We have investigated the mass-metallicity (M-Z ) relation using galaxies at 0:4 < z < 1:0 from the Gemini Deep Deep Survey (GDDS) and Canada-France Redshift Survey (CFRS). Deep K-and z 0 -band photometry allowed us to measure stellar masses for 69 galaxies. From a subsample of 56 galaxies, for which metallicity of the interstellar medium is also measured, we identified a strong correlation between mass and metallicity for the first time in the distant universe. This was possible because of the larger baseline spanned by the sample in terms of metallicity (a factor of 7) and mass (a factor of 400) than in previous works. This correlation is much stronger and tighter than the luminosity-metallicity relation, confirming that stellar mass is a more meaningful physical parameter than luminosity. We find clear evidence for temporal evolution in the M-Z relation in the sense that at a given mass, a galaxy at z $ 0:7 tends to have lower metallicity than a local galaxy of similar mass. We use the z $ 0:1 Sloan Digital Sky Survey M-Z relation and a small sample of z $ 2:3 Lyman break galaxies with known mass and metallicity to propose an empirical redshift-dependent M-Z relation. According to this relation the stellar mass and metallicity in small galaxies evolve for a longer time than they do in massive galaxies. This relation predicts that the generally metal-poor damped Ly galaxies have stellar masses of the order of 10 8:8 M (with a dispersion of 0.7 dex) all the way from z $ 0:2 to 4. The observed redshift evolution of the M-Z relation can be reproduced remarkably well by a simple closed-box model in which the key assumption is an e-folding time for star formation that is higher or, in other words, a period of star formation that lasts longer in less massive galaxies than in more massive galaxies. Such a picture supports the downsizing scenario for galaxy formation.

The galaxy merger and accretion rates, and their evolution with time, provide important tests for models of galaxy formation and evolution. Close pairs of galaxies are the best available means of measuring redshift evolution in these quantities. In this study, we introduce two new pair statistics, which relate close pairs to the merger and accretion rates. We demonstrate the importance of correcting these (and other) pair statistics for selection e †ects related to sample depth and completeness. In particular, we highlight the severe bias that can result from the use of a Ñux-limited survey. The Ðrst statistic, gives N c , the number of companions per galaxy within a speciÐed range in absolute magnitude.is directly N c related to the galaxy merger rate. The second statistic, gives the total luminosity in companions, per L c , galaxy. This quantity can be used to investigate the mass accretion rate. Both and are related to N c L c the galaxy correlation function m and luminosity function /(M) in a straightforward manner. Both statistics have been designed with selection e †ects in mind. We outline techniques that account for various selection e †ects and demonstrate the success of this approach using Monte Carlo simulations. If one assumes that clustering is independent of luminosity (which is appropriate for reasonable ranges in luminosity), then these statistics may be applied to Ñux-limited surveys. These techniques are applied to a sample of 5426 galaxies in the Second Southern Sky Redshift Survey (SSRS2). This is the Ðrst large, well-deÐned low-z survey to be used for pair statistics. Using close (5 h~1 h~1 kpc) kpc ¹ r p ¹ 20 dynamical (*v ¹ 500 km s~1) pairs, we Ðnd andat z \ 0.015. These are the Ðrst secure estimates of low-L _ redshift pair statistics, and they will provide local benchmarks for ongoing and future pair studies. If N c remains Ðxed with redshift, simple assumptions imply that D6.6% of present day galaxies with have undergone mergers since z \ 1. When applied to redshift surveys of more [21 ¹ M B ¹ [18 distant galaxies, these techniques will yield the Ðrst robust estimates of evolution in the galaxy merger and accretion rates.

We examine the cosmic star formation rate (SFR) and its dependence on galaxy stellar mass over the redshift range using data from the Gemini Deep Deep Survey (GDDS). The SFR in the most massive galaxies 0.8 ! z ! 2 ( ) was 6 times higher at than it is today. It drops steeply from , reaching the present-. In contrast, the SFR density of intermediate-mass galaxies ( ) 10.2 10.8declines more slowly and may peak or plateau at . We use the characteristic growth time z ∼ 1.5 t { SFR to provide evidence of an associated transition in massive galaxies from a burst to a quiescent star r /r M SFR * formation mode at . Intermediate-mass systems transit from burst to quiescent mode at , while the z ∼ 2 z ∼ 1 lowest mass objects undergo bursts throughout our redshift range. Our results show unambiguously that the formation era for galaxies was extended and proceeded from high-to low-mass systems. The most massive galaxies formed most of their stars in the first ∼3 Gyr of cosmic history. Intermediate-mass objects continued to form their dominant stellar mass for an additional ∼2 Gyr, while the lowest mass systems have been forming over the whole cosmic epoch spanned by the GDDS. This view of galaxy formation clearly supports "downsizing" in the SFR where the most massive galaxies form first and galaxy formation proceeds from larger to smaller mass scales.

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Hierarchical galaxy formation is the model whereby massive galaxies form from an assembly of smaller units. The most massive objects therefore form last. The model succeeds in describing the clustering of galaxies, but the evolutionary history of massive galaxies, as revealed by their visible stars and gas, is not accurately predicted. Near-infrared observations (which allow us to measure the stellar masses of high-redshift galaxies) and deep multi-colour images indicate that a large fraction of the stars in massive galaxies form in the first 5 Gyr (refs 4-7), but uncertainties remain owing to the lack of spectra to confirm the redshifts (which are estimated from the colours) and the role of obscuration by dust. Here we report the results of a spectroscopic redshift survey that probes the most massive and quiescent galaxies back to an era only 3 Gyr after the Big Bang. We find that at least two-thirds of massive galaxies have appeared since this era, but also that a significant fraction of them are already in place in the early Universe.