Mid-infrared extinction toward the Galactic center

TL;DR

This study measures the mid-infrared extinction toward the Galactic center using MIRI/MRS observations, modeling dust spectra and differential extinction, to refine the extinction law and assess variability and uncertainties.

Contribution

It introduces a direct dust-opacity modeling approach and provides a Python tool for deriving intrinsic dust emission spectra, advancing MIR extinction analysis.

Findings

Confirmed spatial variability of extinction across the field.

Verified absence of PAH emission features in the spectra.

Derived a refined MIR extinction law for the Galactic center.

Abstract

We determine the mid-infrared (MIR, $\sim$5~\mu m--22~\mu m) extinction towards the Galactic center using MIRI/MRS integral field unit (IFU) observations of the central $3''\times3''$ region (near 5~\mu m) to $7''\times7''$ region (near 22~\mu m). To measure the MIR extinction, we employ two approaches: modeling the intrinsic-to-observed dust thermal spectrum and assessing the differential extinction between hydrogen recombination lines. Expanding on prior work, we directly model the dust-opacity distribution along the line of sight, and we make available a Python code that provides a flexible tool for deriving intrinsic dust emission spectra. We confirm the spatial variability of extinction across the field, demonstrating that dusty sources--such as IRS~29N--exhibit higher local extinction. Furthermore, we verify the absence of PAH emission features in the Galactic center MIR spectra. Using the two complementary methods, we derive a refined ``best guess'' MIR extinction law for Sgr A* and the surrounding Galactic-center region. By applying the extinction law to a MIR flare measurement discussed in a companion paper Michail et al. 2025, we estimate a residual relative extinction uncertainty for the short MIRI/MRS grating on the order of $0.2~\mathrm{mag}$ {from $\sim$5 to $\sim$18~\mu m\ and $\sim$0.3~mag from $\sim$18 to $\sim$22~\mu m}, consistent with our uncertainty estimate.