The Integrated Spectrum of M67 and the Spectroscopic Age of M32

Details The Integrated Spectrum of M67 and the Spectroscopic Age of M32 The Integrated Spectrum of M67 and the Spectroscopic Age of M32

Mar 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aj....127.1513s&link_type=abstract

The Astronomical Journal, Volume 127, Issue 3, pp. 1513-1530.

Astronomy and Astrophysics

Astronomy

44

Galaxies: Clusters: General, Galaxies: Individual: Messier Number: M32, Galaxies: Stellar Content, Galaxy: Open Clusters And Associations: Individual: Messier Number: M67, Stars: Evolution

Scientific paper

We construct an integrated spectrum of the intermediate-age, solar-metallicity Galactic cluster M67, from individual spectroscopic observations of bona fide cluster members. The spectrum so obtained is used as a template to test our stellar population synthesis models, in an age and metallicity regime where such models remain largely untested. As a result, we demonstrate that our models predict a spectroscopic age of 3.5+/-0.5 Gyr for M67, which is the same age we obtain from fitting isochrones to the color-magnitude diagram of the cluster. Full consistency is reached when using either Hβ, Hγ, or Hδ as the age indicator. We also check if the models, when applied to the cluster integrated spectrum, predict elemental abundances in agreement with the known detailed abundance pattern of the cluster. The models also pass the latter test, by predicting the abundances of iron, magnesium, carbon, and nitrogen in agreement with detailed abundance analyses of cluster stars to within 0.1 dex. Encouraged by the high degree of consistency of our models, we apply them to the study of the integrated spectrum of the central 3" of the compact elliptical galaxy M32. The resulting luminosity-weighted age of the galaxy ranges between 2 and 3.5 Gyr, depending on the age indicator adopted. According to our models, the center of M32 seems to have a supersolar iron abundance, ranging between [Fe/H]~+0.1 and +0.3, depending on the spectral index adopted. The light element magnesium seems to be underabundant in the center of M32 relative to iron by about ~0.1-0.2 dex, whereas the data are consistent with nearly solar carbon and nitrogen abundances relative to iron. We find that single-age, single-metallicity stellar population models with solar-scaled abundance patterns cannot fit all the Balmer and metal lines in the integrated spectrum of M32. In particular, there is a systematic trend in the sense that bluer absorption lines indicate a younger age and a higher metallicity. This slight inconsistency can be due either to (unaccounted for) abundance ratio effects on blue iron and Balmer line indices or to a spread of the ages of the stellar populations in M32. Current stellar population models cannot break this degeneracy at the level of accuracy required to address this problem.

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