We present new Strömgren and Washington data sets for the Boötes I dwarf galaxy , and combine them with the available SDSS photometry .
The goal of this project is to refine a ground-based , practical , accurate method to determine age and metallicity for individual stars in Boötes I that can be selected in an unbiased imaging survey , without having to take spectra .
With few bright upper-red-giant branch stars and distances of about 35 - 250 kpc , the ultra-faint dwarf galaxies present observational challenges in characterizing their stellar population .
Other recent studies have produced spectra and proper motions , making Boötes I an ideal test case for our photometric methods .
We produce photometric metallicities from Strömgren and Washington photometry , for stellar systems with a range of -1.0 > [ Fe / H ] > -3.5 .
Needing to avoid the collapse of the metallicity sensitivity of the Strömgren m _ { 1 } -index on the lower-red-giant branch , we replace the Strömgren v -filter with the broader Washington C -filter to minimize observing time .
We construct two indices : m _ { * } = ( C - T _ { 1 } ) _ { 0 } - ( T _ { 1 } - T _ { 2 } ) _ { 0 } , and m _ { ** } = ( C - b ) _ { 0 } - ( b - y ) _ { 0 } .
We find that CT _ { 1 } by is the most successful filter combination , for individual stars with [ Fe / H ] < -2.0 , to maintain \sim 0.2 dex [ Fe/H ] -resolution over the whole red-giant branch .
The m _ { ** } -index would be the best choice for space-based observations because the ( C - y ) color is not sufficient to fix metallicity alone in an understudied system .
Our photometric metallicites of stars in the central regions of Boötes I confirm that that there is a metallicity spread of at least -1.9 > [ Fe / H ] > -3.7 .
The best-fit Dartmouth isochrones give a mean age , for all the Boötes I stars in our data set , of 11.5 \pm 0.4 Gyr .
From ground-based telescopes , we show that the optimal filter combination is CT _ { 1 } by , avoiding the v -filter entirely .
We demonstrate that we can break the isochrones ’ age-metallicity degeneracy with the CT _ { 1 } by filters , using stars with \log g = 2.5 - 3.0 , which have less than a 2 per cent change in their ( C - T _ { 1 } ) -colour due to age , over a range of 10-14 Gyr .