Cloud structures in M17 SWex : Possible cloud-cloud collision
Abstract
Using wide-field 13CO (J=1-0) data taken with the Nobeyama 45-m telescope, we investigate cloud structures of the infrared dark cloud complex in M17 with SCIMES. In total, we identified 118 clouds that contain 11 large clouds with radii larger than 1 pc. The clouds are mainly distributed in the two representative velocity ranges of 10 - 20 km s-1 and 30 - 40 km s-1. By comparing with the ATLASGAL catalog, we found that the majority of the 13CO clouds with 10 - 20 km s-1 and 30 - 40 km s-1 are likely located at distances of 2 kpc (Sagittarius arm) and 3 kpc (Scutum arm), respectively. Analyzing the spatial configuration of the identified clouds and their velocity structures, we attempt to reveal the origin of the cloud structure in this region. Here we discuss three possibilities: (1) overlapping with different velocities, (2) cloud oscillation, and (3) cloud-cloud collision. From the position-velocity diagrams, we found spatially-extended faint emission between 20 km s-1 and 35 km s-1, which is mainly distributed in the spatially-overlapped areas of the clouds. We also found that in some areas where clouds with different velocities overlapped, the magnetic field orientation changes abruptly. The distribution of the diffuse emission in the position-position-velocity space and the bending magnetic fields appear to favor the cloud-cloud collision scenario compared to other scenarios. In the cloud-cloud collision scenario, we propose that two 35 km s-1 foreground clouds are colliding with clouds at 20 km s-1 with a relative velocity of 15 km s-1. These clouds may be substructures of two larger clouds having velocities of 35 km s-1 ( 103 M) and 20 km s-1 ( 104 M), respectively.