Formation of the young compact cluster GM 24 triggered by a cloud–cloud collision

Abstract

High-mass star formation is an important step which controls galactic evolution. GM 24 is a heavily obscured star cluster including a single O9 star with more than ∼100 lower mass stars within a 0.3 pc radius toward (l, b) ∼ (350.• 5, 0.• 96), close to the Galactic min-starburst NGC 6334. We found two velocity components associated with the cluster by new observations of 12 COJ= 2-1 emission, whereas the cloud was previously considered to be single. We found the distribution of the two components of 5 km s −1 separation shows complementary distribution; the two fit well with each other if a relative displacement of 3 pc is applied along the Galactic plane. A position-velocity diagram of the GM 24 cloud is explained by a model based on the numerical simulations of two colliding clouds, where an intermediate velocity component created by collision is taken into account. We estimate the collision time scale to be ∼Myr in projection of a relative motion titled to the line of sight by 45 degrees. The results lend further support for cloud-cloud collision as an important mechanism of high-mass star formation in the Carina-Sagittarius Arm.

OB stars and YSO populations in the region of NGC 6334–NGC 6357 as seen withGaiaDR2

“…Fukui et al (2018a,b) suggest that the formation of massives stars in NGC 6357, NGC 6334, and GM1-24 have been triggered by a cloud-cloud collision. However, while NGC 6357 and GM1-24 show the typical depression feature (cavity) and velocity bridge (between the molecular emission at the two velocity components assigned to the two colliding clouds), signature expected from cloud-cloud collision models (e.g., Fukui et al 2018c), this is not so clear for NGC 6334 because the molecular emission at the velocity components assigned to the two colliding clouds show similar spatial distributions. Fukui et al (2018a) estimated that in NGC 6334 and NGC 6357 a cloud of V LSR ∼ −16 km s −1 collided with a main cloud at V LSR ∼ −4 km s −1 (relative collision radial veloc-ity of 12 km s −1 ) while for GM1-24, Fukui et al (2018b) estimate that the colliding and the main clouds have V LSR ∼ −10 km s −1 and −6 km s −1 respectively (relative collision radial velocity of 4 km s −1 ).…”

“…However, while NGC 6357 and GM1-24 show the typical depression feature (cavity) and velocity bridge (between the molecular emission at the two velocity components assigned to the two colliding clouds), signature expected from cloud-cloud collision models (e.g., Fukui et al 2018c), this is not so clear for NGC 6334 because the molecular emission at the velocity components assigned to the two colliding clouds show similar spatial distributions. Fukui et al (2018a) estimated that in NGC 6334 and NGC 6357 a cloud of V LSR ∼ −16 km s −1 collided with a main cloud at V LSR ∼ −4 km s −1 (relative collision radial veloc-ity of 12 km s −1 ) while for GM1-24, Fukui et al (2018b) estimate that the colliding and the main clouds have V LSR ∼ −10 km s −1 and −6 km s −1 respectively (relative collision radial velocity of 4 km s −1 ). Considering the age difference between NGC 6334 (between 0.7 and 2.3 Myr, Getman et al 2014) and NGC 6357 (1-1.3 Myr, Fang et al 2012;Getman et al 2014), Fukui et al (2018b) propose that the main part of NGC 6357 collided first and only then did the collision toward NGC 6334 occur and it is still developing.…”

“…Fukui et al (2018a) estimated that in NGC 6334 and NGC 6357 a cloud of V LSR ∼ −16 km s −1 collided with a main cloud at V LSR ∼ −4 km s −1 (relative collision radial veloc-ity of 12 km s −1 ) while for GM1-24, Fukui et al (2018b) estimate that the colliding and the main clouds have V LSR ∼ −10 km s −1 and −6 km s −1 respectively (relative collision radial velocity of 4 km s −1 ). Considering the age difference between NGC 6334 (between 0.7 and 2.3 Myr, Getman et al 2014) and NGC 6357 (1-1.3 Myr, Fang et al 2012;Getman et al 2014), Fukui et al (2018b) propose that the main part of NGC 6357 collided first and only then did the collision toward NGC 6334 occur and it is still developing.…”

“…Based on cold-dust 1.2 mm continuum emission (Russeil et al 2010) and 13 CO (J = 2−1) line emission (Zernickel 2015), the two regions seem to be connected by a ∼50 pc long filament. For this reason, it has been proposed that the massive star-formation that is observed in these two regions could have been triggered by a cloud-cloud collision process (Fukui et al 2018a).…”

Herschel-HOBYS study of the earliest phases of high-mass star formation in NGC 6357

“…They trace a 100-pc long feature aligned with the Galactic plane at b ∼ 0.67 • (20 pc above the Galactic plane at a distance of 1.75 kpc) which could trace the parental filament of both A134, page 14 of 42 star-forming regions. In this scheme, following Fukui et al (2018) the formation of NGC 6357 and NGC 6334 could have been triggered by a pc-scale cloud-cloud collision.…”