分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: HII regions made of gas ionized by radiations from young massive stars, are widely distributed in the Milky Way. They are tracers for star formation, and their distributions are correlated with the Galactic spiral structure. Radio recombination lines (RRLs) of hydrogen and other atoms allows for the precisest determination of physical parameters such as temperature and density. However, RRLs at around 1.4 GHz from HII regions are weak and their detections are difficult. As a result, only a limited number of detections have been obtained yet. The 19-beam receiver on board of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) can simultaneously cover 23 RRLs for H$n{\alpha}$, He$n{\alpha}$, and C$n{\alpha}$ ($n = 164 - 186$), respectively. This, combined with its unparalleled collecting area, makes FAST the most powerful telescope to detect weak RRLs. In this pilot survey, we use FAST to observe nine HII regions at L band. We allocate 20 minutes pointing time for each source to achieve a sensitivity of around 9 mK in a velocity resolution of 2.0 km/s. In total, 21 RRLs for H$n{\alpha}$ and C$n{\alpha}$ at $1.0 - 1.5$ GHz have been simultaneously detected with strong emission signals. Overall, the detection rates for the H167${\alpha}$ and C167${\alpha}$ RRLs are 100%, while that for the He167${\alpha}$ RRL is 33.3%. Using hydrogen and helium RRLs, we measure the electron density, electron temperature, and pressure for three HII regions. This pilot survey demonstrates the capability of FAST in RRL measurements, and a statistically meaningful sample with RRL detection, through which knowledges about Galactic spiral structure and evolution can be obtained, is expected in the future.
分类: 天文学 >> 天文学 提交时间: 2024-08-14 合作期刊: 《Research in Astronomy and Astrophysics》
摘要: We report a new high-sensitivity H i mapping observation of the NGC 5055 galaxy group over an area of with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our observation reveals that the warped H i disk of NGC 5055 is more extended than what was previously observed by WSRT, out to (61.7 kpc). The total H i mass of NGC 5055 is determined to be ∼1.1 × 1010M⊙. We identified three H i clouds with H i masses of the order of ∼107M⊙ at the southeastern edge of the H i disk, as well as a candidate high-velocity cloud with an H i mass of (1.2 ± 0.5) × 106M⊙ to the north of NGC 5055. The H i content of UGCA 337 is robustly detected for the first time by the FAST observations. It has a narrow H i linewidth of W50 = 17.4 ± 3.8 km s−1 with a total H i mass of (3.5 ± 0.3) × 106M⊙. Comparing the gas content and g−r color of UGCA 337 with typical low-mass dwarf galaxies, UGCA 337 appears relatively gas-poor despite its blue color. This suggests that UGCA 337 may have undergone gas stripping in the past. We also analyzed the possible origin of the diffuse H i clouds located at the outskirts of NGC 5055, and speculate that they might be the remnant features of a merger event in the past.