Searching for converging flows of atomic gas onto a molecular cloud

TL;DR

This study uses [CII], CO, and HI observations to investigate atomic gas flows converging onto a molecular cloud, suggesting accretion of cold atomic gas with a specific infall rate, but no evidence of symmetric inflow.

Contribution

It provides new observational evidence linking [CII] emission to atomic gas accretion onto molecular clouds, highlighting the role of cold neutral medium in cloud formation.

Findings

Detection of [CII] emission associated with photodissociation regions.

Identification of a blue-shifted [CII] component linked to atomic gas inflow.

Estimated mass infall rate of 3.2x10^{-4} Msun/yr onto the cloud.

Abstract

We present new observations of [CII] fine structure line emission from an isolated molecular cloud using the upGREAT instrument onboard SOFIA. These data are analyzed together with archival CO=1-0 and HI 21 cm emission spectra to investigate the role of converging atomic gas flows in the formation of molecular clouds. Bright [CII] emission is detected throughout the mapped area that likely originates from photodissociation regions excited by UV radiation fields produced by newborn stars within the cloud. Upon spatial averaging of the [CII] spectra, we identify weak [CII] emission within velocity intervals where the HI 21 cm line is brightest; these are blue-shifted relative to velocities of the CO and bright [CII] emission by 4 km/s. The brightness temperatures, velocity dispersions, and alignment with HI 21 cm velocities connect this [CII] emission component to the cold, neutral atomic gas of the interstellar medium (CNM). We propose that this CNM feature is an accretion flow onto the far--side of the existing molecular cloud. The mass infall rate is 3.2x10**{-4} Msun/yr. There is no direct evidence of a comparable red--shifted component in the [CII] or HI 21 cm spectral lines that would indicate the presence of a converging flow.