EDP Sciences
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Volume 366, Number 1, January IV 2001
Page(s) 68 - 82
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20000009

A&A 366, 68-82 (2001)
DOI: 10.1051/0004-6361:20000009

The formation of galaxy bulges: Spectrophotometric constraints

Ph. Prugniel1, G. Maubon2 and F. Simien1

1  CRAL-Observatoire de Lyon, CNRS: UMR 142, 69561 St-Genis-Laval Cedex, France
2  OMP, CNRS: UMR 5572, 14 avenue Edouard Belin, 31400 Toulouse, France

(Received 23 May 2000 / Accepted 24 October 2000)

We have measured Mg2, Fe 5270 and Fe 5335 spectrophotometric indices (LICK system) in the bulge of 89 galaxies, mostly spirals from the Héraudeau (1996) sample. The indices are reduced to a null velocity dispersion and normalized to an aperture of 0.2 h-1 kpc. The mean errors are 0.009 mag on Mg2, and 0.3 Åon the iron indices. These measurements almost double the amount of similar data already available on spiral galaxies. Our data confirm the existence of the relation between Mg2 and $\sigma_0$, the central stellar velocity dispersion; we find an even tighter relation between Mg2 and $V^{\rm m}_{\rm rot}$, the maximum rotational velocity of the galaxy, deduced from HI observations. For the most massive bulges, these correlations may be interpreted as a mass-metallicity relation. However, the presence of young stellar populations, traced by the detection of [OIII] $\lambda$ 5007 Åemission, provides clear evidence that age effects do play a role. Since the contribution of the young population is anti-correlated to the mass of the galaxy, it continues the Mg2 vs. $\sigma_0$ relation toward the low-$\sigma_0$ region and globally increases its slope. We also present evidence for a new positive correlation between Fe indices and $\sigma_0$, and for a significant correlation between the line-strength indices and the total or disk luminosity. We propose to model the whole sequence of bulges within the following framework: bulges are composed of a primary population formed prior to the disk, during the initial collapse, and of a secondary population formed during its evolution. The whole family of bulges can be classified into three classes: (A) the bulges dominated by young populations are generally small, have ionized gas, low velocity dispersion and low line strengths; (B) the bulges dominated by the primary population lie along the mass-metallicity sequence defined for elliptical galaxies; and (C) the bulges where the secondary population is significant are less Mg-over-abundant than (B)-type bulges and deviate from the Mg2 vs. $\sigma_0$ relation of elliptical galaxies.

Key words: galaxies: general -- galaxies: spiral -- galaxies: fundamental parameters -- galaxies: stellar content -- galaxies: evolution

Offprint request: Ph. Prugniel, prugniel@obs.univ-lyon1.fr

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