The Near-Infrared Extinction Law at High and Low Galactic Latitudes (2312.11762v1)

Abstract: The Milky Way extinction curve in the near-infrared (NIR) follows a power law form, but the value of the slope, $\beta_\text{NIR}$, 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 essentially unconstrained at high Galactic latitudes where most extragalactic work takes place. In this paper, we construct extinction curves from 56,649 stars with SDSS and 2MASS 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 significant variation of $\beta_\text{NIR}$ across low-to-moderate dust columns ($0.02<E(B-V)\lesssim 1$), and report average $\beta_\text{NIR}=1.85\pm0.01$, in agreement with Fitzpatrick et al. (2019), but steeper than Cardelli et al. (1989) and Fitzpatrick (1999). We also find no intrinsic correlation between $\beta_\text{NIR}$ and $R_V$ (there is an apparent correlation which 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 $R_H=0.345\pm0.007$ and comment on its bearing on Cepheid calibrations and the determination of $H_0$.