Scanning the Milky Way for 10 years: Discoveries from the Sino-German 6 cm polarization survey of the Galactic plane

Abstract

<p indent=0mm>Radio continuum polarization observation is a powerful tool to study the properties of the Galactic interstellar medium. By using the 25-m radio telescope of Xinjiang Astronomical Observatories of Chinese Academy of Sciences the Sino-German 6 cm polarization survey of the Galactic plane was carried out by the National Astronomical Observatories of Chinese Academy of Sciences and the Max-Planck-Institute for Radio Astronomy. The observed sky coverage is the Galactic plane in the region from 10° to 230° in the Galactic longitude and within ±5° in the Galactic latitude. The survey is by far at the highest frequency for a Galactic plane polarization survey conducted by a ground-based radio telescope. Through the ten-year efforts by the Chinese and German scientists, the survey revealed the distribution of the total intensity emission and the orientation in the sky plane of the Galactic magnetic fields, which allows us to paint a scroll of the Galaxy with the data obtained with a telescope in China. At such a higher frequency than that of other surveys over the world, the <sc>6 cm</sc> observations experience less Faraday rotations, and are thus able to detect the polarized emission from a further distance. Based on the survey data, we have obtained a series of results on Galactic supernova remnants (SNRs), HII regions and Faraday screens with high rotation measures. Four new Galactic SNRs have been discovered, among which G178.2–4.2 and G25.1–2.3 published in 2011 are the first discovery of Galactic SNRs by a Chinese radio telescope using the first-hand data. Measurements of flux densities, polarization and spectra were performed for more than 70 known SNRs. The polarized emission is observed, for the first time, for the SNRs G16.2–2.7, G69.7+1.0, G84.2–0.8, G85.9–0.6, G205.5+0.5 and G206.9+2.3. The accurate measurements of the <sc>6 cm</sc> flux densities are crucial to determine the spectra of 20 SNRs. Measurements and statistic studies were made for 401 HII regions toward which the flux densities can be precisely estimated. The result complemented the WISE HII region catalog with regards on the extended sources and fixed the mistakes in the commonly used radio HII region catalog. For the diffuse polarized emission, we have detected many new intriguing structures with high Faraday rotation measures which cannot be observed at low radio frequencies because of severe depolarization. These structures, referred to as Faraday screens, do not have total intensity correspondents, indicating the necessity of polarization observations in understanding the Galactic interstellar medium. The <sc>6 cm</sc> survey data have been made public and the results have already generated high impacts on relevant research fields. The project has demonstrated that a medium size telescope such as the <sc>25 m</sc> dish used for the survey is also capable of achieving great science, and the key is to find the niche in this case which is the polarization survey of the Galactic plane. The survey not only produced exciting results on the polarized emission from the Galaxy but also trained next generation radio astronomers who are now active for large facilities such as the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Square Kilometre Array (SKA).