Sugata Kaviraj

On the formation of massive galaxies: a simultaneous study of number density, size and intrinsic colour evolution in GOODS

The evolution of number density, size and intrinsic colour is determined for a volume-limited sample of visually classified early-type galaxies selected from the Hubble Space Telescope/Advanced Camera for Surveys images of the Great Observatories Origins Deep Survey (GOODS) North and South fields (version 2). The sample comprises 457 galaxies over 320 arcmin 2 with stellar masses above 3 x 10 10 M ⊙ in the redshift range 0.4 3 x 10 11 M ⊙ ) do not show any appreciable change in comoving number density or size in our data. Furthermore, when including the results from 2dF galaxy redshift survey, we find that the number density of massive early-type galaxies is consistent with no evolution between z = 1.2 and 0, i.e. over an epoch spanning more than half of the current age of the Universe. We find large discrepancies between the predictions of semi-analytic models. Massive galaxies show very homogeneous intrinsic colour distributions, with nearly flat radial colour gradients, but with a significant negative correlation between stellar mass and colour gradient, such that red cores appear predominantly in massive galaxies. The distribution of half-light radii - when compared to z ∼ 0 and z > 1 samples - is compatible with the predictions of semi-analytic models relating size evolution to the amount of dissipation during major mergers.

Star formation and nuclear activity in close pairs of early-type galaxies

We extract from the Sloan Digital Sky Survey a sample of 347 systems involving early type galaxies separated by less than 30 kpc, in projection, and 500 km/s in radial velocity. These close pairs are likely progenitors of dry mergers. The (opti cal) spectra is used to determine how the interaction affects the star formation history and n uclear activity of the galaxies. The emission lines (or lack thereof) are used to classify the sam ple into AGN, star forming or quiescent. Increased AGN activity and reduced star formation in early-type pairs that already appear to be interacting indicate that the merging process changes the nature of nebular activity, a finding that is also supported by an increase in AGN l uminosity with decreasing pair separation. Recent star formation is studied on the absorpt ion line spectra, both through principal component analysis as well as via a comparison of the spectra with composite stellar population models. We find that the level of recent star forma tion in close pairs is raised relative to a control sample of early-type galaxies. This excess of residual star formation is found throughout the sample of close pairs and does not correlate with pair separation or with visual signs of interaction. Our findings are consistent with a scen ario whereby the first stage of the encounter (involving the outer parts of the halos) trigger r esidual star formation, followed by a more efficient inflow towards the centre ‐ switching to an AGN phase ‐ after which the systems are quiescent.

The role of environment on the formation of early-type galaxies

The effect of environment on galaxy formation poses one of the best constraints on the interplay between mass assembly and star formation in galaxies. We present here a detailed study of the stellar populations of a volume-limited sample of early-type galaxies from the Sloan Digital Sky Survey, across a range of environments ‐ defined as the mass of the hos t dark matter halo, according to the groups catalogue of Yang et al. The stellar populations are explored through the SDSS spectra, via projection onto a set of two spectral vectors determined from Principal Component Analysis. This method has been found to highlight differences not seen when using standard, model-dependent comparisons of photo-spectroscopic data. We find the velocity dispersion of the galaxy to be the main driver behind the different star formation histories of early-type galaxies. However, environmental effects are seen to play a role (although minor). Our Principal Components allow us to distinguish between the effects of environment as a change in average age (mapping the time lapse of assembly) or the presence of recent star formation (reflecting environment-related interactions) . Galaxies populating the lowest mass halos have stellar populations on average �1 Gyr younger than the rest of the sample. The fraction of galaxies with small amounts of recent star formation is also seen to be truncated when occupying halos more massive than MH >

What drives the star formation in early-type galaxies at late epochs? - the case for minor mergers

Multi-wavelength photometry of early-type galaxies (ETGs) in the COSMOS survey is used to demonstrate that the low-level star formation activity in the ETG population at late epochs ( z z