1 Introduction

Observations of several spectral indices and their calibration had led to a deeper insight of their dependence on the chemical history of stellar populations of the galaxies and globular clusters. A lot of observational work was concentrated on the strong absorption feature of Mg around 5175 Å. The most widely measured parameter is the Mg₂ index which describes the flux deficit at the region of Mg H and Mg I b triplet compared with the neighboring continuum.

To date, among the most comprehensive examples of such works are the studies of:

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$7\ Samurai$ (Davies et al. 1987 - hereafter 7Sam, and Faber et al.1989) for 479 galaxies;

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Jørgensen et al.(1995 - hereafter JFK95) with new data for 182 galaxies and recalibrated data for 90 galaxies;

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Jørgensen (1997) with Mg₂ data for 222 E and S0 galaxies in total;

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Lucey et al.(1997) with 141 new measurements for 78 galaxies and recalibrated data for 72 galaxies; and

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Smith et al.(1997) with Mg₂ data for 137 galaxies.

Most recently the full Lick/IDS (Image Dissector Scanner) database of Mg₂ indices for 381 galaxies and 38 globulars were published by Trager et al.(1998 - hereafter Lick98).

The purpose of this work is to gather all measurements of the Mg₂ index available and, when it is possible, to transform them after appropriate corrections to a standardized homogeneous system.

The Mg₂ index definition introduced initially by Faber (1973), and Faber et al.(1977), can be found in Burstein et al.(1984); Faber et al.(1985) and in Brodie & Huchra (1990). This definition is continuously refined in newer works (see, for instance, Worthey et al.1994 and Lick98 for more detailed description). Following Worthey et al.(1994), the raw value of the Mg₂ index, (Mg$_2)_\mathrm{obs}$, in magnitudes is extracted as:

$$\displaystyle (\mathrm{Mg}_2)_\mathrm{obs} = - 2.5\,\log\,\... ...ac{F_{\rm I\lambda}}{F_{\rm C\lambda}} {\rm d}\lambda \right],$$ (1)

where $F_{\rm I\lambda}$ is the flux (in ergs s^-1 cm^-2 Å^-1) in the MgH and Mg b which is located at $\lambda\lambda\/5154.125{-}5196.625$ Å. Here the local continuum, ${F_{\rm C\lambda}}$, represents the run of flux defined by a stright line connecting the flux levels at midpoints of the corresponding blue and red "continuum'' bandpasses (which are really pseudocontinua) at $\lambda\lambda\/4895.125{-}4957.625$ Å and $\lambda\lambda\/5301.125{-}5366.125$ Å. Throughout the paper, this definition is used.

A total of 41 publications, corresponding to 55 datasets were used to compile the catalogue. Here a dataset is a series of measurements sharing common characteristics: same observational setup, same reduction method.

The measurements of Mg₂ indices of Milky Way's globular clusters are also incorporated into the catalogue using the data of Burstein et al.(1984) and Lick98, because they are fundamental for the calibration of the age and metallicity of extragalactic stellar populations.

The extended set of Mg₂ observations of objects in Canada-France Redshift Survey (up to $z\,\approx\,1$), published by Hammer et al.(1997), is not included in our catalogue since it concerns mostly distant galaxies for which the corrections into our standard system are uncertain.

The homogenisation of the Mg₂ indices is described in Sect. 2, and the catalogue is presented in Sect. 3. The completeness and some statistical properties of the sample of galaxies in this catalogue are discussed in Sect. 4.

This catalogue is maintained up-to-date in the database HYPERCAT, developed at the Observatoire de Lyon (http://www-obs.univ-lyon1.fr/~prugniel/cgi-bin/hyper-cat/). Check this URL for recent version.