Galaxy formation and evolution

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.

Abstract:

We discuss the selection of star-forming galaxies at z6 through the Lyman-break technique. Spitzer imaging implies many of these contain older stellar populations (>200Myr) which produce detectable Balmer breaks. The ages and stellar masses (1010M) imply that the star formation rate density at earlier epochs may have been significantly higher than at z6, and might have played a key role in re-ionizing the universe. © 2007 International Astronomical Union.

Abstract:

We exploit the gravitational potential of massive cluster lenses to probe the emission-line properties of six galaxies which appear as highly magnified luminous arcs. Using the Gemini Multi-Object Spectrograph (GMOS) integral field spectrograph together with detailed cluster lens models, we reconstruct the intrinsic morphologies and two-dimensional velocity fields in these galaxies on scales corresponds to ∼0.5 kpc (unlensed) at . Four of the galaxies have stable disc-like kinematics, whilst the other two resemble interacting or starburst galaxies. These galaxies lie close to the mean rest-frame I-band Tully-Fisher relation for nearby spirals suggesting a clear preference for hierarchical growth of structure. In the rest-frame B band, the observations suggest of brightening, consistent with increased star-formation activity at . However, the galaxies with stable disc kinematics have more slowly rising rotation curves than expected from galaxies with similar surface brightness in the local Universe. We suggest that this may arise because the distant galaxies have lower bulge masses than their local counterparts. Whilst this study is based on only six galaxies, the gain in flux and in spatial resolution achieved via gravitational magnification provides a much more detailed view of the high-redshift Universe than that possible with conventional surveys. © 2006 RAS.

Abstract:

We present a generalization of surface photometry to the higher-order moments of the line-of-sight velocity distribution of galaxies observed with integral-field spectrographs. The generalization follows the approach of surface photometry by determining the best-fitting ellipses along which the profiles of the moments can be extracted and analysed by means of harmonic expansion. The assumption for the odd moments (e.g. mean velocity) is that the profile along an ellipse satisfies a simple cosine law. The assumption for the even moments (e.g. velocity dispersion) is that the profile is constant, as it is used in surface photometry. We test the method on a number of model maps and discuss the meaning of the resulting harmonic terms. We apply the method to the kinematic moments of an axisymmetric model elliptical galaxy and probe the influence of noise on the harmonic terms. We also apply the method to SAURON observations of NGC 2549, NGC 2974, NGC 4459 and NGC 4473 where we detect multiple co- and counter-rotating (NGC 2549 and NGC 4473, respectively) components. We find that velocity profiles extracted along ellipses of early-type galaxies are well represented by the simple cosine law (with 2 per cent accuracy), while possible deviations are carried in the fifth harmonic term which is sensitive to the existence of multiple kinematic components, and has some analogy to the shape parameter of photometry. We compare the properties of the kinematic and photometric ellipses and find that they are often very similar, but a study on a larger sample is necessary. Finally, we offer a characterization of the main velocity structures based only on the kinemetric parameters which can be used to quantify the features in velocity maps. © 2006 RAS.