Lifting the Dusty Veil With Near- and Mid-Infrared Photometry: I. Description and Applications of the Rayleigh-Jeans Color Excess Method

The Milky Way (MW) remains a primary laboratory for understanding the structure and evolution of spiral galaxies, but typically we are denied clear views of MW stellar populations at low Galactic latitudes because of extinction by interstellar dust. However, the combination of 2MASS near-infrared (NIR) and Spitzer-IRAC mid-infrared (MIR) photometry enables a powerful method for determining the line of sight reddening to any star: the sampled wavelengths lie in the Rayleigh-Jeans part of the spectral energy distribution of most stars, where, to first order, all stars have essentially the same intrinsic color. Thus, changes in stellar NIR-MIR colors due to interstellar reddening are readily apparent, and (under an assumed extinction law) the observed colors and magnitudes of stars can be easily and accurately restored to their intrinsic values, greatly increasing their usefulness for Galactic structure studies. In this paper we explore this "Rayleigh-Jeans Color Excess" (RJCE) method and demonstrate that use of even a simple variant of the RJCE method based on a single reference color, (H-[4.5um]), can rather accurately remove dust effects from previously uninterpretable 2MASS color-magnitude diagrams of stars in fields along the heavily reddened Galactic mid-plane, with results far superior to those derived from application of other dereddening methods. We also show that "total" Galactic midplane extinction looks rather different from that predicted using 100um emission maps from the IRAS/ISSA and COBE/DIRBE instruments as presented by Schlegel et al. Instead, the Galactic mid-plane extinction strongly resembles the distribution of 13-CO (J=1->0) emission. Future papers will focus on refining the RJCE method and applying the technique to understand better not only dust and its distribution, but the distribution of stars intermixed with the dust in the low-latitude Galaxy.