Reddening and extinction toward the galactic bulge from OGLE-III: The Inner Milky Way's R_V ∼ 2.5 extinction curve

We combine VI photometry from OGLE-III with VISTA Variables in The Via Lactea survey and Two Micron All Sky Survey measurements of E(J-K_s ) to resolve the longstanding problem of the non-standard optical extinction toward the Galactic bulge. We show that the extinction is well fit by the relation A_I = 0.7465 × E(V-I) + 1.3700 × E(J-K _s ), or, equivalently, A_I = 1.217 × E(V-I)(1 + 1.126 × (E(J-K_s )/E(V-I)-0.3433)). The optical and near-IR reddening law toward the inner Galaxy approximately follows an R_V ≈ 2.5 extinction curve with a dispersion σR_v, consistent with extragalactic investigations of the hosts of Type Ia SNe. Differential reddening is shown to be significant on scales as small as our mean field size of 6′. The intrinsic luminosity parameters of the Galactic bulge red clump (RC) are derived to be (M_I,RC, σ_I,RC,0, (V-I) _RC,(J-K_s)RC,0). Our measurements of the RC brightness, brightness dispersion, and number counts allow us to estimate several Galactic bulge structural parameters. We estimate a distance to the Galactic center of 8.20 kpc. We measure an upper bound on the tilt α ≈ 40° between the bulge's major axis and the Sun-Galactic center line of sight, though our brightness peaks are consistent with predictions of an N-body model oriented at α ≈ 25°. The number of RC stars suggests a total stellar mass for the Galactic bulge of ∼2.3 × 10¹⁰ M _⊙ if one assumes a canonical Salpeter initial mass function (IMF), or ∼1.6 × 10¹⁰ M _⊙ if one assumes a bottom-light Zoccali IMF.