Multi-element Abundance Measurements from Medium-resolution Spectra. IV. Alpha Element Distributions in Milky Way Satellite Galaxies
We derive the star formation histories of eight dwarf spheroidal (dSph) Milky Way satellite galaxies from their alpha element abundance patterns. Nearly 3000 stars from our previously published catalog comprise our data set. The average [α/Fe] ratios for all dSphs follow roughly the same path with increasing [Fe/H]. We do not observe the predicted knees in the [α/Fe] versus [Fe/H] diagram, corresponding to the metallicity at which Type Ia supernovae begin to explode. Instead, we find that Type Ia supernova ejecta contribute to the abundances of all but the most metal-poor ([Fe/H] < −2.5) stars.We have also developed a chemical evolution model that tracks the star formation rate, Types II and Ia supernova explosions, and supernova feedback. Without metal enhancement in the supernova blowout, massive amounts of gas loss define the history of all dSphs except Fornax, the most luminous in our sample. All six of the best-fit model parameters correlate with dSph luminosity but not with velocity dispersion, half-light radius, or Galactocentric distance.
Additional Information© 2011 American Astronomical Society. Received 2010 September 15; accepted 2010 November 16; published 2011 January 5. We thank John Johnson, Hai Fu, Julianne Dalcanton, Chris Sneden, and Bob Kraft for insightful discussions. Support for this work was provided by NASA through Hubble Fellowship grant 51256.01 awarded to E.N.K. by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. S.R.M. acknowledges support from NSF grants AST-0307851 and AST-0807945, and from the SIM Lite key project "Taking Measure of the Milky Way" under NASA/JPL contract 1228235. P.G. acknowledges NSF grants AST-0507483, AST-0607852, and AST-0808133. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
Published - Kirby2011p12852Astrophys_J.pdf
Accepted Version - 1011.5221.pdf