Starlight often reaches Earth altered by interstellar dust that dims and reddens it, a phenomenon that complicates efforts to accurately assess the intrinsic properties of celestial bodies. This reddening and dimming, known as extinction, stems from dust absorbing more light at shorter (blue) wavelengths than at longer (red) ones. To disentangle these effects, researchers need precise knowledge of the dust lying between Earth and their observational targets. The new 3D map serves as a remedy for this cosmic distortion, likened to cleaning a dirty window that impedes a clear view of the universe.
Cosmic dust interacts with light in a wavelength-dependent manner, with its absorption characteristics represented by an "extinction curve." The shape of this curve provides valuable clues about dust composition and the surrounding interstellar environment. Leveraging this principle, PhD student Xiangyu Zhang and his advisor Gregory Green, who leads an independent research group at MPIA, compiled the new dust map by analyzing spectral data from 130 million stars.
The Gaia mission's third data release included 220 million spectra, from which Zhang and Green identified 130 million suitable for dust analysis. Though Gaia's spectra are low-resolution, they cross-referenced this data with high-resolution spectral information from the LAMOST survey, which offered reliable benchmarks for stellar properties such as temperature and spectral type. This allowed them to train a neural network capable of modeling stellar spectra modified by dust.
Using Bayesian statistical methods, the astronomers inferred the properties of dust along each line of sight to these stars. The outcome is a detailed, three-dimensional representation of the extinction curve throughout the Milky Way, surpassing previous efforts by an order of magnitude in terms of data volume and resolution. While prior studies incorporated around one million stars, this project expanded the dataset to 130 million, enabling an extraordinary level of detail.
Beyond aiding observations, cosmic dust plays a critical role in star and planet formation. Dust clouds shield star-forming regions from radiation, and dust grains themselves act as the seeds for planetary bodies. Moreover, the majority of heavier elements in the galaxy reside within these dust grains, highlighting their significance in cosmic evolution.
Unexpectedly, Zhang and Green discovered a phenomenon contrary to previous assumptions. Scientists had anticipated that in denser regions, the extinction curve would flatten due to dust grain growth. However, they observed that in areas of intermediate dust density, the extinction curve steepened instead, with blue light being absorbed more than expected. This suggests a different mechanism at play, possibly linked to the proliferation of polycyclic aromatic hydrocarbons (PAHs), organic molecules abundant in interstellar space and potentially relevant to the origin of life.
Zhang and Green aim to investigate this further, using future observations to explore the role of PAHs in shaping extinction properties. Their groundbreaking map not only enhances our understanding of interstellar dust but also opens new avenues for research into the chemistry and physics of our galaxy.
Research Report:Three-dimensional maps of the interstellar dust extinction curve within the Milky Way galaxy
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