3D Extinction Mapping the Galactic Plane
(Drew, Barentsen, Lucas, Thompson)
Our knowledge of the organisation of the Milky way, is more uncertain than that of our nearest neighbours in the Local Group. But as this is our Galaxy, accessible at higher spatial resolution than any other, it is the place to demonstrate that concepts in star formation, galaxy and stellar evolution, apparently working at fuzzier spatial resolution in the Local Group and beyond, really do work. The major obstacle is the high and spatially-variable reddening within the Galactic Plane (GP).
In recent years there has been a renaissance in surveys of the GP from the optical/infrared to the radio, within which we playing a leading role. This anticipates and complements the astronomical revolution expected as data begins to flow from the ESA Gaia mission, from ~2015-16 on, eventually delivering astrometric distance determinations for objects at V < 20. The payoff for our understanding of star formation and evolution depends on a similar revolution taking place in our quantitative knowledge of GP extinction - without reliable extinctions, stellar luminosities will remain indeterminate.
Reliable extinction measurement has to be based on photometric SED fitting across a wide extinction-sensitive wavelength range, rather than on spectroscopic proxies. The optical-NIR spectrum is optimum, since it is in this wavelength domain that the transition from negligible to high obscuration commonly occurs. To understand physical dust properties, dust emission at longer wavelengths must be studied as well - the full story is thus inherently multi-wavelength. We are working toward linking together IPHAS, UKIDSS/GPS, VPHAS+ and VVV data to estimate extinctions for large numbers of normal stars to enable 3D maps of the stellar and dust content of the Milky Way's disk. The products of this mapping can be linked to far-infrared (Herschel) and submm (JCMT/SCUBA-2) data to better portray the organisation of the star-forming interstellar medium and make a qualitatively new study of the relationship between star formation, dust and molecular cloud properties.
The left hand panel shows data from the IPHAS survey for two different fields in the Aquila Rift that exhibit markedly different maximum reddenings (the green data points are from the more reddened pointing). The right hand panel shows the synthetic main sequence and giant tracks for three different reddenings (E(B-V)=0, 1, 2 as computed for an R(V)=3.1 reddening law), illustrating how increased extinction causes the main sequence track to move almost orthogonally, showing that each point on the colour-colour plane is effectively encoded for reddening and intrinsic stellar colour. This property is very useful in 3D extinction mapping. (Diagrams from the IPHAS survey paper, Drew et al 2005)