Variation of the 2175 {\AA} extinction feature in Andromeda galaxy

TL;DR

This study investigates the variability of the 2175 Å extinction feature across 20 sightlines in Andromeda, revealing diverse dust properties and providing new insights into interstellar dust characteristics in M31.

Contribution

It presents the first comprehensive analysis of the 2175 Å bump variation in M31 using UV spectroscopy and photometry, highlighting the diversity and correlations in dust extinction features.

Findings

Wide variation in bump strength and profile among sightlines.

Average UV extinction curve with R_V ≈ 3.53 in M31.

Identification of sightlines with absent or weak 2175 Å bump.

Abstract

Extinction curves contain key information on interstellar dust composition and size distribution, with the 2175 {\AA} bump being the most prominent feature. We analyze 20 sightlines toward M31 using HST/STIS UV spectroscopy combined with multi-band photometry to characterize this feature. The extinction curves show substantial diversity, from MW-like shapes to flatter profiles with $R_V$ reaching up to $\sim5.8$. The strength of the 2175 {\AA} feature varies widely, including two sightlines where the bump is essentially absent. The bump central wavelength spans a broader range than previously reported, while its width remains consistent with earlier studies. A moderate positive correlation is found between bump strength ($c_3$) and width ($\gamma$). We derive an average UV extinction curve toward M31 with $R_V \approx 3.53$. These results provide new constraints on dust properties and their spatial variations in this galaxy.