A Direct Measurement of the Dust Extinction Curve in an Intermediate-Redshift Galaxy

TL;DR

This study demonstrates a method to directly measure dust extinction curves in distant galaxies using multi-wavelength observations of GRB afterglows, without relying on predefined extinction models.

Contribution

It introduces a novel approach to derive dust extinction curves from IR to X-ray data of GRB afterglows, enabling analysis without assuming known extinction laws.

Findings

Disfavored Milky Way extinction law for the host galaxy

Possible presence of LMC-like 2175 Å bump in the extinction curve

Dust-to-gas ratio similar to the LMC in the host galaxy

Abstract

We present a proof-of-concept study that dust extinction curves can be extracted from the infrared (IR), optical, ultraviolet (UV) and X-ray afterglow observations of GRBs without assuming known extinction laws. We focus on GRB 050525A (z = 0.606), for which we also present IR observations from the Spitzer Space Telescope at about 2.3 days post-burst. We construct the spectral energy distribution (SED) of the afterglow and use it to derive the dust extinction curve of the host galaxy in 7 optical/UV wavebands. By comparing our derived extinction curve to known templates, we see that the Galactic or Milky Way extinction laws are disfavored versus the Small and Large Magellanic Cloud (SMC and LMC) ones, but that we cannot rule out the presence of a LMC-like 2175 angstrom bump in our extinction curve. The dust-to-gas ratio present within the host galaxy of GRB 050525A is similar to that found in the LMC, while about 10 to 40% more dust is required if the SMC template is assumed. Our method is useful to observatories that are capable of simultaneously observing GRB afterglows in multiple wavebands from the IR to the X-ray.