which is similar to the young stars in our Sun’s vicinity. Notably, when comparing this result with the earlier result from Devin Chu’s work, we confirmed for the first time that the fraction of young stars in binary systems actually significantly decreases as they get closer to the central supermassive black hole.
Chu used spectroscopic data to search for binary star systems in the region closest to the Milky Way’s supermassive black hole. No binary systems were detected in our sample of stars. Using simulations, we place an upper limit that 47% of these young stars may be in binaries, which is inconsistent with similar types of stars found in our solar neighborhood. We determine that this reduced number of binary star systems may be explained by these systems merging or being torn apart due to the supermassive black hole.
In other recognition of the group’s work, Postdoc Matthew Hosek was awarded the Brinson Prize Fellowship in September 2022 and is working with the UCLA GCG in the area of observational astrophysics to understand star formation in the extreme environment near the supermassive black hole at the center of our galaxy.
Two graduate students were awarded their PhDs during this time period. Zhuo Chen’s defense was held in July 2022; her work focused on the star formation history in the environs around the supermassive black hole and revealed a significantly younger population and larger population of stellar mass black holes compared to other groups. Zhuo is now a postdoc at the University of Washington. Kelly Kosmo O’Neill was awarded her PhD in August 2023, and her work revealed tentative evidence for the presence of dark matter surrounding the supermassive black hole. She is now a faculty at the University of Nevada, Reno.
The Galactic Center group has been able to further expand our academic outreach program by mentoring six undergraduate students this year. Research experience is increasingly important at the undergraduate level, and we have seen a rise in students requesting to work with us after Professor Ghez’s 2020 Nobel Prize. These students are not only learning vital skills to help their academic futures but are also allowing the Galactic Center Group to expand their research areas.