Researchers have made new progress in the direction of galaxy structure research

Recently, a research team led by the Shanghai Astronomical Observatory of the Chinese Academy of Sciences proposed a new quantitative model of galaxy structure, depicting the geometry of the general disk galaxy similar to the Milky Way as a “chocolate chips cookie” – the old stars in the galaxy and the diffuse galactic medium between them constitute the main component of the “cookie”, while the cluster-like distribution of star formation regions constitutes a darker “chocolate chip”. This study quantitatively analyzes the observation characteristics of discoid galaxies in different line of sight, provides a global geometric distribution model for the spatial distribution of stars and dust in disc-shaped galaxies, and lays a solid foundation for the complete description of the radiation transfer process of dust in disc-shaped galaxies. The findings were published Oct. 21 in the Astrophysical Journal.

Most galaxies in the universe have undergone star formation and evolution over tens of billions of years. For most galaxies, although they have passed the “prime” of their star formation, they continue to form new stars to this day. These new stars are born in low-temperature clouds of gas, and their blazing radiation ionizes some of the surrounding gas, allowing astronomers to track their birth regions through flashes of these ionized gases. During the evolution of galaxies, generations of stars have produced various heavy elements in nuclear fusion. These heavy elements gradually form small solid particles in cold gas clouds through complex physical and chemical reactions – what astronomers call “dust”. This dust not only constitutes the world in the eyes of humans, but also because of their absorption, scattering and re-radiation of starlight, galaxies appear completely different in different electromagnetic bands (Figure 1).

Figure 1. The morphology of the Milky Way in different electromagnetic bands (Image source: NASA)[1]

Today, astronomers have discovered through a large-scale “census” that there are a large number of disk galaxies similar to the Milky Way in the nearby universe. In these discoid galaxies, old stars and interstellar dust diffuse between stars show a continuous flat structure distribution, while new star-forming regions and their surrounding dust envelopes appear as clumps and are distributed in a flatter disk structure. It is also due to the absorption of starlight by dust and the complex geometric distribution between dust and stars that the disc-shaped galaxy looks like “peaks on the side of the ridge” from different angles, adding to the magnificent scenery in the universe (Figure 2).

Figure 2. Frontal view of the disk galaxy: Pinwheel galaxy M101 (left); Side view of the disk galaxy: Straw Hat Galaxy M104 (right). The faint areas in the galaxy images are caused by the light reduction process of dust (Image source: NASA)[2]。

French scholar Stéphane Charlot and American scholar Michael Fall proposed a geometric model of the distribution of local stars and dust in 2000 (Figure 3). In this model, the young star-forming regions are surrounded by successive old stars and diffuse dust between them, so it can qualitatively explain that the star formation regions in extragalactic galaxies are on average more dust extinction than those observed by older stars. But on a larger scale, what is the geometric distribution of stars of different ages and their corresponding dust components in disk galaxies as a whole?

Figure 3. Charlot & Fall proposed a geometric model of the regions of star-to-dust distribution in a galaxy in 2000: shadow regions represent dust, large snowflakes represent newly formed stars, and small snowflakes represent older stars[3]。

Recently, a joint research team from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and the Purple Mountain Observatory has built a new overall structural model of the disk galaxy based on Charlot & Fall’s local model, the “chocolate chips cookie model” (Figure 4). Through the model’s analysis, the researchers found that the aging star and the interstellar medium that pervades it make up the bulk of the cookie, while the chocolate fragments embedded in it visually depict the region where the new star was born. These new stars have not completely broken away from their parent nebula, and the dust in their peripheral nebulae still absorbs radiation from the nebula’s interior significantly, making these star-forming regions look like a darker “chocolate chip.” Since observers can only see images of galaxies at a certain angle, the difficulty for a whole “cookie” like a typical disk galaxy is to accurately give the content and distribution characteristics of the “chocolate chips”, that is, the region of new stars. In order to break through this difficulty, the researchers carefully selected a group of disk galaxies of comparable mass to the Milky Way in a large sample of galaxies in the adjacent universe as the study object. The disks of these galaxies point randomly to the observer — which is somewhat equivalent to an all-round view of a typical galaxy like the Milky Way from all angles. Based on such an idea characterized by “cosmological principles”, the researchers successfully built a quantitative “chocolate chips” model that can perfectly reproduce the different observational effects of dust in disc-shaped galaxies on old and young star regions at different observation angles. In this model, for a typical disk galaxy like the Milky Way, the disk thickness of the star-forming region (“chocolate chips”) is only half the thickness of the distribution of older stars (the “biscuit body”), but its radial ductility is 60% higher.

“The model presented in this study can not only statistically explain the observed features of disk galaxies caused by dust extinction, but also predict their typical physical characteristics, so that they can be further compared with the detailed observations of typical disk galaxies such as the Milky Way.” Lu Jiafeng, the first author of the study and a doctoral student at the Shanghai Astronomical Observatory, said, “For example, through careful analysis of the model, we found that there are about 10,000 visible ‘chocolate chips’ in the ‘cookies’ of disk galaxies like the Milky Way.” Interestingly, this model-based theoretical analysis and observations of star-forming regions and corresponding young clusters in the Milky Way are in good agreement. This also partly explains the general nature of our galaxy in the universe. ”

Shen Shiyin, the corresponding author of the study and a researcher at the Shanghai Astronomical Observatory, said: “The model proposed in this study will be further expanded in the future, and it is expected to be applied to the radiation process of dust in disk galaxies. In this way, we can further reproduce and interpret the observed characteristics of typical disk galaxies like the Milky Way in the infrared band. ”

Figure 4. The “chocolate chips” model of discoid galaxies: old stars are continuously distributed and constitute the main body of cookies; The star-forming regions are distributed in clumps and look like chocolate chips because there is more dust on the periphery.

This project is mainly supported by the Natural Science Foundation of China.

Related Paper Information:


[1] Multiwavelength Milky Way Images:

[2] Hubbles Messier Images:

[3] Charlot, Stéphane ; Fall, S. Michael, 2000,ApJ, 539,718

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