safely ignore the RRL distance uncertainties in our analysis. Note that we have checked using mock stellar haloes from the Auriga simulation suite (see Section \[sec:sims\]) that statistical distance uncertainties of $\sim 10\%$ make little difference to our results.

Blue Horizontal Branch
----------------------

Blue Horizontal Branch (BHB) stars, like RRL, are an old, metal poor population used widely in the literature to study the distant halo (e.g. @xue08 [@deason12b]). BHBs have relatively bright absolute magnitudes ($M_g \sim 0.5$), which can be simply parametrised as a function of colour and metallicity (e.g. @deason11c [@fermani13a]). However, unlike their RRL cousins, photometric samples of BHB stars are often significantly contaminated by blue straggler stars, which have similar colours but higher surface gravity. Spectroscopic samples of BHBs can circumvent this problem by using gravity sensitive indicators to separate out the contaminants (e.g. @clewley02 [@sirko04; @xue08; @deason12b]).

In this work we use the spectroscopic SEGUE sample of BHB stars compiled by [@xue11]. This sample was selected to be relatively “clean” of higher surface gravity contaminants, and has already been exploited in a number of works to study the stellar halo (e.g. @xue11 [@deason12a; @kafle13; @hattori13]). By cross-matching this sample with the SDSS-*Gaia* catalog, we identify 