I’ve had a great time studying how far-infrared (FIR) lines probe interstellar gas properties.
This week, I attended the 69th Dutch Astronomy Conference, held in Noordwijkerhout, The Netherlands. I presented a poster based on the latest work from my next paper in preparation, entitled “Insights into gas heating and cooling in the disc of NGC 891 from Herschel far-infrared spectroscopy” (yes, a second paper on 891), which I share with you below.
I’ve been getting my hands dirty working with FIR spectroscopy from Herschel observations, investigating the heating and cooling of the interstellar gas in the edge-on galaxy NGC 891. I examine these processes on sub-kiloparsec scales using emission maps of the [CII] 158, [OI] 63 and 145, [OIII] 88, and [NII] 122 and 205 micron FIR fine-structure lines obtained with the Herschel PACS and SPIRE instruments. We compare the observed flux of the FIR cooling lines and total IR emission with the predicted flux from a photo dissociation region model to determine the gas density, surface temperature and the strength of the incident far-ultraviolet radiation field, G. For the first time in NGC 891, following recent studies attempted for other galaxies like M51 and Centaurus A, I perform a pixel-by-pixel analysis to map the physical characteristics of the gas, resolving details on physical scales of 0.6 kpc.
So far, my analysis reveals that the majority of the PDRs in NGC 891’s disc have hydrogen densities and experience FUV radiation field strengths that are similar to the physical conditions of the gas in the spiral arm and inter-arm regions of the M51 galaxy. We estimate the FUV radiation field is stronger on average in the far north-eastern side compared to the rest of the disc, coinciding with the above-average star formation rate surface densities and gas-to-dust ratios observed in this region, yet a conclusive interpretation remains difficult due to observational uncertainties. The main problem I’m having at the moment is disentangling optical depth effects with the [OI] 63 micron line, which becomes optically thick faster than the [CII] 158 line, and so will likely manifest effects as we integrate along the line-of-sight through the disc (a recurring problem with interpreting edge-on galaxies). Check out the poster below for more details.
Grab a copy of the pdf by clicking the image.