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[Chinese instrument network instrument research and development] The galaxy is a disk-shaped galaxy. There is a lot of gas and dust on the silver disk, thus forming a spectacular "black nebula" visible to the naked eye. In the direction along the platter (low silver latitude), due to the severe extinction of dust, galaxies beyond the Milky Way are barely visible at the optical wavelengths. Therefore, in general, research on extragalactic (out-of-galactic) celestial bodies is mainly concentrated in the sky area away from the silver plate (high silver latitude). In spite of this, due to the thickness of the Milky Way's plate and the sun's central position on the thickness of the silver plate, we still see a certain degree of galactic dust extinction from any direction.
The visible galactic dust (black nebula)
Therefore, for extragalactic objects, the elimination of dust in the Milky Way is an effect that must be corrected. In the past two decades, the most widely used extinction map of the Milky Way in the field of astronomical research was the result given by Schlegel et al. (1998) based on infrared satellite measurements of dust radiation in the Milky Way (hereinafter referred to as the SFD extinction map). Since the publication of the SFD extinction map, there has been a lot of work to independently measure the extinction of the Milky Way and to further test the SFD chart. Among these inspection methods, a clever method is to use cosmology principles to independently measure the extinction of the Milky Way.
The so-called cosmological principle means that there is no special position in the universe, that is, on a large scale, the universe is uniform and isotropic. Therefore, we look at celestial bodies outside the Milky Way in different directions, and their statistical properties should be exactly the same on a large scale. Specifically, at the same observation depth, we should see the same number of celestial bodies, and the statistical physical properties of these objects, such as the average color, should also be exactly the same. However, as mentioned earlier, the galactic extinction will observably break the isotropy predicted by this cosmological principle. In higher extinction areas, the same objects become darker in observation. In other words, when the same depth is observed, the number of celestial bodies that the higher extinction sky area can see will become less. Not only that, because the extinction of dust in the short waveband is stronger than in the long waveband, the color of the celestial body in the high extinction sky will therefore become redder.
SFD extinction map
Recently, Li Linlin, a doctoral student at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, under the guidance of the instructor Shen Shiyin, based on cosmological principles, based on the characteristics of dust extinction in the short wavelength band, used for the first time the massive observation of u-band data in the shortest wavelength in the optical range for the extinction of the Milky Way. Conducted systematic statistical studies. The data used in this work comes from the Southern Silver U-band survey project led by Chinese astronomers. The innovation of this work is to combine the extinction of galactic dust obtained through galaxy counting with the reddening effect obtained through galaxy colors for the first time, and obtain high-precision statistical results with the support of massive data.
The results of the study indicate that there is a significant overestimation of the general SFD galactic dust extinction map in high extinction areas. Not only that, through the combination of extinction and reddening of dust from the Milky Way, the work has further used statistical methods to place restrictions on the physical properties of the Galaxy dust: the extinction curve of the Galaxy dust can be modeled by certain models, such as O'Donnell in 1994. The classical extinction curves given are precisely described, and some models, such as the modified extinction curves given by Fitzpatrick in 1999, deviate significantly from the observed data.
(Original title: Shanghai Observatory accurately measures the dust extinction of the Milky Way)