Photometric metallicities in Boötes I

We present new Strömgren and Washington data sets for the BoÖtes I dwarf galaxy, and combine them with the available Sloan Digital Sky Survey 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 (UDFs) 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 m1-index on the lower red giant branch, we replace the Strömgren μ filter with the broader Washington C filter to minimize observing time. We construct two indices: m_* = (C - T₁)0-(T₁ - T₂)0 and m_** = (C - b)₀ - (b - y)₀. We find that CT₁by is the most successful filter combination, for individual stars with [Fe/H] < -2.0, to maintain ~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) colour 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 there is a metallicity spread of at least -1.9 > [Fe/H] > -3.7. The best-fitting Dartmouth isochrones give a mean age, for all the Boötes I stars in our data set, of 11.5 ± 0.4 Gyr. From ground-based telescopes, we show that the optimal filter combination is CT₁by, avoiding the v filter entirely. We demonstrate that we can break the isochrones' age- metallicity degeneracy with the CT₁by filters, using stars with log g = 2.5-3.0, which have less than a 2 per cent change in their (C - T₁) colour due to age, over a range of 10-14 Gyr.