M101: Spectral Observations of HII Regions and Their Physical Properties

Abstract: By using the Hectospec 6.5 m Multiple Mirror Telescope (MMT) and the 2.16 m telescope of National Astronomical Observatories, Chinese Academy of Sciences (NAOC), we obtained 188 high signal-to-noise ratio (S/N) spectra of HII regions in the nearby galaxy M101, which are the largest spectroscopic sample of HII regions for this galaxy so far. These spectra cover a wide range of regions on M101, which enables us to analyze two dimensional distributions of its physical properties. The physical parameters are derived from emission lines or stellar continuum, including stellar population age, electron temperature, oxygen abundance and etc. The oxygen abundances are derived using two empirical methods based on O3N2 and R$_{23}$ indicators, as well as the direct Te method when OIII$\lambda4363$ is available. By applying the harmonic decomposition analysis to the velocity field, we obtained line-of-sight rotation velocity of 71 km s$^{-1}$ and a position angle of 36 degree. The stellar age profile shows an old stellar population in galaxy center and a relative young stellar population in outer regions, suggesting an old bulge and a young disk. Oxygen abundance profile exhibits a clear break at $\sim$18 kpc, with a gradient of $-$0.0364 dex kpc$^{-1}$ in the inner region and $-$0.00686 dex kpc$^{-1}$ in the outer region. Our results agree with the "inside-out" disk growth scenario of M101.