# Lifting the Dusty Veil II: A Large-Scale Study of the Galactic Infrared Extinction Law

## TL;DR

This large-scale study maps the infrared extinction law across the Milky Way's midplane, revealing significant variations linked to Galactocentric radius and dust grain properties, with implications for understanding Galactic dust composition.

## Contribution

It provides the first comprehensive analysis of IR extinction law variations across nearly 150 degrees of the Galactic midplane using combined 2MASS and Spitzer data.

## Key findings

- Extinction law varies monotonically with Galactic longitude.
- Steeper extinction curves are observed towards the outer Galaxy.
- The extinction law's shape correlates with Galactocentric radius.

## Abstract

We combine near-infrared (2MASS) and mid-infrared (Spitzer-IRAC) photometry to characterize the IR extinction law (1.2-8 microns) over nearly 150 degrees of contiguous Milky Way midplane longitude. The relative extinctions in 5 passbands across these wavelength and longitude ranges are derived by calculating color excess ratios for G and K giant red clump stars in contiguous midplane regions and deriving the wavelength dependence of extinction in each one. Strong, monotonic variations in the extinction law shape are found as a function of angle from the Galactic center, symmetric on either side of it. These longitudinal variations persist even when dense interstellar regions, known a priori to have a shallower extinction curve, are removed. The increasingly steep extinction curves towards the outer Galaxy indicate a steady decrease in the absolute-to-selective extinction ratio (R_V) and in the mean dust grain size at greater Galactocentric angles. We note an increasing strength of the 8 micron extinction inflection at high Galactocentric angles and, using theoretical dust models, show that this behavior is consistent with the trend in R_V. Along several lines of sight where the solution is most feasible, A_lambda/A_Ks as a function of Galactic radius is estimated and shown to have a Galactic radial dependence. Our analyses suggest that the observed relationship between extinction curve shape and Galactic longitude is due to an intrinsic dependence of the extinction law on Galactocentric radius.