Dust Extinction Law in Nearby Star-Resolved Galaxies. I. M31 Traced by Supergiants

TL;DR

This study derives detailed dust extinction laws in M31 using supergiants and multiband photometry, revealing a range of extinction properties similar to the Milky Way and providing valuable correction data for future research.

Contribution

It introduces a direct modeling approach for extinction curves based on dust grain models, applied to M31's interstellar environment, highlighting its complexity and inhomogeneity.

Findings

Extinction curves in M31 range from R_V ≈ 2 to 6.

Average extinction curve resembles the Milky Way but with less steep far-UV rise.

Median V-band extinction in M31 is about 1 magnitude.

Abstract

The dust extinction laws and dust properties in M31 are explored with a sample of reddened O-type and B-type supergiants obtained from the LGGS. The observed spectral energy distributions (SEDs) for each tracer are constructed with multiband photometry from the LGGS, PS1 Survey, UKIRT, PHAT Survey, Swift/UVOT and XMM-SUSS. We model the SED for each tracer in combination with the intrinsic spectrum obtained from the stellar model atmosphere extinguished by the model extinction curves. Instead of mathematically parameterizing the extinction functions, the model extinction curves in this work are directly derived from the silicate-graphite dust model with a dust size distribution of $dn/da \sim a^{-\alpha}{\rm exp}(-a/0.25),~0.005 < a < 5~\mu {\rm m}$. The extinction tracers are distributed along the arms in M31, with the derived MW-type extinction curves covering a wide range of $R_V$ ($\approx 2 - 6$), indicating the complexity of the interstellar environment and the inhomogeneous distribution of interstellar dust in M31. The average extinction curve with $R_V \approx 3.51$ and dust size distribution $dn/da \sim a^{-3.35}{\rm exp}(-a/0.25)$ is similar to those of the MW but rises slightly less steeply in the far-UV bands, implying that the overall interstellar environment in M31 resembles the diffuse region in the MW. The extinction in the $V$ band of M31 is up to 3 mag, with a median value of $ A_V \approx 1$ mag. The multiband extinction values from the UV to IR bands are also predicted for M31, which will provide a general extinction correction for future works.