Star Formation in the Center of the Milky Way Started at the Core and Then Worked its way out

One of the biggest questions facing astronomers today concerns star formation and its role in the evolution of galaxies. In particular, astronomers are curious whether the process began in the central regions of galaxies, where stars are more tightly bound. Previous observations have shown that numerous galaxies experienced intense periods of star formation in their centers roughly one billion years after the Big Bang. For some time, astronomers have wanted to conduct similar observations of the Milky Way’s Galactic Center to study rapid star formation more closely.

Unfortunately, it has been very difficult for astronomers to study the center of the Milky Way because of how bright and densely packed the region is, which makes it difficult to discern individual stars and clusters. Thanks to a new analysis of a high-resolution infrared survey, a team of astronomers has created the first reconstruction of the star formation history in the Galactic Center. According to their findings, most young stars in this region formed in loose stellar associations that dispersed outwards to fill the Galactic Disk over the course of many eons (as opposed to tightly-knit massive clusters).

The research was led by Dr. Francisco Nogueras-Lara, an independent Humboldt research fellow with the Max-Planck Institute for Astronomy (MPIA). He was joined by Dr. Nadine Neumayer, the leader of the Lise Meitner Group at the MPIA (which specializes in the research of Galactic Nuclei), and Dr. Rainer Schödel – the leader of the Galactic Centre Group at Instituto de Astrofísica de Andalucía (CSIC). The paper that describes their findings, titled “Detection of an excess of young stars in the Galactic Centre Sagittarius B1 region,” recently appeared in the journal Nature Astronomy.

This is an image of the center of the Milky Way. The bright white area right of center is home to the supermassive black hole Sagittarius A star. 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. For starters, our galaxy has a relatively low rate of stellar formation (only a few solar masses a year), whereas “starburst” galaxies experience episodes that last a few million years where they produce tens or even hundreds of solar masses per year. Interestingly, that high formation rate was the norm among galaxies ten billion years ago, with tens of solar masses produced every year.

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

Our observatories must contend with the massive amounts of light-obscuring dust between Earth and the Galactic Center. To circumvent this problem, astronomers rely on instruments that observe the Universe in the infrared, millimeter-wave, or radio wavelengths. These can image the radiation absorbed by the dust, or passes through it, thus revealing objects that are otherwise obscured in visible light. Another issue (already mentioned) is how the Galactic Center is so crowded, making it difficult to discern individual stars (except for the very bright ones that stand out from the rest).

Astronomers know that stars continue to form in the Galactic Center, as indicated by ionized radiation and x-ray emissions. But it has been extremely difficult to spot young stars, those that formed in the past few million years. Prior to this analysis, astronomers could only account for two massive star clusters and a few isolated young stars at the center of our galaxy – about 10% of the expected stellar mass. This has left many questions unanswered about the locations of all the other young stars and their properties.

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

To address this question, Nogueras-Lara, Neumayer, and Schödel consulted data from the GALACTICNUCLEUS campaign, a survey that uses the HAWK-I infrared camera – part of the Very Large Telescope (VLT) at the Paranal Observatory in Chile. Together they took nearly 150 short-exposure pictures of the Milky Way’s central region in the J, H, and Ks infrared bands and examined an area totaling 64,000 square light-years around the Galactic Center. These pictures were then combined via holographic imaging to correct for atmospheric distortion and map the region in much finer detail than ever before.

Whereas only a few tens of stars had been previously mapped, the GALACTICNUCLEUS survey provided individual data for 3 million stars in the Galactic Center. Moreover, the team noticed that the area known as Sagittarius B1 contains considerably more young stars than other regions, made evident 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 luminosity distribution changes gradually (and in a predictable fashion) for stars that formed around the same time. Given such a distribution, it is possible to reconstruct a history of star formation based on those that formed more than 7 billion years ago, between 2 and 7 billion years ago (the “intermediate bracket”), and within the last 2 billion years. Upon analyzing their data, the team found that Sag B1 had an older “intermediate bracket” 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 big step forward in finding the young stars in the Galactic Center. The young stars we found have a total mass of more than 400,000 solar masses. That is nearly ten times higher than the combined mass of the two massive star clusters that were previously known in the central region.”

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

Interestingly, the stars were also found to be dispersed and not part of a massive cluster, which suggests they were born in one or more looser stellar associations that rapidly dissolved as they orbited the Galactic Center over several million years. While these results pertain specifically to Sag B1, they could mean that young stars in the Galactic Center were generally born in loose associations that have since dispersed into separate stars. This would explain why the young populations are so much harder to resolve and require 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 more than 7 billion years old but virtually no stars in the intermediate range. This could mean that star formation began in the innermost part of the Galactic Center and then spread to the “nuclear disk,” the small disk of stars surrounding the center. Indications of this inside-out mechanism of star formation have already been observed in other galaxies, and these latest results suggest this is also 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 luminosity distribution they observed, they hope to identify some of the very young stars in the Galactic Center directly. In addition, there are plans to track the proper motions of the newly-discovered stars based on data obtained by missions like the ESA’s Gaia Observatory. While stars that formed in the same association get dispersed over time, their motion is still likely to be very similar, indicating a common origin.

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

“Both kinds of measurements will serve to hopefully confirm, but definitely refine, the results of the now-published work. At the same time, we and our colleagues will start exploring what the new insights into star formation in the Galactic Center can tell us about high-productivity star formation in other galaxies.”

Further Reading: MPIA, Nature Astronomy