Cosmology

Abstract:

We have studied ~2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX UV photometry with corollary optical data from the SDSS, we find that, at a 95 percent confidence level, at least ~30 percent of galaxies in this sample have UV to optical colours consistent with some recent star formation within the last Gyr. In particular, galaxies with a NUV - r colour less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semi-analytical LCDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ~1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ~300 to 500 Myrs. We also find that monolithically evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colours (NUV - r < 5.5) seen in a significant fraction (~30 percent) of our observed sample.

Abstract:

We discuss the relative merits of mid-infrared and X-ray selection of type 2 quasars. We describe the mid-infrared, near-infrared and radio selection criteria used to find a population of redshift z ∼ 2 type 2 quasars which we previously argued suggests that most supermassive black hole growth in the Universe is obscured. We present the optical spectra obtained from the William Herschel Telescope, and we compare the narrow emission-line luminosity, radio luminosity and maximum size of jets to those of objects from radio-selected samples. This analysis suggests that these are genuine radio-quiet type 2 quasars, albeit the radio-bright end of this population. We also discuss the possibility of two different types of quasar obscuration, which could explain how the ∼2-3:1 ratio of type 2 to type 1 quasars preferred by modelling our population can be reconciled with the ∼1:1 ratio predicted by unified schemes. © 2006 RAS.

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.