The star formation and chemical evolution history of the Fornax dwarf spheroidal galaxy

• Published in: Astronomy and astrophysics (Berlin) Vol. 544; p. A73
• Main Authors: de Boer, T. J. L., Tolstoy, E., Hill, V., Saha, A., Olszewski, E. W., Mateo, M., Starkenburg, E., Battaglia, G., Walker, M. G.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.08.2012
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: dwarf; galaxies: evolution; galaxies: stellar content; Hertzsprung-Russell and C-M diagrams; Local Group; Galaxy evolution; Red giant stars; Stellar content; Red stars; galaxies: evolution; Abundance; Local group; Age metallicity relation; Hertzsprung-Russell and C-M diagrams; Young stars; galaxies: dwarf; Dwarf spheroidal galaxies; Giant stars; Star formation; Stellar mass; Dwarf galaxies; Metallicity; Stellar population; Elements (chemical); Radial gradient; Chemical evolution; Age; Photometry; galaxies: stellar content
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201219547

We present deep photometry in the B, V and I filters from CTIO/MOSAIC for about 270 000 stars in the Fornax dwarf spheroidal galaxy, out to a radius of rell ≈ 0.8 degrees. By combining the accurately calibrated photometry with the spectroscopic metallicity distributions of individual red giant branch stars we obtain the detailed star formation and chemical evolution history of Fornax. Fornax is dominated by intermediate age (1−10 Gyr) stellar populations, but also includes ancient (10−14 Gyr), and young (≤1 Gyr) stars. We show that Fornax displays a radial age gradient, with younger, more metal-rich populations dominating the central region. This confirms results from previous works. Within an elliptical radius of 0.8 degrees, or 1.9 kpc from the centre, a total mass in stars of 4.3 × 107   M⊙ was formed, from the earliest times until 250 Myr ago. Using the detailed star formation history, age estimates are determined for individual stars on the upper RGB, for which spectroscopic abundances are available, giving an age-metallicity relation of the Fornax dSph from individual stars. This shows that the average metallicity of Fornax went up rapidly from  [Fe/H]  ≤ −2.5 dex to  [Fe/H]  = −1.5 dex between 8−12 Gyr ago, after which a more gradual enrichment resulted in a narrow, well-defined sequence which reaches  [Fe/H]  ≈ −0.8 dex,  ≈3 Gyr ago. These ages also allow us to measure the build-up of chemical elements as a function of time, and thus determine detailed timescales for the evolution of individual chemical elements. A rapid decrease in [Mg/Fe] is seen for the stars with  [Fe/H]  ≥ −1.5 dex, with a clear trend in age.