2020-10-14

Cloud-cloud collisions and triggered star formation
in the Central Molecular Zone

Speaker: Rei Enokiya (Keio University)

Abstract:

We will discuss the massive cluster formation in the Central Molecular Zone by applying a new method to identify a cloud-cloud collision which has been developed in the Galactic HII regions and the young massive clusters in the Local Group and the Antennae galaxies.
The Central Molecular Zone harbors outstanding clusters including the Arches and Quintuplet, and it is an issue of keen interest to understand the mechanism of the cluster formation (e.g., Longmore+ 2013, MNRAS, 433, 15). Recently, some authors reported that cloud-cloud collisions (CCCs) trigger the young massive cluster formation in the LMC (R136; Fukui+ 2017, PASJ, 69, 5) and in the Antennae Galaxies (Tsuge+ 2019, ApJ, 871, 44). The CMZ, which holds the highest volume density of molecular gas in the Local Group, is expected to have very frequent CCCs and it is important to test if  CCC provides a viable mechanism of cluster formation in the CMZ at the highest resolution thanks to its small distance. There are already several studies which reported CCCs in localized areas in the CMZ (Sgr B2, Hasegawa+ 1994, ApJ, 429, 77; the 50 km/s cloud, Uehara+ 2019, ApJ, 872, 121). In order to obtain a unified picture on the role of CCC over the whole CMZ, we undertook a new analysis of collision signatures based on the archival CO datasets by applying the identification method of CCC, which is developed in the Galactic disk clouds including the outstanding HII regions M42, M17, and NGC6334 (Fukui+ 2018, ApJ, 859, 166; Nishimura+ 2018, PASJ, 70, 42; Fukui+ 2018, PASJ, 70, 41). We have found evidence for CCCs in the three major, active star-forming regions in the CMZ, namely Sgr A, Sgr B2, and Sgr C, at different evolutionary stages, where Sgr B2 is the youngest and most active site of high-mass star formation. The mass, relative velocity, and collision time scale in the CCC are 105-6 Msun, several ten km/s, and 105-6 yrs, respectively. We discuss that these GMC collisions may play a significant role in the context of the star forming history in the CMZ and moreover in the evolution of the Galaxy. The CCCs in the present analysis can be driven by the bar potential model (e.g., Binney et al. 1991, MNRAS, 252, 210), whereas an alternative mechanism is the magnetic instability which can drive gas motion with the Alfven velocity of ~30-50 km/s; the field strength of the inner several 100 pc is estimated to be at least 50 micro Gauss (Crocker+ 2010, Nature, 463, 65), and the huge molecular loops 1, 2, and 3 discovered within several 100 pc of the GC provide observational evidence for the Parker instability driven by the field with ~100 mG (Fukui+ 2006, Science, 314, 106). Suzuki+ (2015), MNRAS, 454, 3049 and Kakiuchi+ (2018), MNRAS, 476, 5629 demonstrated that the Parker instability generates large velocity dispersions at the foot points of magnetically floated loops, leading to a collision between the magnetic driven gas with the disk.