Star formation on the heart of the Milky Method started on the core after which labored its means outwards

One of the biggest questions facing astronomers today concerns star formation and its role in the evolution of galaxies. Astronomers are particularly curious as to whether the process began in the central regions of galaxies, where stars are more closely bound. Previous observations have shown that numerous galaxies experienced intense periods of star formation at their centers about a billion years after the Big Bang. For some time, astronomers have wanted to make similar observations of the Milky Way’s galactic center to study rapid star formation in more detail.

Unfortunately, astronomers have found it very difficult to study the center of the Milky Way because the region is so bright and densely packed, making it difficult to spot individual stars and clusters. Thanks to a new analysis of a high-resolution infrared survey, a team of astronomers has produced the first reconstruction of the Galactic Center’s star formation history. They found that most of the young stars in this region formed in loose stellar clusters that spread outward over many eons to fill the Galactic disk (as opposed to tightly meshed, massive clusters).

The research was carried out by Dr. Francisco Nogueras-Lara, an Independent Humboldt Research Fellow at the Max Planck Institute for Astronomy (MPIA). He was joined by Dr. Nadine Neumayer, leader of the Lise Meitner group at MPIA (which specializes in studying galactic nuclei), and Dr. Rainer Schödel – the head of the Galactic Center Group at the Instituto de Astrofísica de Andalucía (CSI). The paper describing their findings, titled “Detection of an excess of young stars in the Galactic Center Sagittarius B1 region,” appeared recently in the journal Nature Astronomy.

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This is an image of the center of the Milky Way. The bright white area to the right of center is home to the supermassive black hole Sagittarius A star. Photo Credit: By NASA/JPL-Caltech/ESA/CXC/STScI

While astronomers use our galaxy to learn about the properties of galaxies in general, there are notable differences between the Milky Way and others. First of all, our galaxy has a relatively low star formation rate (just a few solar masses per year), while “starburst” galaxies experience episodes lasting a few million years, during which they produce tens or even hundreds of solar masses per year. Interestingly, ten billion years ago this high rate of formation was the norm among galaxies, with tens of solar masses being produced every year.

But in the central region of the Milky Way, about 1,300 light-years from our galaxy’s supermassive black hole (SMBH), star formation rates 10 times higher than average have been observed over the past 100 million years. In short, the core of our galaxy is as productive as a starburst galaxy, or as productive as galaxies ten billion years ago. Astronomers have hoped to study this region to learn more about the factors affecting star formation in galaxies. Unfortunately, this was far more difficult than studying other galaxies because our solar system is embedded in the disk of the Milky Way.

Our observatories must contend with the vast amounts of opaque dust between Earth and the Galactic Center. To get around this problem, astronomers rely on instruments that observe the universe in infrared, millimeter wave, or radio wave bands. These can map the radiation absorbed or passed through by the dust and thus make objects visible that are otherwise hidden in visible light. Another problem (mentioned earlier) is that the Galactic Center is so crowded, making it difficult to spot individual stars (except for the very bright ones that stand out from the rest).

Astronomers know that stars continue to form at the galactic center, as indicated by ionized radiation and X-ray emissions. But it’s been extremely difficult to spot young stars that formed in the last few million years. Before this analysis, astronomers could only explain two massive star clusters and a few isolated young stars at the center of our galaxy — about 10% of the expected stellar mass. This left many questions unanswered about the locations of all the other young stars and their characteristics.

The central region of the Milky Way in infrared light as imaged by NASA’s Spitzer Space Telescope. Photo credit: NASA/JPL-Caltech/S. Stolovy (Spitzer Science Center/Caltech)

To answer this question, Nogueras-Lara, Neumayer and Schödel consulted data from the GALACTICNUCLEUS campaign, an investigation using the HAWK-I infrared camera – part of the Very Large Telescope (VLT) at the Paranal Observatory in Chile. Together, they captured nearly 150 short-duration images of the central region of the Milky Way in the J, H, and Ks infrared bands, surveying a total area of ​​64,000 square light-years around the Galactic Center. These images were then combined using holographic imaging to correct for atmospheric distortions and map the region in much finer detail than ever before.

While previously only a few dozen stars had been mapped, the GALACTICNUCLEUS survey provided individual data for 3 million stars in the Galactic Center. In addition, the team noted that the region known as Sagittarius B1 contains significantly more young stars than other regions, as evidenced by the way they ionize surrounding gas clouds. With these high-resolution observations, Nogueras-Lara and his colleagues were able to study the region’s stars in detail for the first time – including the statistical distribution of stellar luminosity.

This is particularly important because stars that formed around the same time change their brightness distribution gradually (and in a predictable way). Given such a distribution, it is possible to reconstruct a star-forming history based on those formed more than 7 billion years ago, 2 to 7 billion years ago (the “intermediate class”), and within the last 2 billion years. Analyzing their data, the team found that Sag B1 had an older “interlayer” population and a large population of stars that were 10 million years old or younger. As Nogueras-Lara said in an MPIA press release:

“Our study represents a major step forward in finding the young stars in the Galactic Center. The young stars we have found have a total mass of more than 400,000 solar masses. That is almost ten times the combined mass of the two massive star clusters previously known in the central region.”

The all-sky view that the Gaia survey would have of a simulated Milky Way-like galaxy. Photo credit: Sanderson et al.

Interestingly, the stars were also found to be scattered rather than part of a massive cluster, suggesting they were born into one or more looser stellar clusters that quickly disintegrated as they orbited the galactic center for several million years. Although these results are specific to Sag B1, they could mean that young stars in the Galactic Center were generally born in loose clusters that have since broken up into separate stars. This would explain why the young populations are so much more difficult to resolve, requiring high-resolution surveys in multiple wavelengths.

Another interesting finding was that there is also an older population of stars in Sag B1. In the innermost regions of the Galactic Center there are stars older than 7 billion years, but practically no stars in the middle range. This could mean that star formation began in the innermost part of the Galactic Center and then spread to the “nucleus disk,” the small star disk that surrounds the center. Evidence of this inside-out mechanism of star formation has already been observed in other galaxies, and these latest results suggest the same is true of the Milky Way.

Looking ahead, the team hopes to conduct follow-up observations using the K-band Multi-Object Spectrograph (KMOS) instrument on the VLT. By adding spectral observations to the overall brightness distribution they are observing, they hope to be able to directly identify some of the very young stars at the Galactic Center. In addition, it is planned to track the proper motions of the newly discovered stars using data obtained from missions such as ESA’s Gaia Observatory. While stars formed in the same association will disperse over time, their motion is likely still very similar, suggesting a common origin.

Ergo, tracking the stars’ proper motion in Sag B1 will allow astronomers to deduce whether the young stars observed there were actually born in one or more loose associations. As Nadine Neumayer summarized:

“Both types of measurements will serve to hopefully confirm, but definitely refine, the results of the work now published. At the same time, we and our colleagues will begin to explore what the new insights into star formation in the Galactic Center can tell us about highly productive star formation in other galaxies.”

Further reading: MPIA, Naturastronomie

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