  An updated catalog consisting of 1092 6.7-GHz methanol maser sources was
reported in this work. Additionally, the NH3 (1, 1), NH3 (2, 2), and NH3 (3, 3)
transitions were observed towards 214 star forming regions using the Shanghai
Tianma radio telescope (TMRT) in order to examine the differences in physical
environments, such as excitation temperature and column density of molecular
clouds associated with methanol masers on the Galactic scale. Statistical
results reveal that the number of 6.7 GHz methanol masers in the Perseus arm is
significantly lower than that in the other three main spiral arms. In addition,
the Perseus arm also has the lowest gas column density among the main spiral
arms traced by the NH3 observations. Both of these findings suggest that the
Perseus arm has the lowest rate of high-mass star formation compared to the
other three main spiral arms. We also observed a trend in which both the
luminosity of 6.7 GHz methanol masers and the ammonia gas column density
decreased as the galactocentric distances. This finding indicates that the
density of material in the inner Milky Way is generally higher than that in the
outer Milky Way. It further suggests that high-mass stars are more easily
formed at the head of spiral arms. Furthermore, we found that the column
density of ammonia gas is higher in the regions on the arms than that in the
inter-arm regions, supporting that the former is more likely to be the
birthplace of high-mass stars.
