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Astron. Astrophys. 357, 61-65 (2000)

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Is the [CO] index an age indicator for star forming galaxies?

L. Origlia 1 and E. Oliva 2

1 Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
2 Osservatorio di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received 1 November 1999 / Accepted 6 March 2000


The classical [CO] index, i.e. the strength of the [FORMULA]=2 CO absorption bands starting at 2.29 µm, is sometimes used to constrain the maximum age of star formation events in galaxies. In this paper we critically analyze

  • i)theoretical models which could predict either a factor of [FORMULA]2 drop or a pronounced increase of [CO] at ages older than 100 Myr, depending on the evolutionary tracks one adopts (see Fig. 1).

  • ii) observational data for young clusters in the LMC which do not show any strong relationship between the CO index and cluster age (see Fig. 3).

The above scenario indicates that the value of [CO] does not provide a reliable tool for estimating the age of stellar populations older than [FORMULA]10 Myr, i.e. after the first red supergiants have been formed. The contradictory results of theoretical models reflect problems in treating the evolution along the Asymptotic Giant Branch (AGB). In particular, those evolutionary synthesis models using stellar tracks which do not include the thermal pulsing AGB phase produce too weak CO features at 100-1000 Myr, i.e. in the range of ages when the near infrared emission is dominated by thermal pulsing AGB stars.

Key words: galaxies: starburst – galaxies: star clusters – galaxies: stellar content – infrared: stars

Send offprint requests to: L. Origlia (origlia@astbo3.bo.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000