IDEOS Observables

Here we describe the 80 IDEOS observables that are available for the targets in the IDEOS galaxy sample. Most quantities have an uncertainty associated with them, which is provided in a variable OBSERVABLE_ERR. Similarly, there may be an associated flag denoting a detection (0), upper limit (+1), lower limit (+2), or the lack of a measurement (-1). This value would be stored in the variable OBSERVABLE_FLAG.

Additional quantities describing the data set can be found on the non-observables page.

PAH fluxes and equivalent widths obtained using PAHFIT (Smith et al. 2007, ApJ 656, 770) or QUESTFIT (Veilleux et al. 2009, ApJS 182, 628) differ strongly from those measured using the spline-based method in the following ways:
1) PAH emission bands are assumed to have Drude profiles, which have broader wings than Gauss profiles or Pearson-IV profiles. The PAH fluxes are therefore larger than from the spline-based method.
2) The continuum underneath the PAH emission bands lies below the observed/pseudo continuum, because part of the observed continuum level is formed by the overlapping wings of the Drude shaped PAH profiles. The equivalent widths of PAH features are therefore larger.

The DRUDE observables in the table below originate from either PAHFIT or QUESTFIT depending on whether the source has a strong silicate absorption feature or not. In the former case the DRUDE observables were supplied by QUESTFIT, in the latter case by PAHFIT.

We have used a modified version of PAHFIT (Gallimore et al. 2010, ApJS 187, 172), which assumes PAH features and emission lines to be un-attenuated. Our PAHFIT PAH fluxes and equivalent widths therefore have not been extinction corrected.

Like PAHFIT, QUESTFIT assumes PAH fluxes and equivalent widths to have Drude shaped feature profiles. The big difference with PAHFIT is that the 24 PAH components are not allowed to vary freely amongst themselves during the fit. Instead they are constrained to two sets of pre-determined PAH component ratios -- or "PAH templates" -- each of which stems from fits to Spitzer-IRS spectra of multiple star forming galaxies (Smith et al. 2007, ApJ 656, 770). The contribution of either PAH template can be varied in the overall QUESTFIT optimization.

Unlike PAHFIT (and the modified version that we used), QUESTFIT includes screen absorption by water ice (6µm), aliphatic hydrocarbon (6.85 and 7.25µm) and crystalline silicate absorption (23µm) on the thermal dust emission components as free parameters. Like for our modified version of PAHFIT, QUESTFIT assumes attenuation on PAH emission to be negligible. Extinction on the PAH templates is hence not a parameter in the fit.

QUESTFIT is only used to fit spectra of moderately to deeply obscured targets, for which the number of degrees of freedom on the shape of the PAH emission spectrum is too large to produce realistic fits in all cases.