Mapping the Milky Way's ISM

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Compared to other galaxies, we know surprisingly little about our home, the Milky Way. We do know that it is a spiral galaxy, that we live in the outer disc about 26 thousand light years from the centre, and -- like many other galaxies -- there is some kind of rotating barred structure in the inner 10,000 light years. We also know lots of little details about the stars close to the sun. However, we do not know what the Milky Way would look like if we could view it face on from another galaxy: the shape and size of the central bar and the distribution of spiral arms are all uncertain. The main cause of this uncertainty is pollution: the space between the stars is almost empty, but the stuff that is there is filthy. Every star is an enormous nuclear furnace, burning gas and belching out soot. Long ago, some of this soot -- astronomers prefer to call it dust -- condensed to form our solar system, the earth and ultimately us. So, dust is worthy of our deepest respect. It is also a nuisance: most of it doesn't condense but instead simply lurks in interstellar space, spoiling our view of the rest of the Galaxy. An important feature of dust is that it absorbs blue light more strongly than red: it makes stars appear redder and fainter than they really are. (Think red sky at night.) The first part of our project takes advantage of this reddening effect to map out the distribution of both stars and dust in our Galaxy by making a statistical analysis of the infrared colours of hundreds of millions of stars within our Galaxy. Infrared light is much less strongly absorbed by dust than visible light, which means that we can use it to detect bright stars on the other side of the Galaxy. Another form of interstellar matter is gas. Unlike dust, gas does not block starlight significantly, but it can be detected directly by its radio emission. We cannot measure the distances to individual gas clouds, but instead can use the Doppler shift of well-known radio emission lines to measure the line-of-sight components of their velocities. These velocities depend in a complicated way on both the unknown distances and on the effect the rotating bar and spiral arms have in stirring up the gas. Nevertheless, one can interpret these observations by comparing them against predictions from computer simulations of the gas flow around the Galaxy. We aim to further develop a scheme for exploring the range of models that can fit the observations and thereby improve our understanding of the Galactic bar, spiral arms and the distribution of gas. Like many projects in modern astronomy, these projects do not require any dedicated telescope time. Instead they make use of existing catalogues of stars and gas that are freely available online. There is a deep connection between these two projects: dust tends t be found in the densest region of interstellar gas clouds, since both are associated with the process of forming stars. We will combine results from both projects to make a coherent map of the interstellar medium. This is mostly aimed at astronomers studying the structure of our own Galaxy, but it will also be of use to extragalactic astronomers and cosmologists who need to understand the effects of foreground contamination from the Milky Way on their observations, and to astronomers who specialise in the study of the detailed properties of dust itself.

Publications

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Barentsen G (2014) The second data release of the INT Photometric Ha Survey of the Northern Galactic Plane (IPHAS DR2) in Monthly Notices of the Royal Astronomical Society

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Barentsen G (2013) Bayesian inference of T Tauri star properties using multi-wavelength survey photometry in Monthly Notices of the Royal Astronomical Society

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Drew J (2014) The VST Photometric H  Survey of the Southern Galactic Plane and Bulge (VPHAS+) in Monthly Notices of the Royal Astronomical Society

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Huckvale L (2014) Reference image selection for difference imaging analysis* in Monthly Notices of the Royal Astronomical Society

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Raddi R (2013) First results of an Ha based search of classical Be stars in the Perseus Arm and beyond in Monthly Notices of the Royal Astronomical Society

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Ridley M (2017) Nuclear spirals in the inner Milky Way in Monthly Notices of the Royal Astronomical Society

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Sale S (2015) Marginal likelihoods of distances and extinctions to stars: computation and compact representation in Monthly Notices of the Royal Astronomical Society

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Sale S (2012) 3D extinction mapping using hierarchical Bayesian models Hierarchical Bayesian extinction mapping in Monthly Notices of the Royal Astronomical Society

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Sale S (2014) A 3D extinction map of the northern Galactic plane based on IPHAS photometry in Monthly Notices of the Royal Astronomical Society

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Sale S (2018) Large-scale three-dimensional Gaussian process extinction mapping in Monthly Notices of the Royal Astronomical Society

 
Description The space between the stars in our Galaxy is filthy. It is full of "dust" (actually closer to soot) that is belched out by stars as a by product of their nuclear fuelling process. Most astronomers consider this soot a nuisance, as it obstructs our view of the rest of the Galaxy. On the other hand, some others are very interested in it, because it will be caught up in the formation of the next generation of stars.

This project developed a new way of mapping the 3d distribution of this soot within our Galaxy, starting from the well-established fact that absorption of starlight by these soot grains make stars appear redder than they truly are. The novel feature was the inclusion of a simple model of turbulence within the interstellar medium to allow us to consider 3d chunks of the Galaxy coherently, instead of treating single lines of sight individually.
Exploitation Route Application to real data to construct a coherent 3d dust map of the full Galaxy. This is a tough computational challenge though.
Sectors Education,Environment