The Milky Way is a spiral galaxy with the Schechter characteristic luminosity L*,thus an important anchor point of the Hubble sequence of all spiral galaxies.Yet the true appearance of the Milky Way has remained elusive for centuries.We review the current best understanding of the structure and kinematics of our home galaxy,and present an updated scientifically accurate visualization of the Milky Way structure with almost all components of the spiral arms,along with the COBE image in the solar perspective.The Milky Way contains a strong bar,four major spiral arms,and an additional arm segment(the Local arm)that may be longer than previously thought.The Galactic boxy bulge that we observe is mostly the peanut-shaped central bar viewed nearly end-on with a bar angle of^25°-30°from the SunGalactic center line.The bar transitions smoothly from a central peanut-shaped structure to an extended thin part that ends around R^5 kpc.The Galactic bulge/bar contains^30%-40%of the total stellar mass in the Galaxy.Dynamical modelling of both the stellar and gas kinematics yields a bar pattern rotation speed of^35-40 km s-1 kpc-1,corresponding to a bar rotation period of^160-180 Myr.From a galaxy formation point of view,our Milky Way is probably a pure-disk galaxy with little room for a significant merger-made,"classical"spheroidal bulge,and we give a number of reasons why this is the case.
We present results of 13CO（1-0）,C18O（1-0）,and HCO＋（1-0） map observations and N2H＋（1-0） single point observations directed towards a sample of nine low-luminosity 6.7-GHz masers.N2H ＋ line emission has been detected from six out of nine sources,C18O line emission has been detected from eight out of nine sources,and HCO ＋ and 13CO emission has been detected in all sources.In particular,a ＂blue profile＂ of the HCO ＋ spectrum,a signature of inflow,is found towards one source.From integrated intensity emission maps,we identified 17 cores in the sample.Among them,nine cores are closely associated with low-luminosity methanol masers.For these cores,we derive the column densities,core sizes,masses and molecular abundances.Comparison of our results with similar molecular line surveys towards the southern sky methanol masers indicates that linewidths of our sample,including only the low-luminosity masers,are smaller than the sample that includes both lowand high-luminosity masers.For the maser associated cores,their gas masses have the same order of magnitude as their virial masses,indicating that these cores are gravitationally bound systems.In addition,we have found from our observations that the low-luminosity methanol masers tend to coexist with H2O masers and outflows rather than with OH masers.
Yuan-Wei Wu,Ye Xu and Ji Yang Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210008,China
For the first time, the OMC-2/3 region was mapped in C2H （1–0）, HC3N （10–9） and HNC （1–0） lines. In general, the emissions from all the three molecular species reveal an extended filamentary structure. The distribution of C2H cores almost follows that of the 1300μm condensations, which might suggest that C2H is a good tracer to study the core structure of molecular clouds. The core masses traced by HNC are rather ？at, ranging from 18.8 to 49.5 M , while also presenting a large span for those from C2H, ranging from 6.4 to 36.0 M . The line widths of both HNC and C2H look very similar, and both are wider than that of HC3N. The line widths of the three lines are all wider than those from dark clouds, implying that the former is more active than the latter, and has larger turbulence caused by winds and UV radiation from the surrounding massive stars.
