Astron. Astrophys. 359, 18-28 (2000)

Models for the interpretation of CaT and the blue spectral indices in elliptical nuclei ^*

M. Mollá ¹ and M.L. García-Vargas <sup>2

1</sup> Departement de Physique, Université Laval, Chemin St-Foy, G1K 7P4-Quebec, Canadá ^**
2 Instituto de Astrofísica de Canarias, GRANTECAN Project, Vía Láctea S/N, 38200 La Laguna, Tenerife, Spain

Received 2 February 2000 / Accepted 26 April 2000

Abstract

We present a grid of theoretical models where the calculation of absorption line spectral indices in both the blue and red wavelength ranges is done with the same evolutionary synthesis code. We have computed some of these indices: CaT, Na I , Mg I in the near infrared and Mgb, Mg₂, Fe52, Fe53, NaD and H, in the blue-visible range, for Single Stellar Population (SSP) of 6 different metallicities, (Z=0.0004, 0.001, 0.004, 0.008, 0.02 and 0.05), and ages from 4 Myr to 20 Gyr.

From the comparison of these evolutionary synthesis models with a compilation of elliptical galaxy data from the literature, we find that the observed CaT index follows the blue index rather than Mg₂ as the models predict. If this implies an over-abundance [Mg/Ca] and we take into account the masses of stars which produce Mg and Ca, these stars could form in a time scale shorter than 5 Myr from the beginning of the star formation process. Alternatively, an IMF biased towards very massive stars (M) at the early epoch of star formation in elliptical nuclei has to be assumed. We also suggest to revise the calculation of the nucleosynthesis yield of Magnesium.

By using the diagnostic diagram CaT-H to disentangle age and metallicity in such populations, we obtain around solar abundances and a sequence of ages between 4 and 16 Gyr for the galaxy sample.

Key words: galaxies: elliptical and lenticular, cD

* Complete set of Table 2 is only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr/J/A+A/359/018 (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/359/018
** Present address: Dep. de Física Teórica, Universidad Autónoma de Madrid, 28049 Cantoblanco, Madrid, Spain

Send offprint requests to: Mercedes Mollá

Correspondence to: mercedes@pollux.ft.uam.es

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Contents

• 1. Introduction
• 2. The evolutionary synthesis model
  • 2.1. Model description
  • 2.2. Model results
• 3. Data analysis
  • 3.1. Comparison of data with models
  • 3.2. Disentangling age and metallicity: the CaT-H plane
• 4. Discussion
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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