Milky Way Extinction

Measurements of dust extinction in our Galaxy provides a rich view of dust grains due to the large number of sightlines with measurements. My work has focused on spectroscopic measurements of extinction to constrain the average behavior and the characteristics of features. I have done this with larger spectroscopic samples in the ultraviolet (Valencic et al. 2004), far-ultraviolet (Gordon et al. 2009), optical (Fitzpatrick et al. 2019; Massa et al. 2020), near-infrared (Decleir et al. 2020), and mid-infrared (Gordon et al. 2021). These results have been used to determine a fully spectroscopic based R(V) extinction relationship that extends from 912 A to 32 micron (Gordon et al. 2023). This relationship provides the average extinction for a range of R(V) values where R(V) traces average dust grain size. This useful relationship is included in the dust_extinction astropy affiliated python package.

We have found two sightlines of particular interest as both do not have a 2175 Å bump - one possibly due to grain destruction (Valencic et al. 2003) and one due to grain coagulation (Whittet et al. 2004)

Local Group Ultraviolet Extinction

Measuring the dust extinction in the Large and Small Magellanic Clouds provides a view into dust grains in environments different than seen in our Galaxy. For example, the SMC shows the most different ultraviolet dust extinction curves, most do not have the 2175 Å feature (Gordon & Clayton 1998 ; Gordon et al. 2003). The LMC shows large variations, including weak 2175 Å bumps near the 30 Dor starburst region (Misselt et al. 1999). With collaborators, I am working to expand the number of measured extinction curves in the SMC (paper in prep.), M31 (Clayton et al. 2015; ongoing HST program), and M33 (ongoing HST program).

BEAST

I have been working on a new technique to measure dust extinction in nearby galaxies based on photometry of many, many resolved stars. This work was motivated by the large HST M31 survey named PHAT (Dalcanton et al. 2012), that has measured photometry in 6 bands from ultraviolet through near-infrared for 100+ million stars (Williams et al. 2014). The Bayesian Extinction And Stellar Tool (BEAST) fits the photometry for each star with a model based on the physics of stars, an empirical model of dust extinction based on Milky Way and Magellanic Cloud measurements, and an empirical observation model (Gordon et al. 2016). Current work is focused on creating the MegaBEAST - a Hierarchical Bayesian model that directy interfaces with the BEAST to model the ensemble properties of a group of dust extinguished stars to produce maps of dust extinction properties as well as stellar population parameters.