CO observation of the Galactic bubble N4
  We presented a study on the Galactic bubble N4 using the 13.7 m millimeter
telescope of Purple Mountain Observatory at the Qinghai Station. N4 is one of
the science demonstration regions for the Milky Way Imaging Scroll Painting
(WMISP). Simultaneous observations of $^{12}$CO (J = 1$-$0), $^{13}$CO (J =
1$-$0) and C$^{18}$O (J = 1$-$0) line emission towards N4 were carried out. We
analyzed the spectral profile and the distribution of the molecular gas.
Morphologically, the CO emissions correlate well with Spitzer IRAC 8.0 $\mu$m
emission. The channel map and velocity-position diagram shows that N4 is more
likely an inclined expanding ring than a spherical bubble. We calculated the
physical parameters of N4 including the mass, size, column density and optical
depth. Some massive star candidates were discovered in the region of N4 using
(J, J$-$H) color-magnitude diagram. We found an energy source candidate for the
expansion of N4, a massive star with a mass of ${\sim} 15\,M_{\odot}$ and an
age of $\sim$ 1 Myr. There exists infall motion signature in N4, which can be a
good candidate of infall area. Combined mm and infrared data, we think there
may exists triggered star formation in N4.
