Recently, the Milky Way Imaging Scroll Painting (MWISP) survey group members of Purple Mountain Observatory of CAS, using the Delingha 13.7m millimeter wave telescope, first revealed the relatively complete molecular structure of the Outer Arm in the second galactic quadrant. The result is shown in Fig. 1. This work will soon be published in The Astrophysical Journal Supplement Series. 

  Outer arm is a major spiral arm of the Milky Way. At the second galactic quadrant, its distance to us is about 5-7 kpc, namely about 16,000-23,000 light year. Because of its further distance, the molecular emissions of the arm are so weak that the past survey could only detect a few bright ones. But today, using the Delingha 13.7m millimeter wave telescope, the MWISP group have detected 457 Outer arm molecular clouds in the region of galactic longitude from 100° to 150° and galactic latitude from -3° to 5°. Among them, 332 (about 69%) are newly detected. The total mass of the detected Outer arm clouds is more than 3,000,000 solar mass. 

  The on-going MWISP project is the world’s highest sensitivity large scale no-bias CO molecular spectral line survey. This project has helped us achieving many progress about the Milky Way structure, such as the discovery of the new spiral arm reported in 2015 (Sun et al. 2015). As the survey carrying on, more secrets about the Milky Way structure will be gradually revealed. 

By DU Xinyu

  Fig.1. The Milky Way plan view. The red circles are the Outer arm molecular clouds. The red dashed curve indicates the fitting result of the Outer arm. And the red solid curve indicates the translationally moved result by taking the fitting result from Reid et al.           (2014) into account.

  The article link：http://arxiv.org/abs/1602.08229