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Appendix A: Comparison with the column density obtained from Herschel data

The extinction map derived in this work relies on the reddening of background sources detected at H and K_s by UKIDSS. A totally independent estimation of the column density is proposed by Schneider et al. (2012) from dust emission. Their map is based on the gray-body fitting from 160 to 500 μm of the 1.8 deg² observed by the Herschel key program HOBYS (Motte et al. 2010). Using our extinction map as reference, we convert the Herschel column density (noted CD and expressed in cm^-2) into extinction as A_V(FIR) = (CD − 3.71 × 10²⁰)/5.11 × 10²⁰. This relation depends mainly on the dust opacity at all wavelengths, and the gas-to-dust ratio through the value of N_H/A_V that is supposed constant. However, the situation is more complicated, as pointed out by a recent study by Roy et al. (2013), which discusses the variation in the dust opacity with column density in Orion. They conclude that no single opacity can be applied to the whole cloud, which has quantitative implications for interpreting of the column density from Herschel data. Differences between the two column density maps are therefore expected.

Figure A.1 presents the difference between the two extinctions as a function of the distance to NGC 2244, i.e. the OB stars. The excess of extinction for the region at less than ~50′ from the NGC 2244 suggests that the optical depth derived from dust emission is overestimated near the massive star cluster. The spatial distribution of the excess is shown better in Fig. A.2, which presents the two extinction maps and their difference. The diffuse excess, which appears in green in Fig. A.2c recalls the ionization front of Fig. 6. The extinction excess likely results from the emission of warmer grains in the diffuse envelop surrounding the dense molecular cloud. A similar effect has been observed by Russeil et al. (2013) in NGC 6334 when excluding the 70 μm to derive the column density. In addition, the two maps exhibit strong differences of several tens of magnitudes for limited areas that happen to match the star clusters. It suggests local heating by YSOs, although it is true that real dense cores with A_V > 150 mag would not be revealed using H − K_s since no background star could be detected through such a column density.

	Fig. A.1 Difference between the extinction obtained from dust emission in the FIR and from dust reddening in the NIR versus the distance to NGC 2244. The x-axis corresponds roughly to the galactic longitude and is reversed to match the map orientation with east pointing left.
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	Fig. A.2 a) Extinction derived from dust emission using Herschel data. b) Extinction derived from UKIDSS H − Ks color excess. c) Difference between the two extinction map a)−b) . The color-map cuts are identical for a) and b) and set to display the visual extinction from 0 to 30 mag. Contours for AV = 6 and 12 mag are overlaid.
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	Fig. A.3 Cumulative mass distribution from Herschel dust emission (dashed line) and UKIDSS reddening (plain line) for the same field of view.
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Despite the obvious differences between the two extinction maps, it is worth noting the total mass estimates shown in Fig. A.3 are consistent. We obtained 1.7 × 10⁵ and 1.4 × 10⁵  M_⊙ for the Herschel and the UKIDSS-based maps, respectively. The high extinctions make only a very limited contribution to the total mass estimate because they correspond to a small surface area. Our analysis for the RMC confirms that Herschel column density maps must be used with caution. Further comparisons between column density maps derived with Herschel and extinction methods are required to better understand the impact of parameters, such as the opacity (i.e. dust emissivity), variations, and the presence of several dust components along the line of sight.

© ESO, 2013