Atlas and Catalog of Dark Clouds Based on the 2 Micron All Sky Survey. II. Correction of the Background Using the Besançon Galaxy Model

Abstract

In this paper, we give a correction to the large scale color excess maps of $ E$ ($ J$$-$$ H$ ) and $ E$ ($ H$$-$$ K_{\rm S}$ ) derived by Dobashi (2011, PASJ, 63, S1) based on the 2 Micron All Sky Survey Point Source Catalog (2MASS PSC). These maps were produced using a new technique named $ X$ percentile method, and they cover all of the sky at the $ 1'$ grid. The maps, however, suffer from an apparent error on a large scale arising from an ambiguity in determining the background star colors. The error is relatively large in the inner region of the Galaxy at $ \vert l\vert$$ \lesssim$ 90$ ^\circ$ , and the maps may overestimate the true extinction by a few magnitudes in $ A_V$ in this region. To improve the background determination, we performed a Monte Carlo simulation to generate a star catalog equivalent to the 2MASS PSC based on the Besançon Galaxy Model described by Robin et al. (2003, A&A, 409, 523). The simulated catalog contains $ \sim$ 7.7 $ \times$ 10$ ^{8}$ stars whose apparent magnitudes in the $ J$ , $ H$ , and $ K_{\rm S}$ bands are calculated assuming no interstellar dust throughout the Galaxy. We applied the $ X$ percentile method to the simulated star catalog, and regarded the resulting star color maps as the background. As a result, the overestimation in the original color excess maps has been significantly improved. Extinction maps of $ A_J$ , $ A_H$ , and $ A_{K_{\rm S}}$ made by Dobashi (2011) were also improved utilizing the resulting color excess maps. In this paper, we further investigated possible errors arising from the $ X$ percentile method itself by setting an artificial diffuse dust disk in the simulated star catalog, and found that the diffuse dust on a large scale can be underestimated by $ \sim$ 20% for the galactic latitude range $ \vert b\vert$$ >$ 5$ ^\circ$ at most, which should be noted when the color excess maps are compared with other dataset including the far-infrared dust emission detected by Planck and Herschel satellites.