Formation and evolution of early-type dwarf galaxiesa two-dimensional perspective

Resumo

Dwarf elliptical galaxies (dEs) are the most common galaxy class in dense environments, outnumbering all other classes combined. Their low masses make them ideal test beds for studying different mechanisms that shape galaxies, since both external influences and internal feedback mechanisms are far more extreme in dwarfs than in massive galaxies. They also make them challenging to observe. dEs are a surprisingly inhomogeneous class: which has made it difficult to relate different dE subtypes to each other, as well as to place the whole class in the larger context of galaxy assembly and (trans)formation processes. In this thesis we present the kinematic, dynamical and stellar population analysis of 12 dEs observed using the SAURON integral-field unit mounted on the 4.2m William Herschel Telescope (Roque de los Muchachos Observatory, La Palma). The idea behind the project was to obtain large-scale two-dimensional maps of kinematic and stellar population properties for objects for which (with very few and small-scale exceptions) only one-dimensional profiles were available before. With integral-field data it is possible not only to reduce the uncertainties on radial properties, but also to characterize potential substructures far more accurately, and to provide detailed information on the dynamical structure of observed systems. % Our integral field survey of Virgo and field dEs provides the first stellar kinematic and population maps of such quality and spatial extent for cluster dEs. We show that no two galaxies in our sample are alike. We see that the level of rotation is not tied to flattening (we have, e.g. round rotators and flattened nonrotators), we observe kinematic twists in one Virgo and one field object; we discover large-scale kinematically-decoupled components in two field galaxies, and we see varying gradients in line-strength maps, from nearly flat to strongly peaked in the center. The great variety of morphological, kinematic, and stellar population parameters seen in our data points to a formation scenario in which properties are shaped stochastically. We construct Jeans axisymmetric models and obtain total dynamical masses enclosed within one effective radius. We use the obtained values to show that the validity of the dynamical scaling relations of massive early-type galaxies (ETGs) can be extended to these low-mass systems (virial mass estimator vs. $(M/L)_{dyn}$, $V_{circ}-\sigma$, and Mass Plane relations), except that dEs seem to contain relatively larger fraction of dark matter in their inner parts. We use these dynamical results together with the line-strength-based stellar population analysis to confirm that, in the single stellar population (SSP) sense, dEs are slightly younger and more metal-poor than the more massive early-type galaxies, with their abundances scattered around the solar value. Also, dEs lie on the extension of the line-stength - sigma relations of ETGs, especially clearly seen for the well-know Mgb - sigma relation. We demonstrate that dEs have lower angular momenta than the present-day analogs of their presumed late-type progenitors and we show that dE circular velocity curves are steeper than the rotation curves of galaxies with equal and up to an order of magnitude higher luminosity. Additionally, we see that galaxies in the cluster outskirts tend to have a higher dark-to-stellar matter ratio. Transformation due to tidal harassment is able to explain all of the above, unless the dE progenitors were already compact and had lower angular momenta at higher redshifts. We then look at the star formation histories (SFHs) of our galaxies and find that for the majority of them star formation activity was either still strong at a few Gyr of age or they experienced a secondary burst of star formation roughly at that time. This latter possibility would be in agreement with the scenario where tidal harassment drives the remaining gas inwards and induces a secondary star formation episode. Additionally, our data shows that there exists a correlation between metallicity gradient and clustrocentric distance for Virgo dEs, with stronger gradients towards the center. This, again, favors the harassment scenario as it can be seen in the context of central star formation bursts caused by tidal interactions. Finally, one of our galaxies appears to be composed exclusively of an old population (>~12 Gyr). Combining this with our earlier dynamical results, we conclude that it either was ram-pressure stripped early on in its evolution in a group environment and subsequently tidally heated (which lowered its angular momentum and increased compactness), or that it evolved in situ in the cluster's central parts, compact enough to avoid tidal disruption. This work has therefore provided a number of important insights into the studied objects, presenting the community with both a carefully analyzed high-quality dataset for dwarf early-type galaxies, as well as providing stronger constraints on their formation and evolution scenarios. From the variety of dE properties we find and confirm with our data, and through its analysis, we are able to show tidal harassment signatures in the galaxies' dynamical and stellar population properties, paying particular attention to radial trends within the cluster. We have thus been able to establish dE progenitor class with more accuracy and define the place of dEs among other galaxy types.