Professor Roger Davies

Abstract:

We present the fundamental plane (FP) for 38 early-type galaxies in the two rich galaxy clusters RX J0152.7-1357 (z = 0.83) and RX J1226.9+3332 (z = 0.89), reaching a limiting magnitude of MB = -19.8 in the rest frame of the clusters. While the zero-point offset of the FP for these high-redshift clusters relative to our low-redshift sample is consistent with passive evolution with a formation redshift of zform ≈ 3.2, the FP for the high-redshift clusters is not only shifted as expected for a mass-independent zform but rotated relative to the low-redshift sample. Expressed as a relation between the galaxy masses and the mass-to-light ratios, the FP is significantly steeper for the high-redshift clusters than for our low-redshift sample. We interpret this as a mass dependency of the star formation history, as has been suggested by other recent studies. The low-mass galaxies (10 10.3 M⊙) have experienced star formation as recently as z ≈ 1.35 (1.5 Gyr prior to their look-back time), while galaxies with masses larger than 1011.3 M⊙ had their last major star formation episode at z > 4.5. © 2006, The American Astronomical Society. All rights reserved.

Abstract:

Following the pilot study of Kuntschner et al., we have investigated the properties of a volume-and magnitude-limited (cz > 10 000 km s-1, bJ > 16) sample of early-type galaxies that were carefully selected from the Anglo-Australian Observatory (AAO) two-degree field galaxy redshift survey (2dFGRS) to have no more than one and five companions within 1 and 2 Mpc, respectively. We used images from the Digital Sky Survey (DSS) to confirm the E/SO morphologies. We augmented this sample with field galaxies from Colbert et al. selected as having no neighbour within 1 Mpc and ±1000 km s-1. We present spectroscopic observations of 22 galaxies from the combined sample, from which central velocity dispersions and the Lick stellar population indices were measured. After carefully correcting the spectra for nebular emission, we derived luminosity-weighted ages, metallicities and α-element abundance ratios. We compare these isolated galaxies with samples of early-type galaxies in the Virgo and Coma clusters, and also with the previous sample of galaxies in low-density regions of Kuntschner et al. We find that galaxies in low-density environments are younger and have a greater spread of ages compared to cluster galaxies. They also show a wider range of metallicities at a given velocity dispersion than cluster galaxies, which display only supersolar metallicities. On average cluster, as well as, isolated galaxies show non-solar abundance ratios in α elements, suggesting that, independent of galactic environment, star formation occurred on short time-scales. However, the abundance ratios for our low-density environment sample galaxies do not scale with the stellar velocity dispersion as observed in clusters. In fact we detect a large spread at a given velocity dispersion even reaching solar abundance ratios. The metallicity of isolated early-type galaxies is found to correlate weakly with σ. We reason that early-type galaxies in low-density environments experienced merging-induced star formation episodes over a longer and more recent period of time compared to a cluster environment, and speculate that a considerable fraction of their stars formed out of low-metallicity halo gaseous material during the slow growth of a stellar disc between merging events. © 2006 RAS.

Abstract:

We seek to understand whether the stellar populations of galactic bulges show evidence of secular evolution triggered by the presence of the disc. For this purpose, we re-analyse the sample of Proctor and Sansom, deriving stellar population ages and element abundances from absorption-line indices as functions of central velocity dispersion and Hubble type. We obtain consistent constraints on ages from the three Balmer-line indices Hβ, Hγ and Hδ, based on stellar population models that take the abundance ratio effects on these indices into account. Emission-line contamination turns out to be a critical aspect, which favours the use of the higher order Balmer-line indices. Our derived ages are consistent with those of Proctor and Sansom based on a completely different method. In agreement with other studies in the literature, we find that bulges have relatively low luminosity weighted ages, the lowest age derived being 1.3 Gyr. Hence, bulges are not generally old, but actually rejuvenated systems. We discuss evidence that this might be true also for the bulge of the Milky Way. The data reveal clear correlations of all three parameters luminosity weighted age, total metallicity and α/Fe ratio with central velocity dispersion. The smallest bulges are the youngest with the lowest α/Fe ratios owing to late Fe enrichment from Type Ia supernovae. Using models combining recent minor star formation with a base old population, we show that the smallest bulges must have experienced significant star formation events involving 10-30 per cent of their total mass in the past 1-2 Gyr. No significant correlations of the stellar population parameters with Hubble type are found. We show that the above relationships with σ coincide perfectly with those of early-type galaxies. In other words, bulges are typically younger, metal poorer and less α/Fe enhanced than early-type galaxies because of their smaller masses. At a given velocity dispersion, bulges and elliptical galaxies are indistinguishable as far as their stellar populations are concerned. These results favour an inside-out formation scenario and indicate that the discs in spiral galaxies of Hubble types Sbc and earlier cannot have a significant influence on the evolution of the stellar populations in the bulge component. The phenomenon of pseudo-bulge formation must be restricted to spirals of types later than Sbc. © 2005 RAS.