Astronomers examining a star cluster near Sgr A*, the Milky Way’s supermassive black hole, found that the cluster has some unusually young members for its location. That’s difficult to explain since the region so close to the powerful black hole is infused with powerful radiation and dominated by the black hole’s extremely powerful gravitational force. According to our understanding of stellar formation, young stars shouldn’t be there.
The cluster is named IRS 13, and astronomers have been puzzling over it for years. It’s sometimes called GCIRS 13, for Galactic Centre. It’s home to huge stars, including Wolf-Rayet stars, which are massive, hot stars in a class of their own. For a cluster, and especially one so close to an SMBH, IRS 13 has an exceptionally high density of stars.
New research shows that the stars are only a few million years old and less, which is young by stellar standards. It also found 33 new, previously unknown objects in the cluster. The researchers suggest that many of IRS 13’s stars formed elsewhere, and then the cluster was pulled towards Sgr A*. That triggered a second episode of star formation.
The research was published in The Astrophysical Journal. The title is “The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. I. Detection of a Rich Population of Dusty Objects in the IRS 13 Cluster.” Dr. Florian Peißker from the Institute of Physics at the University of Cologne is the lead author.
IRS 13 is only about 0.15 parsecs from Sgr A*, or about half of one light-year. That’s extraordinarily close. This region is a busy place, and astronomers have been working to understand the galactic center more thoroughly for decades. When this team of researchers examined IRS 13 more closely, they found that the 33 newly-formed stars, called young stellar objects (YSOs), are located in only one part of the cluster. And all but one of the 33 new YSOs are massive.
There are several stumbling blocks facing astronomers trying to understand IRS 13 and how it formed. Much of what lies inside the cluster is embedded in thick gas and dust. There are two distinct generations of stars in the cluster, and there’s also a disk structure containing the youngest stars. How does it all fit together?
These figures from the study show some of what astronomers see when they observe IRS 13 and the region around Sgr A *. The image on the left shows the black hole and several stars in the IRS 13 cluster. The image on the right shows a zoomed-in view with many of the dusty sources labelled. The numbered ones were newly identified in this research. Each dusty source is a YSO. Image Credit: Peißker et al. 2023.Another difficulty is that the YSOs in the cluster emit different types of radiation from different locations. It takes different telescopes to sense the different bands of radiation. The researchers performed a multiwavelength photometric analysis of the light coming from the YSOs to understand them better.
This sketch from the study shows how the young stellar objects in the cluster emit different wavelengths of energy. As different astronomers observed IRS 13 over the years, they arrived at different conclusions. Image Credit: Peißker et al. 2023.After intense scrutiny by different teams of astronomers over the years and after even more observations by Peißker and his colleagues, the researchers think they can explain what IRS 13 is and how it formed.
IRS 13 was likely migrating toward Sgr A* because of interaction with other clusters, the interstellar medium, and because of other internal processes. At a certain distance, it was drawn toward the SMBH by the object’s powerful gravity. The cluster is surrounded by dust and gas, and a bow shock was formed. The increased density in the bow shock triggered the cluster’s second episode of star formation. This explains why the young stars are in the tip.
This image from the study shows IRS 13’s unusual shape. The older stars are in the core, and the YSOs are in the tip. The researchers think the tip is a bow shock created when the cluster was pulled toward the SMBH by its powerful gravity. The cluster’s supersonic speed, its stellar winds, and the ISM created a bow shock. The increased pressure at the bow shock drove the cluster’s second round of star formation a few hundred thousand years ago. Image Credit: Peißker et al. 2023.Astronomers have been observing IRS 13 and the entire region near Sgr A* for decades. In fact, the SMBH was discovered by tracking the motion of a group of stars called the S-cluster, whose members are also younger than expected. Their highly eccentric orbits indicated the presence of something massive. This study may finally explain the young ages of the stars in the galactic centre.
This table from the research sums up some of the findings. There are two generations of stars in IRS 13. The 1st generation is older, about four million years, and they’re located in the core and were formed in the circumnuclear disk. The second generation is less than one million years old, is located in the tip, and formed in the bow-shock shell as the cluster moved through space. Image Credit: Peißker et al. 2023.“The analysis of IRS13 and the accompanying interpretation of the cluster is the first attempt to unravel a decade-old mystery about the unexpectedly young stars in the Galactic Center,” said Dr. Peißker. “In addition to IRS13, there is a star cluster, the so-called S-cluster, which is even closer to the black hole and also consists of young stars. They are also significantly younger than would be possible according to accepted theories.”
The JWST contributed to this research, too, with its powerful NIRSPEC spectrometer, which can observe 100 objects at the same time. Some of the telescope’s observing time was already set aside to observe the galactic centre. When pointed at YSOs in IRS 13, it found strong water absorption features. Water ice is typically found in the disks of dust around YSOs because they’re not very hot. Finding water ice there is another indication of the young ages of the objects.
By establishing the nature of IRS 13 and its members, Peißker and his colleagues in this work may have done what other researchers haven’t been able to do. They may have explained not only IRS 13 but the other stars that reside in the Milky Way’s centre, too. And their results likely apply to other galaxies with SMBHs.
“The star cluster IRS13 seems to be the key to unravelling the origin of the dense star population at the centre of our galaxy,” said Dr. Michal Zajacek, second author of the study and a scientist at Masaryk University in Brno, Czechia (Czech Republic.) “We have gathered extensive evidence that very young stars within the range of the supermassive black hole may have formed in star clusters such as IRS13. This is also the first time we have been able to identify star populations of different ages – hot main sequence stars and young emerging stars – in the cluster so close to the centre of the Milky Way.”
