Herschel [CII] 158μm Observations of “CO-Dark” Gas in the Perseus Giant Molecular Cloud

We have observed velocity-resolved [CII] 158 micron emission from both a dense and a more diffuse photodissociation region (PDR) in the Perseus giant molecular cloud (GMC) using the Heterodyne Instrument for the Far-Infrared on board the Herschel Space Telescope. These observations were undertaken to trace diffuse H₂ gas that does not have corresponding ¹²CO emission, “CO-dark” molecular gas. We detected [CII] emission from 80% of our total positions, with a 95% detection rate from the dense boundary region. The [CII] emission remains at a constant integrated intensity across each boundary, despite the observations spanning a wide range of optical extinction between 1 mag and 10 mag. In addition, the [CII] emission extends to 80 arcmin (or 7 pc) away from the Perseus center, demonstrating that a significant fraction of the [CII] emission is associated with the extended Perseus envelope. A flat integrated intensity indicates a constant heating and cooling rate within each of the two boundary regions observed. The dense region has a factor of two higher integrated [CII] intensity relative to the more diffuse region, possibly due to a more intense ambient radiation field. The integrated intensity of [CII] emission is reasonably well correlated with the HI column density, as well as total gas column density, suggesting that HI plays an important role in explaining the [CII] emission emanating from Perseus. We compared the integrated intensity of [CII] and ¹²CO emission with predictions from a 1-D, two-sided slab PDR model and showed that a simple core + envelope model without the “CO-dark” component can reproduce observations well. However, due to model degeneracies, a more realistic density distribution which includes the “CO-dark” gas would likely produce just as accurate of predictions. Additional observations, from diffuse layer species such as OH and CH, are needed to disentangle how much of the [CII] emission is associated with the “CO-dark” molecular gas.