2018ApJ...854...68H

By using the Hectospec 6.5 m Multiple Mirror Telescope and the 2.16 m telescope of the National Astronomical Observatories, of the Chinese Academy of Sciences, we obtained 188 high signal-to-noise ratio spectra of HII regions in the nearby galaxy M101, which is 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 continua, including stellar population age, electron temperature, oxygen abundance, etc. The oxygen abundances are derived using two empirical methods based on O3N2 and R₂₃ indicators, as well as the direct T_e method when [OIII]λ4363 is available. By applying the harmonic decomposition analysis to the velocity field, we obtained a line-of-sight rotation velocity of 71 kms^–1 and a position angle of 36°. The stellar age profile shows an old stellar population in the galaxy center and a relatively young stellar population in outer regions, suggesting an old bulge and a young disk. The oxygen abundance profile exhibits a clear break at ∼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.