The galactic center's high-productivity star factory is revealed for the first time

False-color image of the region Sagittarius B1, as seen by the GALACTICNUCLEUS survey. The data on which this image is based allowed Francisco Nogueras-Lara and colleagues to identify three million stars in the galactic center, and deduce key properties of the high-productivity star formation going on in that region of our home galaxy. Credit: F. Nogueras-Lara et al. / MPIA
False-color image of the region Sagittarius B1, as seen by the GALACTICNUCLEUS survey. The data on which this image is based allowed Francisco Nogueras-Lara and colleagues to identify three million stars in the galactic center, and deduce key properties of the high-productivity star formation going on in that region of our home galaxy. Credit: F. Nogueras-Lara et al. / MPIA

Astronomers have able to gain a first representative sight of the countless newborn stars in the center parts of our home galaxy thanks to thorough studies. The measurements show that star formation in the galactic center began close to the core and later spread outwards. This confirms a style of star creation previously discovered in the cores of other, distant galaxies. The findings also show that the majority of stars in that region did not originate in tightly connected giant clusters, but in loose associations whose individual stars have long since separated. The findings were reported in Nature Astronomy.

The core area of our home galaxy, the Milky Way, is far more populated with stars than other sections of our galaxy. Astronomers have long hoped that this would provide a laboratory for studying fast star formation, a phenomena that happens in many other galaxies, particularly in the early billions of years of cosmic history. However, because of the crowding, stars in the core area are famously difficult to view.

A new study based on a high-resolution infrared scan, published in Nature Astronomy, presents the first representative reconstruction of star formation history in the galactic center. It also demonstrates that the majority of young stars in the galactic core developed in loose stellar associations rather than tightly-knit giant clusters that scattered over millions of years.

Our Milky Way Galaxy is not a particularly prolific galaxy. The new stars formed by our home galaxy in a year add up to only a few solar masses. "star burst galaxies" are far more effective: They create tens or even hundreds of solar masses worth of stars every year during short outbursts that span a few million years! More broadly, that type of rapid creation rate, with tens of solar masses generated each year, appears to have been the norm among galaxies 10 billion years ago.

Astronomers use the Milky Way all the time to learn about galaxy features in general. After all, the Milky Way is the only galaxy where scientists can examine processes and features up close and in detail. Given the Milky Way's poor star-formation efficiency, one may suppose that high-productivity star formation is one area where this formula of studying what happens locally does not work. But you'd be wrong: Over the past 100 million years, star formation rates in the Milky Way's center regions have been ten times greater than on average, corresponding to the central 1300 or so light-years around our galaxy's central black hole. The centre of our galaxy is as productive as a starburst galaxy or the hyper-productive galaxies of 10 billion years ago.

But there is a problem if we wish to learn about high-productivity star creation from our galaxy's center regions: These areas are infamous for being difficult to observe. To begin with, they are obscured by a thick layer of dust as viewed from Earth. However, this is a simple problem to solve: employ infrared, millimeter wave, or radio measurements. The light at those wavelengths will flow straight through the dust, allowing us to see the galactic core. That is how Andrea Ghez and Reinhard Genzel's groups completed their Nobel Prize-winning near-infrared studies of stars orbiting our galaxy's center black hole, and how the Event Horizon Collaboration obtained the first image of our galaxy's central black hole's shadow (millimeter waves at 1.3 mm).

With the first issue resolved, the second arises: the galactic core is so densely packed with stars that distinguishing one from the next is impossible. The exception is certain really brilliant big stars, which are especially luminous, stand out from the throng, and can thus be distinguished from the rest quite readily. For years, astronomers have struggled to make sense of high-productivity star formation near the galactic core. The presence of hydrogen gas split into its components (ionized) by ultraviolet light from hot, young stars, as well as the presence of X-rays characteristic of certain types of young, extremely massive stars, attests to the existence of such star formation over the last one to ten million years.

However, because of the crowding issue, it has been difficult to answer the question "... so where are the resulting young stars, then?" Prior to this new research, astronomers had only discovered around 10% of the predicted total stellar mass in the galactic core, in the form of two huge star clusters and a few solitary young stars. So, where were the other stars, and what were their characteristics?

That was the question posed by the authors of a recently published article. Francisco Nogueras-Lara, an independent Humboldt research fellow in Nadine Neumayer's Lise Meitner group at the Max Planck Institute for Astronomy, and Rainer Schödel of the Instituto de Astrofsica de Andaluca in Granada, Spain, were in a unique position to go about finding the missing young stars in the galactic center: Schödel is the PI of GALACTICNUCLEUS, a survey that used the HAWK-I infrared camera at the European Southern Observatory's Very Large Telescope (VLT) to take nearly 150 images (in the infrared bands J, H, and Ks) of the Milky Way's central region, covering a total area of 64,000 square light-years around the galactic center.

The search was led by Nogueras-Lara. Resolution, or the capacity to detect microscopic features in the sky, is required to recognize individual stars in a crowded location. The VLT is made up of telescopes with mirrors of 8 meters in length. The survey was able to map its target region in far greater detail than ever before because to a process known as holographic imaging, which included merging numerous short-exposure photographs in a proper way to reduce the blurring effects of Earth's atmosphere (with a resolution of 0.2 arc seconds). Whereas just a few stars had previously been mapped, GALACTICNUCLEUS gave individual data for 3 million.

When the researchers examined the (false-color) photos from the GALACTICNUCLEUS survey, they saw that the region known as Sagittarius B1 in the galactic core was different. It includes far more young stars, which ionize the surrounding gas, than other areas, an effect that was not unexpected: Earlier studies, particularly of light associated with hydrogen gas being ionized by bright stars, had suggested as much. Nogueras-Lara and his colleagues were now able to investigate the region's stars in depth for the first time, thanks to the highly resolved GALACTICNUCLEUS observations.

Even while astronomers could only investigate massive stars individually (not so-called main-sequence stars like our Sun) with their high-resolution survey, the data from the 3 million stars they could analyze independently already provided a lot of information. The researchers were able to calculate each star's brightness, correcting for darkening caused by dust between us and a certain star. All of the stars in Sagittarius B1 are roughly the same distance from Earth, and the distance from Earth to the galactic center is known; using that knowledge, the researchers were able to recreate each star's luminosity, or the amount of light a star produces per unit time.

The statistical distribution of stellar luminosity for those stars was particularly intriguing, as was the number of stars in each "brightness bracket." That luminosity distribution evolves over time in a regular and predictable manner for stars that develop at the same moment. Given such a distribution, one may conclude at least an approximate history of star formation: How many stars were born more than 7 billion years ago? How many fall somewhere between between 2 and and 7 billion years? How many years ago was that? The luminosity distribution gives at least a statistical response the most probable star formation history.

When Nogueras-Lara, Neumayer, and Schödel examined their luminosity distribution, they discovered that Sagittarius B1 had gone through several phases of star formation: an older population that formed between 2 and 7 billion years ago, and a large population of much younger stars that were only 10 million years old or younger. "Our study represents a significant step forward in the discovery of young stars in the galactic center," says Nogueras-Lara. "The young stars we discovered have a total mass of more than 400,000 solar masses, which is nearly ten times greater than the combined mass of the two massive star clusters previously known in the central region."

Surprisingly, the stars discovered in Sagittarius B1 are scattered and not part of a huge cluster. This implies that they were created in one or more looser stellar associations, less closely connected by the stars' mutual attraction, which later swiftly disintegrated as they orbited the galactic center on scales of millions of years, leaving behind numerous independent stars. While this conclusion is exclusive to Sagittarius B1, it might also explain why the young stars in the galactic core can only be detected through high-resolution investigations like this one: they were born in loose associations that have since split into distinct stars.

The presence of an older population of stars in Sagittarius B1 is also intriguing. There are stars older than 7 billion years old in the galactic core, however there are almost no stars in the 2 to 7 billion year age range. This might suggest that star creation in the center area began in the innermost region and subsequently expanded to the outer regions giving an overall pattern for the chronology of star formation in those regions. This inside-out technique to form the so-called nuclear disk, a small-scale disk comprised of stars encircling the galactic center, has already been detected in other galaxies. The latest findings suggest that the same process is happening in the center area of our own galaxy.

As compelling as the evidence from infrared photos is already, both for the reconstruction of star formation history and for the overall inside-out pattern of star formation, astronomers are keen to place their conclusions on a solid basis. To that aim, Nogueras-Lara and his colleagues intend to expand on their findings using the VLT's KMOS instrument, a high-precision spectrograph. The deductions in this study were derived based on the overall luminosity distribution. Spectral observations would allow the scientists to identify some of the extremely young stars directly, given the look of their spectra. That would be a crucial check on the results that have already been published.

Furthermore, the scientists will monitor the movements of the newly discovered stars in the sky ("proper motion"). Stars move rather quickly toward the galactic center. Because of this, even though these stars are almost 26,000 light-years from Earth, detailed measurements over a few years will be able to measure their position changes. Stars that formed in the same stellar association get scattered over time, but their mobility remains fairly similar, therefore measuring proper motion might allow conclusions about whether the stars in Sagittarius B1 were truly born in one or multiple loose associations.

Finally, according to Nadine Neumayer, both types of measurements will serve to hopefully corroborate, but surely refine, the conclusions of the recently published work. Simultaneously, we and our colleagues will begin to investigate what the new insights into star formation in the galactic core might teach us about high-productivity star creation in other galaxies.

Journal Information: Francisco Nogueras-Lara et al, Detection of an excess of young stars in the Galactic Centre Sagittarius B1 region, Nature Astronomy (2022). DOI: 10.1038/s41550-022-01755-3
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