Constraints on the dust extinction law of the Galaxy with Swift/UVOT, Gaia, and 2MASS

Ignacio Ferreras,Mónica Tress,Vladimir Yershov,Gustavo Bruzual,Paul Kuin,Daisuke Kawata,Stéphane Charlot,Mark Cropper,Mat Page

doi:10.1093/mnras/stab1270

Abstract

ABSTRACT We explore variations of the dust extinction law of the Milky Way by selecting stars from the Swift/UVOT Serendipitous Source Catalogue, cross-matched with Gaia DR2 and 2MASS to produce a sample of 10 452 stars out to ∼4 kpc with photometry covering a wide spectral window. The near ultraviolet passbands optimally encompass the 2175 Å bump, so that we can simultaneously fit the net extinction, quoted in the V band (AV), the steepness of the wavelength dependence (δ), and the bump strength (Eb). The methodology compares the observed magnitudes with theoretical stellar atmospheres from the models of Coelho. Significant correlations are found between these parameters, related to variations in dust composition that are complementary to similar scaling relations found in the more complex dust attenuation law of galaxies – that also depend on the distribution of dust among the stellar populations within the galaxy. We recover the strong anticorrelation between AV and Galactic latitude, as well as a weaker bump strength at higher extinction. δ is also found to correlate with latitude, with steeper laws towards the Galactic plane. Our results suggest that variations in the attenuation law of galaxies cannot be fully explained by dust geometry.

Highlights

Dust is one of the key observable components of galaxies
To illustrate the power of a photometric analysis to constrain the dust extinction law, we show in Fig. 4 three colour–colour diagrams with our sample represented as a density plot
The orange star and dashed lines mark the Milky Way standard extinction law (Eb = 4, δ = −0.05), and a characteristic error bar of the dust parameters is shown in each panel, defined as the median error bar for each parameter from the MCMC-based fits to the whole sample, given at the 1σ level

Summary

Introduction

Dust affects the way starlight reaches the observer, and a good characterization of the wavelength dependence of these processes is fundamental to be able to, for instance, derive accurate stellar masses or star formation rates of galaxies. There are two main ways in which the presence of dust will shape the spectral energy distribution of a galaxy: the dust properties will lead to an extinction law that reflects how the superposition of dust grains of different size and composition absorbs and scatters incoming light The distribution of dust within the galaxy will affect light from the constituent stellar populations in different ways. Light from other regions of the galaxy may be scattered into the line of sight to the observer, complicating the interpretation of attenuation, that no longer depends exclusively on dust composition

Methods

Results

Conclusion