Volume 572, December 2014
|Number of page(s)||14|
|Published online||18 November 2014|
1 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
2 Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
3 Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
4 National Optical Astronomy Observatory, PO Box 26732, Tucson, AZ 85726, USA ⋆⋆⋆
The National Optical Astronomy Observatory is operated by AURA, Inc., under cooperative agreement with the National Science Foundation.
5 Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA
6 Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090, USA
7 INAF − Osservatorio Astronomico di Bologna via Ranzani 1, 40127 Bologna, Italy
Received: 2 May 2014
Accepted: 10 September 2014
We present deep photometry of the Carina dwarf spheroidal galaxy in the B and V filters from CTIO/MOSAIC out to and beyond the tidal radius of rell ≈ 0.48 degrees. The accurately calibrated photometry is combined with spectroscopic metallicity distributions of red giant branch (RGB) stars to determine the detailed star formation and chemical evolution history of Carina. The star formation history (SFH) confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail as a function of radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2−8 Gyr) ages, both enriching stars starting from low metallicities ([Fe/H] < − 2 dex). By dividing the SFH into two components, we determine that 60 ± 9 percent of the total number of stars formed within the intermediate-age episode. Furthermore, within the 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. By combining the detailed SFH with spectroscopic observations of RGB stars, we determined the detailed age-metallicity relation of each episode and the timescale of α-element evolution of Carina from individual stars. The oldest episode displays a tight age-metallicity relation during ≈6 Gyr with steadily declining α-element abundances and a possible α-element “knee” visible at [Fe/H] ≈ − 2.5 dex. The intermediate-age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in α-element abundances 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 declining to Mg-poor values ([Mg/Fe] ≤ − 0.5 dex). This clearly indicates that the two episodes of star formation formed from gas with different abundance patterns, which is inconsistent with simple evolution in an isolated system.
Key words: Local Group / galaxies: stellar content / galaxies: evolution / galaxies: dwarf / Hertzsprung-Russell and C-M diagrams
Tables 1–3 are only available in electronic at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A10
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014
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