Physical Properties of H II Regions in M51 from Spectroscopic
  Observations
  M51 and NGC 5195 is an interacting system that can be explored in great
details with ground-based telescopes. The H II regions in M51 were observed
using the 2.16 m telescope of the National Astronomical Observatories of the
Chinese Academy of Sciences and the 6.5 m Multiple Mirror Telescope with
spatial resolution of less than $\sim100$ pc. We obtain a total of 113 spectra
across the galaxy and combine the literature data of Croxall et al. to derive a
series of physical properties, including the gas-phase extinction, stellar
population age, star formation rate (SFR) surface density, and oxygen
abundance. The spatial distributions and radial profiles of these properties
are investigated in order to study the characteristics of M51 and the clues to
the formation and evolution of this galaxy. M51 presents a mild radial
extinction gradient. The lower gas-phase extinction in the north spiral arms
compared to the south arms are possibly caused by the past encounters with the
companion galaxy of NGC 5195. A number of H II regions have the stellar age
between 50 and 500 Myr, consistent with the recent interaction history by
simulations in the literatures. The SFR surface density presents a mild radial
gradient, which is ubiquitous in spiral galaxies. There is a negative
metallicity gradient of $-0.08$ dex $R_{e}^{-1}$ in the disk region, which is
also commonly found in many spiral galaxies. It is supported by the
"inside-out" scenario of galaxy formation. We find a positive abundance
gradient of 0.26 dex $R_{e}^{-1}$ in the inner region. There are possible
reasons causing the positive gradient, including the freezing of the chemical
enrichment due to the star-forming quenching in the bulge and the gas infall
and dilution due to the pseudobulge growth and/or galactic interaction.
