The episodic star formation history of the Carina dwarf spheroidal galaxy

We present deep photometry of the Carina dwarf spheroidal galaxy in theB and V filters from CTIO/MOSAIC out to and beyond the tidal radius ofrell ≈ 0.48 degrees. The accurately calibrated photometryis combined with spectroscopic metallicity distributions of red giantbranch (RGB) stars to determine the detailed star formation and chemicalevolution history of Carina. The star formation history (SFH) confirmsthe episodic formation history of Carina and quantifies the duration andstrength of each episode in great detail as a function of radius fromthe centre. Two main episodes of star formation occurred at old (>8Gyr) and intermediate (2-8 Gyr) ages, both enriching stars starting fromlow metallicities ([Fe/H] <- 2 dex). By dividing the SFH into twocomponents, we determine that 60 ± 9 percent of the total numberof stars formed within the intermediate-age episode. Furthermore, withinthe tidal radius (0.48 degrees or 888 pc) a total mass in stars of 1.07± 0.08 × 106 M⊙ was formed,giving Carina a stellar mass-to-light ratio of 1.8 ± 0.8. Bycombining the detailed SFH with spectroscopic observations of RGB stars,we determined the detailed age-metallicity relation of each episode andthe timescale of α-element evolution of Carina from individualstars. The oldest episode displays a tight age-metallicity relationduring ≈6 Gyr with steadily declining α-element abundances anda possible α-element "knee" visible at [Fe/H] ≈ - 2.5 dex. Theintermediate-age sequence displays a more complex age-metallicityrelation starting from low metallicity and a sequence in α-elementabundances with a slope much steeper than observed in the old episode,starting from [Fe/H] = -1.8 dex and [Mg/Fe] ≈ 0.4 dex and decliningto Mg-poor values ([Mg/Fe] ≤ - 0.5 dex). This clearly indicates thatthe two episodes of star formation formed from gas with differentabundance patterns, which is inconsistent with simple evolution in anisolated system.Tables 1-3 are only available in electronic at the CDS via anonymous ftpto http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A10Appendicesare available in electronic form at http://www.aanda.org