The Near-infrared Extinction Law at High and Low Galactic Latitudes

Abstract

The Milky Way dust extinction curve in the near-infrared (NIR) follows a power-law form, but the value of the slope, β NIR, is debated. Systematic variations in the slope of the Milky Way UV extinction curve are known to be correlated with variations in the optical slope (through R V ), but whether such a dependence extends to the NIR is unclear. Finally, because of low dust column densities, the NIR extinction law is poorly understood at high Galactic latitudes where most extragalactic work takes place. In this paper, we construct extinction curves from 56,649 stars with Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey photometry, based on stellar parameters from SDSS spectra. We use dust maps to identify dust-free stars, from which we calibrate the relation between stellar parameters and intrinsic colors. Furthermore, to probe the low-dust regime at high latitudes, we use aggregate curves based on many stars. We find no systematic variation of β NIR across low-to-moderate dust columns (0.02 < E(B − V) ≲ 1), and report average β NIR = 1.85 ± 0.01, in agreement with the law in the 2019 Fitzpatrick et al. study, but steeper than the Cardelli et al. and 1999 Fitzpatrick laws. Star-to-star scatter in β NIR is relatively small (σ(β NIR) = 0.13). We also find no intrinsic correlation between β NIR and R V (there is an apparent correlation that is the result of the correlated uncertainties in the two values). These results hold for typical sightlines; we do not probe very dusty regions near the Galactic Center, nor rare sightlines with R V > 4. Finally, we find R H = 0.345 ± 0.007 and comment on its bearing on Cepheid calibrations and the determination of H 0.