Extinction and Polycyclic Aromatic Hydrocarbon Intensity Variations Across the H Ii Region Iras 12063–6259

Abstract

The spatial variations in polycyclic aromatic hydrocarbon (PAH) band intensities are normally attributed to the physical conditions of the emitting PAHs, however in recent years it has been suggested that such variations are caused mainly by extinction. To resolve this question, we have obtained near-infrared (NIR), mid-infrared (MIR), and radio observations of the compact H ii region IRAS 12063−6259. We use these data to construct multiple independent extinction maps and also to measure the main PAH features (6.2, 7.7, 8.6, and 11.2 μm) in the MIR. Three extinction maps are derived: the first using the NIR hydrogen lines and case B recombination theory; the second combining the NIR data with radio data; and the third making use of the Spitzer/IRS MIR observations to measure the 9.8 μm silicate absorption feature using the Spoon method and PAHFIT (as the depth of this feature can be related to overall extinction). The silicate absorption over the bright, southern component of IRAS 12063−6259 is almost absent while the other methods find significant extinction. While such breakdowns of the relationship between the NIR extinction and the 9.8 μm absorption have been observed in molecular clouds, they have never been observed for H ii regions. We then compare the PAH intensity variations in the Spitzer/IRS data after dereddening to those found in the original data. It was found that in most cases, the PAH band intensity variations persist even after dereddening, implying that extinction is not the main cause of the PAH band intensity variations.

Mid-infrared spectroscopy of the Andromeda galaxy

“…While extinction does influence the individual PAH bands to different degrees (e.g. Brandl et al 2006;Stock et al 2013), the observed spread in PAH intensity ratios is dominated by the PAH charge balance (e.g. Galliano et al 2008).…”

The Infrared Emission Bands

“…Exceptions to these well know correlations exist: two recent studies report that the 6.2 and 7.7 seem to decouple inside massive star forming regions (Whelan et al 2013;Stock et al 2013). In addition, weak or no 7.7 and 8.6 μm emission is observed towards some T-Tauri stars and HAeBe stars (Acke & van den Ancker 2004;Geers et al 2006) and similarly, very weak emission in the 6 to 9 μm region relative to the 11.2 μm band intensity is observed towards several galaxies harboring an AGN (Kaneda et al 2005;Smith et al 2007;Bregman et al 2008).…”

Characterizing Spatial Variations of PAH Emission in the Reflection Nebula NGC 1333

“…To correct our IRS SL spectra for extinction, we apply the 'modified Spoon method' (Stock et al 2013(Stock et al , 2014(Stock et al , 2016. The Spoon method (Spoon et al 2007) is an iterative fitting procedure that interpolates a power law continuum with anchor points at 5.5 and 14.5 m of the form y = a x and then calculates the natural log of the ratio of this continuum at 9.8 m to the observed flux at 9.8 m. This is referred to as the optical depth of the 9.8 m silicate absorption feature, 9.8 .…”

“…We define a linear continuum of the form, y = a + b x, where b is the slope. We then deredden our spectral data by dividing by our final silicate extinction, A = 1.079 9.8 (Stock et al 2013). We assume an uncertainty in silicate extinction of 10%.…”