Spectral Study Of The HII Region In The Nearby Spiral Galaxy NGC 240

Nearby spiral galaxies are ideal targets for studying physical processes such as star formation,element enrichment,and photoionization in galaxies,due to their large apparent sizes and richness in HII regions.Chemical evolution of galaxies can be inferred from observational properties exhibited by these physical processes.As a type of interstellar medium,HII regions are ionized nebulae in galaxies,containing a large amount of gas and dust,where star formation actively occurs.The study presented in this thesis focuses on spectral properties of HII regions in the nearby spiral galaxy NGC 2403.By means of observations with the 2.16 m telescope mounted at the Xing Long station of the National Astronomical Observatories of China,in combination with published data in literature,we compile a sample of 159 HII regions.Through an inspection with photoionization model,the spectral index N2O2 is found to be the best indicator of the oxygen abundance and employed to estimate the oxygen abundance in this study.Spatial distributions in three aspects including one-dimensional(radial),two-dimensional,and azimuth distributions are drawn for the oxygen abundance in NGC 2403.In the results,the oxygen abundance exhibits an obvious radial gradient decreasing from the center to the outer part,but the slope of the variation appears variable;there is no trend of the oxygen abundance with the position angle of the galaxy.The bending distribution of the oxygen abundance in NGC 2403 is found not caused by systematic errors in the derivation of the oxygen abundance but possibly related with the discontinuity in kinematic properties of the galaxy;the traditional definition of the galactic center is also likely to induce an uncertainty in the radial distribution of the oxygen abundance;in addition,gas inflow has impact on the radial distribution of the oxygen abundance,which should not be ignored as well.Future work is planned to deeply investigate potential reasons for the bend in the radial gradient of the oxygen abundance,so as to reveal specific physical processes affecting the oxygen-abundance gradient;at the same time,different spectral indices and methods for estimating the oxygen abundance will be compared for examining the applicability and reliability of these diagnostics.All the studies will lay a foundation for an accurate and comprehensive understanding of chemical properties and evolution of galaxies.