Variation of the 2175 Å extinction feature in Andromeda galaxy

TL;DR

The paper investigates the 2175 Å extinction feature in M31 by combining HST/STIS UV spectroscopy with multi-band photometry to map bump properties and the UV extinction law across sightlines. It adopts the FM90 parametrization for the UV curve, couples stellar atmosphere models with a Bayesian two-step MCMC fitting to extract bump parameters ($c_3$, $\gamma$, $x_0$, $c_4$) and global metrics ($R_V$, $A_V$), and constructs an average UV extinction curve for M31. Key findings include substantial extinction curve diversity, two sightlines with almost no bump, a bump center wavelength ranging beyond previous MW estimates, and a positive $c_3$–$\gamma$ correlation; the average $R_V$ is $\approx 3.53$ (or $\approx 3.62$ after Galactic foreground removal), with UV deviations from the CCM law. These results imply significant spatial variation in dust composition and grain size in M31 and challenge the universality of a single extinction law for external galaxies, offering new constraints for dust evolution models in spiral systems.

Abstract

Extinction curves contain key information on interstellar dust composition and size distribution, with the 2175 Å 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 Å 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 ($γ$). 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.

Figures (12)

• Figure 1: Comparison of extinction curves for M31, the MW, and different regions of the MCs. The black and brown solid line denotes the average diffuse ISM curve of the MW from CCM89 and Wang2019. The blue and green dotted lines correspond to the SMC Bar and Wing, respectively, while the cyan dotted line shows the LMC average from Gordon2003. Violet triangles indicate the M31 extinction from Bianchi1996, and magenta squares represent the average curves of the M31 bulge from Dong2014. Orange and red dashed lines show the average curves from Clayton2025 and Wang2022, respectively.
• Figure 2: HST/STIS UV spectra for the 20 sample stars. Red and green solid lines show the G140L and G230L grating spectra, respectively. Light-gray segments indicate regions excluded from the analysis, while black lines represent the spectra after resolution degradation to $R=100$ for fitting. The overall SNR of each spectrum is noted in the upper-left corner of each panel.
• Figure 3: The research workflow adopted in this study. Details of the observational data are provided in Section \ref{['sec:data']}. The extinction curve model and the intrinsic stellar spectra from atmospheric models are described in Sections \ref{['sec:mod_ext']} and \ref{['sec:mod_intri']}, respectively. The construction of the model SEDs and the fitting methodology are presented in Section \ref{['sec:mod sed']}.
• Figure 4: Observed spectra, intrinsic SEDs, and UV extinction curves for 20 sightlines with S2175=1, ordered by decreasing $A_\text{bump}$. (a) Each panel lists the stellar ID and spectral S/N in the upper-left corner. The black solid line denotes the best-fit model spectrum, while the gray dashed line shows the intrinsic (unreddened) spectrum. The red solid line represents the observed HST spectrum, and the colored points correspond to photometric measurements in different bands. Gray points indicate data with missing uncertainties or those deviating by more than $3\sigma$ from the initial fit. (b) UV extinction curves, decomposed into individual components: total extinction (red solid line), linear background (green dotted line), 2175 Å bump (black dashed line), and far-UV rise (blue dotted line). (c) UV extinction curves compared with observed data, using the same plotting styles as in (a).
• Figure 4: -- continued