The Mid-infrared Extinction Law in the Large Magellanic Cloud
Based on photometric data from the Spitzer/SAGE survey, using red giants as extinction tracers, the mid-infrared (MIR) extinction laws in the Large Magellanic Cloud (LMC) are derived for the first time in the form of . This quantity refers to the extinction in the four Infrared Array Camera (IRAC) bands (i.e., [3.6], [4.5], [5.8], and [8.0] μm) relative to the Two Micron All Sky Survey KS band at 2.16 μm. We obtain the near-infrared extinction coefficient to be E(J – H)/E(H – KS ) 1.29 ± 0.04 and E(J – KS )/E(H – KS ) 1.94 ± 0.04. The wavelength dependence of the MIR extinction in the LMC varies from one sightline to another. The overall mean MIR extinction is , , , and . Except for the extinction in the IRAC [4.5]  μm band, which may be contaminated by the 4.6 μm CO gas absorption of red giants used to trace LMC extinction, the extinction in the other three IRAC bands show a flat curve, close to the Milky Way RV = 5.5 model extinction curve, where RV is the optical total-to-selective extinction ratio. The possible systematic bias caused by the correlated uncertainties of KS – λ and J – KS is explored in terms of Monte Carlo simulations. We find that this bias could lead to an overestimation of in the MIR.