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Stockholm, October 23, 2007-10-25 Martin Lindman Galaxies course Seminar 2, October 3, 2007 The topic of this seminar was the evolution of galaxies. Five different papers based on observations of galaxies at different redshifts were discussed. Important discussion topics were the changes in morphology and star formation rate of galaxies at different redshifts and how these changes vary with the masses of the systems. We also discussed how these findings fit with the different models (hierarchical or downsizing) of galaxy formation. The most important points that were discussed are presented for each paper below. Paper 1: Abraham, R. G. and van den Bergh, S The morphological evolution of galaxies The main point of this paper is that galaxies have different morphologies at different redshifts. This indicates that they have gone through some sort of evolution in the past. The redshift range discussed in detail in this paper is 0 < z < 1. At higher redshifts it becomes very difficult to determine the morphology of galaxies. Observational results indicate that the galaxies have only taken on their familiar appearance relatively recently. The galaxy population starts to deviate significantly at z as low as 0.3 and at z ~1 the morphology is so peculiar that about 30 % of the galaxies cannot be fitted into the classical Hubble tuning fork system. Although the internal structure and morphology show significant changes with redshift, the overall space density of galaxies is constant up to z = 1, indicating that the number of galaxies seem to be constant in this redshift range. The nature of the large fraction of peculiar galaxies at higher redshifts remains a mystery, but it seems conceivable that in some of the distant peculiar objects we are seeing the early stages of present day luminous galaxies where most of the baryons in the galaxy are locked up in stars. This classic description of a protogalaxy would support the hierarchical model stating that smaller structure form first. A short summary of the key developments in galaxy morphology: At z < 0.3 grand design spiral galaxies exists and the Hubble scheme applies in full detail. At z ~ 0.5 barred spirals become rare and spiral arms are underdeveloped. At z > 0.6 the fraction of mergers and peculiar galaxies increases rapidly. By z = 1 around 30 % of luminous galaxies are off the Hubble scheme. Paper 2: Bell, E. F. et al. Toward an understanding of the rapid decline of the cosmic star formation rate This paper is based on observations of nearly 1500 galaxies in the redshift range 0.65< z < 0.75. The authors use this sample to compare the star formation rate (SFR) of the past Stockholm, October 23, 2007-10-25 Martin Lindman to what we see in the local universe. There is an evidence of a significant decline in SFR of intermediate and high mass galaxies from. At z ~ 7 about 40 % of the high mass and intermediate mass galaxies are undergoing an intense star formation and in the local universe less than 1 % of the galaxies have the same SFR. Since only a fraction of the observed galaxies at 0.65 < z < 0.75 show signs of strong interactions the conclusion is that a decline in major merger since this redshift cannot be the cause of the decline in the SFR. Since the galaxies in the sample have an undisturbed morphology the cause of the decline in SFR must be changes in the physical properties that do not affect the morphology of the galaxy. Examples of such explanations are gas consumption and weak interactions with smaller galaxies. The contribution of Active Galactic Nuclei was also discussed but more work is needed in this area. It seems however that the dust heating by AGNi is not the explanation of the decline in SFR. Paper3: Juneau, S. et al. Cosmic star formation history and its dependence on galaxy stellar mass This paper examines hoe the cosmic star formation rate (SFR) depends on the mass of a galaxy in the redshift range 0.8 < z < 2. The SFR density of the most massive galaxies show a significant decline and is about 6 times less today than it is at z = 2. The SFR in the intermediate mass galaxies however decline more slowly and the low mass systems show an intense star formation throughout the redshift range of this study. The conclusion from these observational results is that the era of star formation was extended and proceeds from high mass systems to lower mass systems. This indicates that the higher mass systems formed first and the smaller systems later and would thus support the downsizing theory of galaxy formation. This would contradict the hierarchical model. For the hierarchical model to be true the star formation must have been more efficient in the past in the massive galaxies. It could be that different processes dominate at different redshifts. Paper 4: Labbé, I. et al. IRAC mid-infrared imaging of the Hubble deep field-south: Star formation histories and stellar masses of red galaxies at z > 2 This paper study distant red galaxies (DRGs) at z > 2 and compare them with Lyman break galaxies (LBGs) at 2 < z < 3. The DRGs can be divided into two groups; galaxies that are red due to dust reddening (about 70 %) or old and “dead” galaxies (about 30 %). One important conclusion from the observations is that it is impossible to obtain massselected samples photometrically. The mass to light ratio of the DRGs and LBGs in the Stockholm, October 23, 2007-10-25 Martin Lindman sample varies by a factor of 6. These variations could possibly be explained by a relation between total stellar mass and mass to light ratio and thus the differences in stellar mass would give these variations. Another important conclusion is that at high redshifts the red and dead galaxies are the more massive galaxies. This supports the idea that the more massive systems are the oldest and have the highest mass to light ratios. Paper 5: Bouwens, R. & Illingworth G. Rapid evolution of the most luminous galaxies during the first 900 million years. This paper presents results from a search for very high redshift galaxies at z ~ 7 – 8. In addition to these results the authors use simulations to find the expected number of galaxies at these redshifts and compare with the observational results. In the observations only one candidate galaxy was found at this high redshift under conservative selection criteria. The expected number of galaxies under these criteria is ten. Using less conservative criteria they found four candidates where 17 would be expected. Both of these simulations were made under the assumption that there was no galaxy evolution. These results show that the most luminous galaxies are very rare at z ~7 compared to z ~ 6. Other papers conclude that the high mass galaxies have higher star formation rates at higher redshifts. If the SFR is even higher at z ~7 than at z ~6 we would expect to see more galaxies at higher redshifts than would be the case if there was no evolution between these redshifts. The fact that less galaxies than expected are observes indicates that we have an upper limit on the SFR peak. The simplest explanation for the lack of luminous galaxies at z ~7 is that these systems simply have not yet had the time to form. This is in support of the hierarchical model of galaxy evolution.