Title:Spiral pattern speed of M51 with stellar catalog from HST/ACS archived 
images

Abstract:
We have derived the spiral pattern speed of the galactic disk of M51 on the 
basis of stellar catalog extracted from the archived images with HST/ACS 
(Mutchler et al., 2005), to be 45 +- 5 km/s/kpc. We followed the offset method 
devised by Egusa et al. (2004, 2009) who measured the pattern speed at the 
corotation radius where the spiral pattern catches up with the galactic flat 
rotation. To search for such a radius, they measured the azimuthal offset 
between CO emission from molecular gas and H-alpha emission from HII regions, 
from the center to the outer disk. Alternatively, taking advantage of high-
spatial resolution of HST/ACS images, instead of HII regions, we took 
distribution of OB stars, which serve as a more definite proxy of time after 
star formation. The resultant pattern speed is consistent with Egusa et al. 
(2009), however, our result is based more densely sampled along the spiral arm. 
The stellar catalog is available for other applications.