[12CII] and [13CII] 158 mum emission from NGC 2024: Large column densities of ionized carbon

TL;DR

This study investigates the ionized carbon emission in NGC 2024, revealing large column densities and complex velocity structures, and providing insights into the physical conditions of the ionization front and surrounding gas.

Contribution

The paper presents high-resolution [12CII] and [13CII] observations of NGC 2024, revealing detailed structure and large column densities of ionized carbon, with implications for understanding PDRs.

Findings

Detected [13CII] near the ionization front with a single velocity component.

Estimated a total column density of N(H)~1.6×10^23 cm^-2 in the emitting gas.

Identified self-absorption caused by cooler foreground gas.

Abstract

Context: We analyze the NGC 2024 HII region and molecular cloud interface using [12CII] and [13CII] observations. Aims: We attempt to gain insight into the physical structure of the interface layer between the molecular cloud and the HII region. Methods. Observations of [12CII] and [13CII] emission at 158 {\mu}m with high spatial and spectral resolution allow us to study the detailed structure of the ionization front and estimate the column densities and temperatures of the ionized carbon layer in the PDR. Results: The [12CII] emission closely follows the distribution of the 8 mum continuum. Across most of the source, the spectral lines have two velocity peaks similar to lines of rare CO isotopes. The [13CII] emission is detected near the edge-on ionization front. It has only a single velocity component, which implies that the [12CII] line shape is caused by self-absorption. An anomalous hyperfine line-intensity ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2 in the gas emitting the [CII] line. A large fraction of this gas has to be at a temperature of several hundred K. The self-absorption is caused by a cooler (T<=100 K) foreground component containing a column density of N(H)~10^22 cm^-2.