It has long been reported in the literature that emission in the 850 and 450 micron bands can be affected by the emission from molecular lines (Johnstone, D., Boonman, A. M. S., & van Dishoeck, E. F. 2003, A&A, 412, 157). At 450μm the center of the bandpass filter at 664GHz is close to the 12CO(6-5) line (at 691.473 GHz). At 850 μm the center of the bandpass filter at 347 GHz is close to the 12CO(3-2) line (at 345.796 GHz).
Often astronomers wish to disentangle emission from dust at these wavelengths vs molecular emission lines and/or free-free emission (for the latter see Rumble et al. 2016).
This page focuses on how to calculate the contamination in the SCUBA-2 bandpass from emission lines. Specifically it outlines the Conversion Factor Function C which is used to convert data from K/km/s to mJy/beam. C is found to depend on the amount of Precipitable Water Vapor (PWV) in the atmosphere.
In 2012 Drabek et al. produced a paper specifically examining the Molecular line contamination in the SCUBA-2 450 and 850 μm continuum data, outlining the Conversion Factor C.
An updated value of C is presented below and has been published by Parsons et al. 2018.
C factor as a function of PWV. Top: 450 μm. Bottom: 850 μm. The solid lines are the fit as outlined in the paper by Parsons et al. submitted. The data points plotted are the original C factor values as reported by Drabek et al. (2012) for comparison. The shaded region in the C450 plot denote the ± 10% level, and in the C850 plot denote the ± 5% level.
The C factor as a function of PWV is shown in the figure and is given as:
C850 = α + β PWV – γ (PWV2) + δ (PWV3) – ε (PWV4) mJy/beam / K/km/s
The coefficients are provided as:
The C factor function coefficients.
An example look at how to implement this can be found in the SCUBA-2 Data Reduction Tutorial 5.
The C factor function requires the knowledge of the PWV during an observation. The conversion from Tau225GHz (which can be obtained from any JCMT fits header) to PWV is given on the weather page and in Dempsey et al. 2013:
Tau225GHz = 0.04 PWV + 0.017