The formation of galaxy bulges: Spectrophotometric constraints

Free access article

Issue		A&A 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)

Abstract
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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