Cosmology

Abstract:

The colour-magnitude relation (CMR) of cluster ellipticals has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic and merger paradigms of elliptical galaxy formation. We investigate the elliptical CMR predicted in the merger paradigm by using a LCDM hierarchical merger model. We first highlight sections of the literature which indicate that the traditional use of fixed apertures to derive colours gives a distorted view of the CMR due to the presence of colour gradients in galaxies. Fixed aperture observations make the CMR steeper and tighter than it really is. We then show that the star formation history (SFH) of cluster ellipticals predicted by the model is quasi-monolithic, with over 95 percent of the total stellar mass formed before a redshift of 1. The quasi-monolithic SFH produces a predicted CMR that agrees well at all redshifts with its observed counterpart once the fixed aperture effect is removed. More importantly, we present arguments to show that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, because a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.

Abstract:

Kinematic and population studies show that bulges are generally rotationally flattened systems similar to low-luminosity ellipticals. However, observations with state-of-the-art integral field spectrographs, such as SAURON, indicate that the situation is much more complex, and allow us to investigate phenomena such as triaxiality, kinematic decoupling and population substructure, and to study their connection to current formation and evolution scenarios for bulges of early-type galaxies. We present the examples of two SO bulges from galaxies in our sample of nearby galaxies: one that shows all the properties expected from classical bulges (NGC 5866), and another case that presents kinematic features appropriate for barred disk galaxies (NGC 7332).