Spectrally resolved CII emission in M~33 ({\tt HerM33es}): Physical conditions and kinematics around BCLMP 691

TL;DR

This study uses high-resolution spectral observations of the extCII\, line in M33's BCLMP 691 region to analyze physical conditions, kinematics, and the origin of extCII\, emission in relation to other interstellar medium tracers.

Contribution

It provides detailed measurements of extCII\, line profiles and physical conditions in M33, revealing the origin of extCII\, emission and its relation to molecular and atomic gas, which was not previously characterized at this resolution.

Findings

extCII\, and CO line velocities are similar within 0.5 km/s.

extCII\, lines are broader than CO but narrower than extHI\, lines.

The extCII\, emission mainly arises from the warm neutral medium.

Abstract

This work presents high spectral resolution observations of the \CII\ line at 158 \micron, one of the major cooling lines of the interstellar medium, taken with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691, an \HII\ region far north (3.3 kpc) in the disk of M 33, the \CII\ and CO line profiles show similar velocities within $0.5 \kms$, while the \HI\ line velocities are systematically shifted towards lower rotation velocities by $\sim 5\kms$. Observed at the same $12"$ angular resolution, the \CII\ lines are broader than those of CO by about 50% but narrower than the \HI\ lines. The \CII\ line to far-infrared continuum ratio suggests a photoelectric heating efficiency of 1.1%. The data, together with published models indicate a UV field $G_0 \sim 100$ in units of the solar neighborhood value, a gas density $n_H \sim 1000 \cc$, and a gas temperature $T\sim 200$ K. Adopting these values, we estimate the C$^+$ column density to be $N_{C^+} \approx 1.3 \times 10^{17} \cmt$. The \CII\ emission comes predominantly from the warm neutral region between the \HII\ region and the cool molecular cloud behind it. From published abundances, the inferred C$^+$ column corresponds to a hydrogen column density of $N_H \sim 2 \times 10^{21} \cmt$. The CO observations suggest that $N_H = 2 N_{H_2} \sim 3.2 \times 10^{21} \cmt$ and 21cm measurements, also at $12"$ resolution, yield $N_\HI \approx 1.2 \times 10^{21} \cmt$ within the \CII\ velocity range. Thus, some H$_2$ not detected in CO must be present, in agreement with earlier findings based on the SPIRE 250 -- 500 $\mu$m emission.