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Symposium 241

Abstract details

The Nature of Galactic Bulges from SAURON Absorption Line Strength Maps
Reynier Peletier, Jesus Falcon Barroso, et al.

We present absorption line-strength maps of a sample of 24 representative early-type spiral galaxies, mostly of type Sa, obtained with SAURON on the WHT. We present, for the first time, maps of H$\beta$, Fe5015, and Mg b. The absorption line maps show that many galaxies contain some younger populations, distributed in small or large nuclear disks, or in circumnuclear star forming rings, often related to bars. Comparison with H$\alpha$ star formation rates from the literature shows that these young stars are formed in circumnuclear mini-starbursts, which form most of the stars in the centres of early-type spirals. These mini-starburst cause a considerable scatter in index-index diagrams such as Mg b vs. H$\beta$ and Mg b vs. Fe 5015 , more than is measured for early-type galaxies. As a result, there is not only a wide range in ages, even within the galaxies, but also in abundance ratio Mg/Fe. We find that the central regions of early-type spirals are often dusty, with a good correlation between the presence of young central stellar populations and the amount of dust extinction. All galaxies lie on or below the Mg b -- $\sigma$ relation for elliptical galaxies in the Coma cluster by Jorgensen et al. (1996). If that relation is considered to be a relation for old galaxies we see that our sample of spirals has a considerable scatter in age, with the highest scatter at the lowest $\sigma$, i.e. for the faintest galaxies. Our result is in disagreement with results for highly inclined samples, where the galaxy populations are measured a few hundred parsec away from the centre on the dustfree minor axis. Here generally only old stellar populations are found. These two results can only be reconciled if star formation in the inner regions only occurs in a thin disk. Since this inner disk is dominating the light in many cases, one has to speak about a pseudobulge (Kormendy 1993). Above the plane, however, there is another galaxy component, the classical bulge, consisting of old stars. These components together form the bulge, in the same way as the thin and the thick disk co-exist in the solar neighbourhood. Many galaxies (11 or more out of 24) contain velocity dispersion drops in their centre. Relations between the stellar populations and these sigmadrops are discussed.

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