Photometric metallicities in Boötes I. Issue 1 (4th February 2014)

• Record Type: Journal Article
• Authors: Hughes, J.; Wallerstein, G.; Dotter, A.; Geisler, D.
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 439:Issue 1(2014)
• Page Start: 788
• Page End: 809
• Publication Date: 2014-02-04
• Subjects: galaxies: abundances -- galaxies: dwarf -- Local Group; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stt2500
• Languages: English
• ISSNs: 0035-8711

Abstract

Abstract: 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 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 ∼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 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.