What alien astronomers observing our Milky Way galaxy from afar would discover by analyzing its chemical composition has been reconstructed by researchers. The review, which is driven by specialists from the Max Planck Foundation for Cosmology, is applicable to how we might interpret the universe: It makes it possible to make new comparisons between our own galaxy and the many other galaxies we see from space. Part of the answer to the old question about whether our own galaxy is special can be found in the results: The Milky Way, at least in terms of its chemical makeup, is unusual but not unique.
We see far-off universes from an external perspective: Telescope perceptions show us a system’s shape and its range (the rainbow-like deterioration of a world’s light). So, how would a distant alien astronomer see our own galaxy from that vantage point? That appears to be a straightforward inquiry. After all, astronomers on Earth have devised quite inventive methods for deducing the characteristics of a galaxy from our observations, and extraterrestrial astronomers will probably have a similarly sophisticated perspective on the Milky Way.
With the more modern techniques for examination, it isn’t by any stretch simple to determine what outside space experts would find were they to apply those strategies to our home system. However, the rewards can be substantial. Jianhui Lian (Max Planck Establishment for Space Science and Yunnan College), the lead creator of the review that has now been distributed in Nature Stargazing, says, “Tracking down ways of contrasting our home world and more far-off cosmic systems is what we want if we have any desire to know, regardless of whether the Smooth Way is extraordinary. Since astronomers discovered a century ago that the Milky Way is not the only galaxy in the universe, this has remained a question.
“We need to find ways to compare our home galaxy to more distant galaxies if we want to know whether the Milky Way is unique or not. Since scientists discovered a century ago that the Milky Way is not the only galaxy in the cosmos, this has remained an open subject.”
Jianhui Lian (Max Planck Institute for Astronomy and Yunnan University)
Great strides for data and simulations
Advances in data and simulations Despite the question’s age, it appears that astronomy is currently in a position to provide a solid response. First, systematic research on our own galaxy has made significant progress over the past few years. Surveys like APOGEE have used spectra to deduce the chemical composition, physical properties, and three-dimensional motions of millions of individual stars in our Milky Way. The brightness, motion, and distance of nearly 1.5 billion stars in our own galaxy have been tracked by ESA’s Gaia spacecraft.
Additionally, there is significantly more and much better information for faraway worlds. The MaNGA survey looked in depth at nearly 10,000 galaxies. MaNGA provides a “spectral picture,” demonstrating how, for instance, the chemical composition of each galaxy varies from the center to the outer regions, in contrast to previous surveys that targeted that many galaxies and provided only one overall spectrum for each galaxy.
Last but not least, modern simulations of galaxy formation and evolution exist. One example is the TNG50 simulation, which follows the evolution of thousands of galaxies in a model universe from the Big Bang to the present. We needed all of these developments to know what alien astronomers would see when they pointed their telescopes at the Milky Way and tried to reconstruct the chemical composition of the galaxy.
Second-guessing alien astronomers
This is exactly what a new Max Planck Institute for Astronomy study, led by Lian and Maria Bergemann, did: second-guess alien astronomers. In particular, Lian, Bergemann, and their partners thought about the substance organization of stars. The stars we see around us are mostly hydrogen and helium, but there are a few elements that are heavier than helium—elements that are studied in astronomy (but not chemistry!) are classified as “metals.”
A portion of these metals are delivered inside stars and flung into space when enormous stars detonate toward the end of their lives. Others are made in the outermost layers of bloated giant stars and are planned to travel into space from there. Most importantly, a general trend can be observed: Over time, the interstellar medium—the low-density mixture of gas and dust that encircles the stars—increases in metal concentration. Metals are more abundant in later-born stars than in earlier-born stars. You can determine which region formed its stars earlier or later by mapping the regions of a galaxy that have stars with fewer or more metals.
From neighborhood cosmology to an outsider’s point of view
Our home system, the Smooth Way, is as of now the main twisting universe in which we can straightforwardly make an enormous scope overview of individual stars—measure their situations inside our world and, by means of their spectra, their metal substance, surface temperature, and other actual properties. The goal of Lian, Bergemann, and their coworkers was to recreate the scene that extraterrestrial astronomers would see if they were to map the abundance of metals in the Milky Way. Since our home cosmic system is a plate universe, the key inquiry is: How might a far-off outsider stargazer see the overflow of metals fluctuate contingent upon the distance of a locale from the focal point of our cosmic system?
Reconstruction of this kind takes effort. The APOGEE survey data were only a starting point. Next, the researchers needed to account for the “smudgy” view of the Milky Way from Earth: There will be more dust in some directions between us and stars farther away, dimming the light from some of the brightest stars and completely concealing others. This way, there will be less residue. In order to reconstruct the true distribution of stars in our galaxy, the researchers needed to combine the observation data with what we know about dust and star properties.
Our galaxy’s high-metallicity ‘belt’
The findings were somewhat unexpected. Our galaxy’s “belt” of high metallicity At a distance of approximately 23,000 light-years from the galaxy’s center, the average metal content of the stars will rise to a level comparable to that of our sun. (As a comparison, Our sun is approximately 26,000 light-years away from the center of the galaxy.) At a distance of even 50,000 light-years from the center, the average metal content again decreases, falling to about one third of the solar value.
The APOGEE spectra allow for at least a rough estimate of the age of the stars, so the researchers looked at different age groups of stars separately to understand what was going on. They observed a consistent pattern across both younger and older stars, with higher metal content closer to the center and lower metal content further out. Younger stars were becoming more prevalent further out, but older stars—those with a much lower metal content—were only responsible for the increase and maximum of the overall distribution near the center of the galaxy.
Comparing our Milky Way with other galaxies
Lian, Bergemann, and their colleagues compared this intriguing result to the properties of other galaxies in order to compare our Milky Way to other galaxies. On the one hand, they took into account 321 galaxies in the MaNGA survey, all of which have masses comparable to those of the Milky Way, produce the same number of stars, and are visible face-on, making it possible to measure the change in average metallicity. On the other hand, the same criteria were used by the researchers to find 134 Milky Way-like galaxies in the TNG50 simulation’s model universe.
So exactly how exceptional is our home world—or not? The current study provides the following response: Our Milky Way is not the only galaxy with a peculiar distribution of metal abundances. Similar ups and downs in average metallicity were observed in only 11% of the galaxies in the MaNGA sample and in 11% of the galaxies in the TNG50 sample. Uncertainties in the MaNGA data and the lack of realistic simulations in the TNG50 model universe are likely to account for the 1 percent to 1 percent difference.
In addition, when compared to the MaNGA and TNG50 galaxies, the Milky Way has a lower average metallicity in the outer regions as the distance from the center increases.
The question of ‘why’
What are the unusual properties of the Milky Way, and how do they relate to the formation history of our own galaxy? The relative scarcity of metal-rich stars close to the center of the galaxy can be explained in a number of different ways. The so-called bulge, a roughly spherical region of an older star that extends out to a distance of approximately 5,000 light-years from the center of the galaxy, may be connected to the formation of this feature. The use of the majority of the available hydrogen gas for bulk formation would have made later star formation much more challenging. Alternately, the scarcity may be connected to an active phase during which the central supermassive black hole of our galaxy spewed particles and radiation from its immediate vicinity, preventing star formation.
There are a number of possible explanations for the metallicity in the outer regions, all of which combine the history of star formation across the galactic disk with the evolution of gas in our own galaxy. The precarious downfall could be an indication of a surprising episode in our system’s set of experiences—say, our home universe “gulping” a more modest world with gas that contained few metals. When stars with fewer metals in the disk formed later, that gas would have served as the starting material. It’s also possible that our estimate of the Milky Way’s stellar disk’s size is incorrect, skewing the comparison to other galaxies in terms of how steep the decrease is.
Outlook
Maria Bergemann says, “The findings are very exciting! For the first time, we are able to meaningfully compare our galaxy’s chemical composition to that of many other galaxies. The findings will have a significant impact on future comprehensive studies of galaxy formation. Data from upcoming large-scale observational programs focusing on the Milky Way or distant galaxies will be used in these studies. Our study demonstrates how to effectively combine the two types of data sets.”
Overall, the information presented here raises a number of intriguing questions. We can hope to obtain answers and better comprehend our home galaxy’s past by conducting fresh surveys and investigations from a “alien astronomer” perspective.
More information: Jianhui Lian et al, The integrated metallicity profile of the Milky Way, Nature Astronomy (2023). DOI: 10.1038/s41550-023-01977-z
