Milky Way’s chemical composition compared to other galaxies

Scientists from the Max Planck Institute for Astronomy have undertaken a groundbreaking study in which they reconstructed how alien astronomers would perceive our Milky Way galaxy if they were observing it from a distant vantage point. This research not only expands our understanding of the cosmos but also enables a novel approach to comparing our home galaxy with other distant galaxies that we observe externally.

When we observe galaxies from afar using telescopes, we can discern their shape and analyze their spectrum, which involves breaking down the galaxy’s light into its component colors. However, contemplating how our own galaxy would appear to distant alien astronomers is a seemingly straightforward yet complex question. Earth-based astronomers have devised ingenious methods for deducing a galaxy’s properties based on our observations, and it is reasonable to assume that alien astronomers would possess similarly advanced techniques to study the Milky Way.

Determining what these hypothetical extraterrestrial astronomers would uncover by employing sophisticated analytical methods to study our home galaxy is no easy task. Nevertheless, the potential rewards are significant. Jianhui Lian, the lead author of the recently published study in Nature Astronomy from the Max Planck Institute for Astronomy and Yunnan University, explains, “If we aspire to determine whether the Milky Way is a unique entity or not, we must discover means of comparing it with distant galaxies. This question has persisted since astronomers realized a century ago that the Milky Way is just one of many galaxies in the universe.”

Great strides for data and simulations

With recent advancements in astronomy, we find ourselves in a promising position to finally provide a definitive answer to the age-old question about the uniqueness of our Milky Way galaxy. Over the past decade, significant strides have been made in conducting comprehensive studies of our home galaxy. Surveys like APOGEE have played a crucial role by furnishing us with valuable data on the chemical composition, physical properties, and three-dimensional movements of millions of individual stars in the Milky Way, gleaned from their spectra.

ESA’s Gaia spacecraft has also made remarkable contributions, meticulously tracking the brightness, motion, and distances of nearly 1.5 billion stars within our galaxy. This wealth of information has significantly expanded our understanding of the Milky Way’s intricate dynamics.

Moreover, the availability of abundant and high-quality data for distant galaxies has been instrumental in our quest for answers. The MaNGA survey, for instance, has undertaken an in-depth examination of nearly 10,000 galaxies. Unlike previous surveys that provided only an overall spectrum per galaxy, MaNGA has provided a more detailed “spectral picture,” allowing us to discern the variations in chemical composition from the central regions to the outer reaches of each galaxy.

Notably, modern simulations of galaxy formation and evolution, such as the TNG50 simulation, have played a crucial role in this endeavor. These simulations recreate the history of thousands of galaxies in a model universe, spanning from the early stages after the Big Bang to the present day. They have provided us with invaluable insights into the factors influencing the formation and evolution of galaxies.

These advancements, ranging from comprehensive surveys to sophisticated simulations, have been indispensable in our ability to predict how alien astronomers would perceive the Milky Way when directing their telescopes towards it. Through these efforts, we are edging closer to unraveling the secrets of our home galaxy’s chemical composition and gaining a deeper understanding of its place within the cosmic tapestry.

Second-guessing alien astronomers

In a recent study led by Lian and Maria Bergemann from the Max Planck Institute for Astronomy, the focus was primarily on the chemical composition of stars as a key aspect. While stars are primarily composed of hydrogen and helium, there are also trace amounts of elements heavier than helium, commonly referred to as “metals” in the realm of astronomy (distinct from the conventional definition in chemistry).

The origins of these metals can be traced back to stellar processes. Some metals are synthesized within stars and expelled into space when massive stars undergo explosive supernovae at the end of their lives. Others are produced in the outer layers of expansive giant stars and subsequently released into the surrounding space. What makes the analysis of metal concentrations crucial is the existence of a discernible pattern: the interstellar medium, which constitutes the low-density mixture of gas and dust filling the space between stars, exhibits an increasing concentration of metals over time. Stars that formed earlier possess lower metal content, while those that formed later contain higher levels of metals. By mapping out the distribution of stars with varying metal concentrations in different regions of a galaxy, it becomes possible to determine the relative ages of star formation within those regions.

By examining the chemical composition of stars and studying the distribution of metals in the interstellar medium, Lian, Bergemann, and their team have shed light on the temporal sequence of star formation in galaxies. This investigation provides valuable insights into the evolution of galaxies and contributes to our broader understanding of the processes governing the formation and development of celestial structures.

From local cosmology to an alien perspective

The Milky Way, our home galaxy, offers a unique advantage compared to other spiral galaxies in that we have the ability to conduct large-scale surveys of individual stars. By precisely measuring their positions within the galaxy and analyzing their spectra, we can determine various physical properties such as metal content and surface temperature. With the aim of envisioning how alien astronomers would perceive the distribution of metals in the Milky Way, Lian, Bergemann, and their team embarked on a quest to reconstruct this view.

Constructing such a reconstruction involves a meticulous process. The initial step involved gathering data from the APOGEE survey. However, the researchers had to address a significant factor: our observation of the Milky Way from Earth is affected by an inherent blurriness. The presence of interstellar dust can obstruct and dim starlight, making it challenging to observe the faintest stars in certain directions, while other regions may experience less dust interference. To achieve an accurate depiction of the stellar distribution within our galaxy, the researchers needed to integrate the observational data with our understanding of dust properties and the characteristics of stars.

By combining these diverse sources of information, Lian, Bergemann, and their colleagues aimed to create a comprehensive portrayal of how the abundance of metals would vary depending on the distance from the center of the Milky Way. This ambitious undertaking provides insights into the spatial distribution of stars and their metal content, ultimately enabling a glimpse into how the Milky Way would appear to distant alien astronomers scrutinizing our galaxy from afar.

Our galaxy’s high-metallicity ‘belt’

The outcomes of the study yielded intriguing results. When examining the average metal content of stars as one moves outward from the center of the galaxy, a surprising pattern emerged. The metal content gradually rises, reaching levels comparable to that of our sun at a distance of approximately 23,000 light-years from the galactic center. It is worth noting that our sun is situated around 26,000 light-years away from the center. However, as the distance from the center continues to increase, the average metal content declines once again, plummeting to roughly one-third of the solar value at approximately 50,000 light-years from the center.

To comprehend these findings, the researchers delved into the analysis of stars belonging to different age groups, utilizing the APOGEE spectra to make rough estimations of stellar age. By segregating younger and older stars, they made an intriguing observation. Each age group exhibited a consistent trend with higher metal content closer to the galactic center and lower content farther away. The overall distribution’s increase and peak were predominantly influenced by older stars, which possessed significantly lower metal content. These older stars were more abundant near the galactic center, consequently exerting a downward pull on the overall average. Conversely, younger stars became increasingly prevalent as one moved toward the outskirts of the galaxy.

This comprehensive examination of stellar populations with varying ages sheds light on the intricate relationship between galactic location, stellar age, and metal content. It unveils a nuanced picture of how the abundance of metals evolves across different regions of the Milky Way, offering valuable insights into the complex processes that shape and influence the composition of galaxies.

Comparing our Milky Way with other galaxies

To assess the uniqueness of our home galaxy, Lian, Bergemann, and their team conducted a comparative analysis involving other galaxies. They examined 321 galaxies from the MaNGA survey, all of which shared similar characteristics with the Milky Way in terms of mass, star formation rates, and face-on visibility, allowing for the measurement of changes in average metallicity. Additionally, the researchers identified 134 Milky-Way-like galaxies within the simulated universe of the TNG50 simulation, employing the same criteria for comparison.

The question at hand was the extent to which the distribution of metal abundances in our Milky Way sets it apart from other galaxies. The study revealed that while our galaxy displays an intriguing up-and-down pattern in average metallicity, it is not entirely unique in this regard. Only 11% of the galaxies in the TNG50 sample and approximately 1% of the galaxies in the MaNGA sample exhibited a similar fluctuation in average metallicity. The slight disparity between these percentages can be attributed to uncertainties within the MaNGA data and the limitations inherent in realistic simulations within the TNG50 model universe.

Furthermore, the study found that in the outer regions of the galaxy, the decline in average metallicity with increasing distance from the center was notably steeper in the Milky Way compared to both the MaNGA and TNG50 galaxies. This observation underscores another distinct aspect of our galaxy’s metallicity distribution.

The question of ‘why’

The peculiar properties exhibited by the Milky Way can be attributed to several possible explanations, shedding light on its formation history. One key aspect is the comparative scarcity of metal-rich stars near the galactic center. This could be associated with the formation of the galactic bulge, a spherical region comprising older stars that encompasses the galactic center up to a distance of approximately 5,000 light-years. During bulge formation, the available hydrogen gas would have been predominantly utilized, rendering subsequent star formation more challenging. Another possibility is that the scarcity is linked to an active phase involving the central supermassive black hole of our galaxy, emitting particles and radiation that inhibit star formation in its immediate vicinity.

Regarding the metallicity observed in the outer regions, various scenarios arise from the interplay between gas evolution within the Milky Way and the history of star formation across its disk. The steep decline in metallicity could signify an extraordinary episode in our galaxy’s past, such as the assimilation of a smaller gas-rich galaxy that possessed minimal metal content. This gas would have subsequently served as raw material for star formation within the disk, resulting in stars with lower metal content. Alternatively, it is plausible that inaccuracies in estimating the extent of the Milky Way’s stellar disk could influence the comparison with other galaxies, potentially exaggerating the steepness of the metallicity decrease.

These explanations provide insights into the complex interplay of galactic dynamics, gas evolution, and star formation processes that have shaped the unique characteristics of the Milky Way. By unraveling these mechanisms, we gain a deeper understanding of the formation and evolution of our home galaxy within the broader context of the cosmic landscape.


Maria Bergemann expresses her enthusiasm about the groundbreaking findings, stating, “These results are incredibly exciting! It marks the first time we can truly compare the intricate chemical composition of our galaxy with measurements from numerous other galaxies. These findings hold significant importance for future comprehensive investigations into galaxy formation. These studies will utilize data from forthcoming large-scale observational programs, focusing on either the Milky Way or distant galaxies. Our research demonstrates how to effectively combine these two types of datasets.”

Undoubtedly, the research discussed here raises a host of intriguing questions. Through the utilization of new surveys and investigations that adopt the perspective of an “alien astronomer,” we can anticipate uncovering answers that will enhance our understanding of the formation history of our home galaxy. These advancements pave the way for exciting discoveries and offer a unique opportunity to delve deeper into the mysteries surrounding the Milky Way.

Source: Max Planck Society

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