We describe the Sloan

We study the formation of galaxies in a Λ cold dark matter (ΛCDM) universe using high‐resolution hydrodynamical simulations with a multiphase treatment of gas, cooling and feedback, focusing on the formation of discs. Our simulations follow eight isolated haloes similar in mass to the Milky Way and extracted from a large cosmological simulation without restriction on spin parameter or merger history. This allows us to investigate how the final properties of the simulated galaxies correlate with the formation histories of their haloes. We find that, at z= 0, none of our galaxies contains a disc with more than 20 per cent of its total stellar mass. Four of the eight galaxies nevertheless have well‐formed disc components, three have dominant spheroids and very small discs, and one is a spheroidal galaxy with no disc at all. The z= 0 spheroids are made of old stars, while discs are younger and formed from the inside‐out. Neither the existence of a disc at z= 0 nor the final disc‐to‐total mass ratio seems to depend on the spin parameter of the halo. Discs are formed in haloes with spin parameters as low as 0.01 and as high as 0.05; galaxies with little or no disc component span the same range in spin parameter. Except for one of the simulated galaxies, all have significant discs at z≳ 2, regardless of their z= 0 morphologies. Major mergers and instabilities which arise when accreting cold gas is misaligned with the stellar disc trigger a transfer of mass from the discs to the spheroids. In some cases, discs are destroyed, while in others, they survive or reform. This suggests that the survival probability of discs depends on the particular formation history of each galaxy. A realistic ΛCDM model will clearly require weaker star formation at high redshift and later disc assembly than occurs in our models.

The fourth generation of the Sloan

We study galaxy pairs (GPs) in the field selected from the 100‐K public release of the Two Degree Field (2dF) galaxy redshift survey. Our analysis provides a well‐defined sample of 1258 GPs, a large data base suitable for statistical studies of galaxy interactions in the local Universe, z≤ 0.1. GPs were selected by radial velocity (ΔV) and projected separation (rp) criteria determined by analysing the star‐formation activity within neighbours. We have excluded pairs in high‐density regions by removing galaxies in groups and clusters. We analyse the star‐formation activity in the pairs as a function of both relative projected distance and relative radial velocity. We found power‐law relations for the mean star‐formation birth parameter and equivalent widths of the galaxies in pairs as a function of rp and ΔV. We find that star formation in GPs is significantly enhanced over that of isolated galaxies with similar redshifts in the field for rp < 25 h−1 kpc and ΔV < 100 km s−1. We detected that, when compared to isolated galaxies of similar luminosity and redshift distribution, the effects of having a companion are more significant on the star‐formation activity of bright galaxies in pairs, unless the pairs are formed by similar luminosity galaxies. In this case, the star formation is enhanced in both components. The ratio between the fractions of star‐forming galaxies in pairs and in isolation is a useful tool to unveil the effects of having a close companion. We found that about 50 per cent of GPs do not show signs of important star‐formation activity (independently of their luminosities), supporting the hypothesis that the internal properties of the galaxies play a crucial role in the triggering of star formation by interactions.