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Scientists have observed the Galactic Center for the first time thanks to the JWST

An international team of scientists, including astrophysicist Michal Zajaček from the Faculty of Science at Masaryk University, has identified a young stellar cluster in the immediate vicinity of the supermassive black hole Sgr A* at the center of our Galaxy. Scientists were surprised by the age of observed objects and for the first time published the spectrum of the center of our galaxy taken by the James Webb Space Telescope.

11 Oct 2023 Michal Zajaček

This star cluster, known as IRS13, was discovered more than 20 years ago. But only by combining data taken with a variety of telescopes over a period of several decades has it been possible to determine the nature of the star cluster members. Surprisingly, it turns out that the age of the objects is younger than expected. Actually, it should not be possible that such a large number of such young stars is in the direct vicinity of the supermassive black hole. However, the recent detection of the baby star X3, which probably also belongs to IRS13, has already indicated this.

A K- and L-band infrared image of the inner region of our galactic center taken with the infrared camera (NACO detector) on the Very Large Telescope (VLT) system at the European Southern Observatory, Chile. The cluster IRS13 is marked in green. The four squares indicate the pointing areas for spectroscopic observations by the James Webb Space Telescope. Image source: Courtesy of F. Peissker et al.

“IRS 13 cluster seems to hold the keys to unraveling the origin of the Nuclear Star Cluster, the densest association of the stars in our Galaxy. We have accumulated substantial evidence that very young stars in the grasp of the supermassive black hole could have originated in infalling clusters like IRS 13. This is also the first time we can distinguish stellar populations of different age – hot main-sequence stars and still young forming stars – in the cluster so close to the center of the Milky Way,” explained the uniqueness of the new discovery Michal Zajaček, a scientist from the Department of Theoretical Physics and Astrophysics of the Faculty of Science at Masaryk University.

Another outstanding result is the first published spectrum of the Galactic Center taken with the James Webb Space Telescope, currently the largest space telescope operating in the infrared domain and located at 1.5 million kilometers from the Earth. It should be mentioned that a prism on board the James Webb Space Telescope was also developed at the University of Cologne in the group of Prof. Andreas Eckart, a co-author of the publication and PhD supervisor of Dr. Zajaček. This spectrum confirms the assumption that water ice is located towards the Galactic center region.

Infrared spectrum obtained with the James Webb Space Telescope (NIRSpec) between 1.5 and 3.2 micrometers. The spectrum shows prominent emission lines belonging to ionized hydrogen and helium. Absorption bands belonging to the CO molecule and water ice reveal cooler material surrounded by dominantly ionized material. Source: Astrophysical Journal (Peissker et al., 2023)

In addition to the unexpected detection of young stars and water ice by the James Webb Space Telescope, the researchers also found that the star cluster IRS13 probably has a turbulent formation history. It is plausible that IRS13 migrated towards the supermassive black hole through dynamical friction with the circumnuclear medium, including collisions with other star clusters. While plunging deeper, the star cluster was then "captured" by the gravitational potential of the black hole. During this process, a dense bow shock wave, similar to the one at the tip of a ship in water, may have formed ahead of the infalling star cluster. The associated increase in density then stimulated star formation. This is at least one explanation why these young stars are mainly found in front of the cluster.

The analysis of IRS13 and the interpretation of the cluster is the first attempt to solve a decade-old mystery about the unexpected young stars in the Galactic center. In fact, in addition to IRS13, there is a star cluster, the S-cluster, which is also composed of young stars and is even closer to the black hole. Already in the 90's Andreas Ghez (Nobel Prize in 2020) of the University of California has determined that the S-cluster members are significantly younger than it would be possible according to established theories of star formation. In the current study, IRS13 now offers a unique opportunity to establish a connection between the immediate vicinity of the black hole and regions several light-years away.

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