The MopraCO Galactic Survey

Abstract

The MopraCO project (see also Burton et al., 2013, Braiding et al., 2015, Rebolledo et al., 2016) is a CO J=1-0 line survey across the 4th Quadrant of the Milky Way, performed using the 22 m diameter single dish Mopra telescope in Australia. It includes the three main CO molecule isotopologues (12CO , 13CO and C18O) and more than 100 square degrees have been surveyed, spanning from l=265deg to +10deg and b= ± 0.5deg, covering also the Carina complex and the Central Molecular Zone, and extending beyond b= ± 0.5o in several regions. The final plan is to cover from l=265deg to +10deg and b= ± 1deg. The use of the 8 GHz-wide UNSW-MOPS spectrometer and the fast mode of on-the-fly mapping, developed for the Mopra telescope, permits us to reduce the cycle time to 1/4 of a second. <P />Compared with the previous Dame et al. 2001 CO survey of the Southern Galactic plane the MopraCO spectral and angular resolution of 0.1 km/s and 0.6' are an order of magnitude better. This permits us to improve the model of our Galaxy, showing the spirals structures with unprecedented details, in particular in the inner regions where the tracers of spiral arm tangents show an offset from the mid-arms that is matched by maser data, while the real structure of the innermost region remains difficult to define. Furthermore, the 12CO/13CO line ratios are found to be higher in the diffuse gas outside the GMC complexes than inside them, opening new hypothesis on the Xco factor used to estimate the masses of the molecular clouds. <P />Providing a detailed large scale third dimension, across the Southern Galaxy in the wide range of velocity (-500 to 450 km/s), the combination of this survey with the other ones will help to investigate the dark gas fraction not traced by the CO, better characterise the evolution of the molecular clouds and of the dense core inside them, giving more insights on the early stages of massive star formation. <P />In particular, in the direction toward the galactic longitude 332 at a distance of ∼ 3.5 kpc, we discovered a new giant molecular ring, around 40 pc in diameter, which presents strong emissions in the 12O and in 13CO lines as well as in atomic carbon and other tracers.