This work presents high spectral resolution observations of the [C ii] line at 158 μm, one of the major cooling lines of the interstellar medium, taken with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691, an H ii region far north (3.3 kpc) in the disk of M 33, the [C ii] and CO line profiles show similar velocities within 0.5 km s-1, while the H i line velocities are systematically shifted towards lower rotation velocities by ~5 km s-1. Observed at the same 12”angular resolution, the [C ii] lines are broader than those of CO by about 50% but narrower than the H i lines. The [C ii] line intensities also follow those of CO much better than those of H i. A weak shoulder on the [C ii] line suggests a marginal detection of the [13C ii] line, insufficient to constrain the [C ii] optical depth. The velocity coincidence of the CO and [C ii] lines and the morphology at optical/UV wavelengths indicate that the emission is coming from a molecular cloud behind the H ii region. The relative strength of [C ii] with respect to the FIR continuum emission is comparable to that observed in the Magellanic Clouds on similar linear scales but the CO emission relative to [C ii] is stronger in M 33. The [C ii] line to far-infrared continuum ratio suggests a photoelectric heating efficiency of 1.1%. The data, together with published models indicate a UV field G0 ~ 100 in units of the solar neighborhood value, a gas density nH~1000 cm-3, and a gas temperature T ~ 200 K. Adopting these values, we estimate the C+ column density to be NC + ≈ 1.3×1017cm-2. The [C ii] emission comes predominantly from the warm neutral region between the H ii region and the cool molecular cloud behind it. From published abundances, the inferred C+ column corresponds to a hydrogen column density of NH~2×1021cm-2. The CO observations suggest thatNH=2NH2~3.2×1021cm-2 and 21 cm measurements, also at 12”resolution, yield NH i≈1.2×1021cm-2 within the [C ii] velocity range. Thus, some H2 not detected in CO must be present, in agreement with earlier findings based on the SPIRE 250-500 μm emission.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
