Dark Matter Halos
The orbits of stars around galaxies are influenced gravitational attraction of all the mass - stars, gas, dust and dark matter - that they contain. The most mysterious of these types of matter, precisely because we have yet to detect it directly, is dark matter. Cosmological simulations of structure formation (like the one taken from the Via Lactea II simulation to the right) suggest that the dark matter in and around galaxies should be distributed in an ellipsoidal halo that extends roughly ten times further than the bulk of main galaxy's stars. The also suggest that these halos should be filled with substructure - smaller "sub halos" of dark matter in orbit around each galaxy in addition to the larger halos encompassing them.
Measuring the shape, size and mass in our own dark matter halo
Debris from the destruction of satellite galaxies is a particularly powerful probe of where the dark matter is around galaxies because: the debris is so low mass and that it does not appreciably alter the mass distribution - and hence orbits of stars - in the galaxy itself; it is spread out far around the main galaxy to probe deep into the dark matter halo; and the evolution of debris can be simply modeled
A major goal of this research group is to work on methods to map dark matter halos of our own Galaxy using debris we observe around us. One approach is the REWINDER code which takes the positions of stars we observe today in a debris structures and turns back the clock to "rewind" them to where they came from. In a good model of our dark matter halo, the orbits we derive for these stars will eventually intersect with the orbit of their original satellite galaxy. If the model is not good, this will not happen.
Detecting dark "subhalos" of matter that contain no stars
- This work was supported in part by a NASA LTSA grant (1999-2002), NSF CAREER grant (2002-2007), NSF grant AST-1312196 (2013-2016) and NASA grant (2015-2017)
- It is described in Johnston et al (1999), Johnston, Spergel & Haydn (2002), Yoon, Johnston & Hogg (2010), Johnston et al (2005), Law, Majewski & Johnston (2005), Law, Majewski & Johnston (2009), Price-Whelan & Johnston (2013), Price-Whelan et al (2014) - see publications