The inception of star cluster formation revealed by [C ii] emission around an Infrared Dark Cloud

Abstract

We present SOFIA-upGREAT observations of [CII] emission of Infrared Dark Cloud (IRDC) G035.39-00.33, designed to trace its atomic gas envelope and thus test models of the origins of such clouds. Several velocity components of [CII] emission are detected, tracing structures that are at a wide range of distances in the Galactic plane. We find a main component that is likely associated with the IRDC and its immediate surroundings. This strongest emission component has a velocity similar to that of the 13 CO(2-1) emission of the IRDC, but offset by ∼ 3 km s −1 and with a larger velocity width of ∼ 9 km s −1 . The spatial distribution of the [CII] emission of this component is also offset predominantly to one side of the dense filamentary structure of the IRDC. The CII column density is estimated to be of the order of ∼ 10 17 − 10 18 cm −2 . We compare these results to the [CII] emission from numerical simulations of magnetized, dense gas filaments formed from giant molecular cloud (GMC) collisions, finding similar spatial and kinematic offsets. These observations and modeling of [CII] add further to the evidence that IRDC G035.39-00.33 has been formed by a process of GMC-GMC collision, which may thus be an important mechanism for initiating star cluster formation.

The Molecular Cloud Lifecycle

“…Bisbas et al (2017) used radiative transfer calculations to show that the [CII] and CO lines show a significant offset in the process of the cloud collision. Such an offset is reported toward a few possible cloud-cloud collision sites (Bisbas et al 2018;Lim and De Buizer 2019). Nonetheless, it needs to be quantified how this signature differs from hierarchical gravitational collapse or feedback-driven outflows.…”

Surveying the Giant HII Regions of the Milky Way with SOFIA: III. W49A

“…This scenario was originally suggested by Mufson & Liszt (1977) based on molecular and recombination line observations, with follow-up studies supporting the idea (e.g., Buckley & Ward-Thompson 1996;Serabyn et al 1993). Unfortunately, our data cannot prove such a scenario, and more direct evidence of the cloud-cloud collision scenario will need targeted observations, such as comparing velocity profiles of the 158 µm [C II] line emission and CO isotopologue bands (Bisbas et al 2018;Lim et al 2021). However, such a global-triggering scenario could explain why so many sub-regions spread throughout such a large volume could have begun star-formation activities at the same time.…”

Star Cluster Formation in Orion A

“…3.2 Comparing 13 CO(1-0) to [Cii] 158 µm Bisbas et al (2018) detected velocity and position offsets between [Cii] 158 µm and 13 CO (1-0) line emission maps in IRDC G035.39-00.33. From a comparison with results of numerical models, they claimed that a cloudcloud collision could explain these offsets, and thus may be a likely formation mechanism for the IRDC.…”

“…One needs to note that Bisbas et al (2018) predicted position offsets between 13 CO and [Cii] emission which are not distinguishable in the integrated line images. The detailed 13 CO and [Cii] structures enable us to investigate through a pixel by pixel comparison of both line emissions.…”

“…However, these studies have typically been limited to searching for the "bridge effect" (Haworth et al 2015;Bisbas et al 2017) in positionvelocity diagrams from molecular gas tracers, such as 12 CO(1-0). Recently, Bisbas et al (2018) introduced a new method to test the observational evidence of GMC collisions by examining both 13 CO(1-0) emission (from the Galactic Ring Survey [GRS]; Jackson et al 2006) and 158µm [Cii] emission (from SOFIA-upGREAT) toward an IRDC. They found position and velocity offsets between 13 CO(1-0) and [Cii] intensity peaks of IRDC G035.39-00.33, to be consistent with those derived in simulations and synthetic observations of cloud-cloud collisions.…”