M101: Spectral Observations of HII Regions and Their Physical Properties
  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.
