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Astron. Astrophys. 325, 1025-1030 (1997)

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1. Introduction

In order to study the stellar populations of galaxies it is useful to test the capability of some spectral features of revealing the presence of stars of different ages.

The quantities here considered are the break at 4000 Å [FORMULA] and the equivalent widths (EW) of the Balmer lines, in particular [FORMULA]. Unlike several other spectral indices, they do not require high quality spectra and are therefore suitable for studying galaxies at intermediate and high redshifts.

[FORMULA] is largely used to determine the star formation characteristics of distant field and cluster galaxies. Hereafter the 4000 Å break is defined as the ratio between the average flux density in [FORMULA] between 4050 and 4250 Å and that between 3750 and 3950 Å (Bruzual 1983).

The analysis of the Balmer lines has been employed in a countless number of studies of stellar clusters, local and distant galaxies; indices including Balmer lines are useful for estimating the ages of star clusters, in the comparison between globular clusters and ellipticals and in the detection of recent starbursts in galaxies. A strong absorption [FORMULA] line has been detected in several cluster galaxies with a wide range of colours at [FORMULA] (Couch & Sharples 1987, Dressler & Gunn 1992, Caldwell et al. 1993, Belloni et al. 1995). This feature is usually interpreted as evidence of a burst of star formation ended 0.5-1.5 Gyr prior to the moment of observation (Dressler & Gunn 1983), assuming implicitely that the A-type stars responsible for the Balmer lines are in the main sequence phase. Here we investigate if stars in other evolutionary phases could give rise to a strong [FORMULA] line in order to determine if and when this feature is an unambiguous sign of recent star formation.

Although largely used, these features lack a systematic analysis (such a study exists for the [FORMULA] indices, see Buzzoni et al. 1994 and references therein), needed to clarify their dependence on metallicity (compare for instance Dressler & Shectman (1987), Kimble et al. (1989) for the [FORMULA] index and Bica & Alloin (1986), Gregg (1994) for the Balmer lines). This problem is related to the well known age-metallicity degeneracy, investigated by many authors especially for old stellar populations (Jones & Worthey 1995 and references therein). Compared to previous analysis, the spectral quantities here considered are influenced also by more recent and even present star formation.

Considering stellar models instead of observed stellar libraries allows one to investigate the whole range of stellar parameters. Single stellar population (SSP) and galaxy integrated spectra are computed by means of an evolutionary synthesis model that includes both the stellar contribution and the emission of the ionized gas (Barbaro & Poggianti 1997). The advantages in taking into account a wide range of metallicities and the main advanced stellar stages, in particular the horizontal branch (HB) phase, will emerge clear from the subsequent discussion. The problem of the "second parameter", i.e. the fact that stellar clusters of comparable metallicities may have distinct horizontal branch morphologies, is not taken into account, being this question not directly relevant to the present study (see de Freitas Pacheco & Barbuy 1995 for an analysis of the [FORMULA] indices of clusters of different HB morphologies). The total (stellar absorption + gaseous emission) EWs of the Balmer lines were measured by direct numerical integration, using the SPLOT program in IRAF and setting interactively the continuum levels and the integration limits. For SSPs and galaxies a Salpeter (1955) Initial Mass Function (IMF) between 0.1 and 100 [FORMULA] was assumed.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998