Gas and stellar spiral arms and their offsets in the grand-design spiral
  galaxy M51
  Theoretical studies on the response of interstellar gas to a gravitational
potential disc with a quasi-stationary spiral arm pattern suggest that the gas
experiences a sudden compression due to standing shock waves at spiral arms.
This mechanism, called a galactic shock wave, predicts that gas spiral arms
move from downstream to upstream of stellar arms with increasing radius inside
a corotation radius. In order to investigate if this mechanism is at work in
the grand-design spiral galaxy M51, we have measured azimuthal offsets between
the peaks of stellar mass and gas mass distributions in its two spiral arms.
The stellar mass distribution is created by the spatially resolved spectral
energy distribution fitting to optical and near infrared images, while the gas
mass distribution is obtained by high-resolution CO and HI data. For the inner
region (r < 150"), we find that one arm is consistent with the galactic shock
while the other is not. For the outer region, results are less certain due to
the narrower range of offset values, the weakness of stellar arms, and the
smaller number of successful offset measurements. The results suggest that the
nature of two inner spiral arms are different, which is likely due to an
interaction with the companion galaxy.
