Galaxy Formation Spanning Cosmic History

Over the past several decades, galaxy formation theory has met with significant successes. In order to test current theories thoroughly we require predictions for as yet unprobed regimes. To this end, we describe a new implementation of the Galform semi-analytic model of galaxy formation. Our motivation is the success of the model described by Bower et al. in explaining many aspects of galaxy formation. Despite this success, the Bower et al. model fails to match some observational constraints and certain aspects of its physical implementation are not as realistic as we would like. The model described in this work includes substantially updated physics, taking into account developments in our understanding over the past decade, and removes certain limiting assumptions made by this (and most other) semi-analytic models. This allows it to be exploited reliably in high-redshift and low mass regimes. Furthermore, we have performed an exhaustive search of model parameter space to find a particular set of model parameters which produce results in good agreement with a wide range of observational data (luminosity functions, galaxy sizes and dynamics, clustering, colours, metal content) over a wide range of redshifts. This model represents a solid basis on which to perform calculations of galaxy formation in as yet unprobed regimes.Comment: MNRAS accepted. Extended version (with additional figures and details of implementation) is available at http://www.galform.or

Accretion shocks and cold filaments in galaxy formation

A generic expectation for gas accreted by high-mass haloes is that it is shock-heated to the virial temperature of the halo. In low-mass haloes, or at high redshift, however, the gas cooling rate is sufficiently rapid that an accretion shock is unlikely to form close to the virial radius. Instead, the accretion shock will form at smaller radii, perhaps close to the central galaxy. Semi-analytic models have always made a clear distinction between the regimes in which accretion is limited by the cooling time of hot gas and by the dynamical time-scale of the halo, using simple estimates of the mass-scale at which the transition from rapid to slow cooling occurs. In this work, we revisit this issue using the latest understanding and calibration of accretion shock formation. Starting from the well-established Galform code, we investigate the effect of accounting for the presence or otherwise of an accretion shock close to the virial radius using the shock stability model of Birnboim & Dekel. As expected, when we modify the code so that there is no effective feedback from galaxy formation, we find that so-called ‘cold-mode’ accretion is the dominant channel for feeding gas into the galaxies at high redshifts, such that 90 per cent of baryons in galaxies (averaged over all galaxies) arrive via this channel. The mass-scale at which the rapid to slow cooling transition occurs is significantly affected at high redshifts and accretion rates become dominated by cold-mode accretion. However, the impact of this change in the cooling channel on galaxies properties is mitigated by compensating effects in the star formation and feedback cycle. When effective feedback, which reheats gas from galaxies to the virial temperature but which allows no gas to escape from a halo, is included in the model, we find that the ‘cold mode’ is even less apparent because of the presence of gas ejected from the galaxy's disc, although it can still contribute almost 50 per cent of the net inflow rate when averaged over all galaxies. Thus, the inclusion of the latest calibration of accretion shock physics makes little difference to basic results from earlier semi-analytic models, which used a simpler treatment. We conclude that this ‘cold-mode’ physics is already adequately accounted for in semi-analytic models and that feedback represents a much larger uncertainty than any of these effects

Discovery of a very extended X-ray halo around a quiescent spiral galaxy - the "missing link" of galaxy formation

Hot gaseous haloes surrounding galaxies and extending well beyond the distribution of stars are a ubiquitous prediction of galaxy formation scenarios. The haloes are believed to consist of gravitationally trapped gas with a temperature of millions of Kelvin. The existence of such hot haloes around massive elliptical galaxies has been established through their X-ray emission. While gas out-flowing from starburst spiral galaxies has been detected, searches for hot haloes around normal, quiescent spiral galaxies have so far failed, casting doubts on the fundamental physics in galaxy formation models. Here we present the first detection of a hot, large-scale gaseous halo surrounding a normal, quiescent spiral galaxy, NGC 5746, alleviating a long-standing problem for galaxy formation models. In contrast to starburst galaxies, where the X-ray halo can be powered by the supernova energy, there is no such power source in NGC 5746. The only compelling explanation is that we are here witnessing a galaxy forming from gradually in-flowing hot and dilute halo gas.Comment: New Astronomy, in pres

New partial dentaries of amphitheriid mammal Palaeoxonodon ooliticus from Scotland, and posterior dentary morphology in early cladotherians

We describe two partial dentaries of mammals from the Middle Jurassic of Scotland. They belong to the early cladotherian Palaeoxonodon ooliticus. These dentaries comprise the first specimen of P. ooliticus ever found—although its significance was initially unrecognised so it remained undescribed until now—and the most recently discovered specimen, found during fieldwork in 2017. The new specimen preserves part of the coronoid process of the dentary, previously unknown for P. ooliticus, demonstrating the presence of a deep masseteric fossa, with a prominent crest enclosing the fossa anteriorly, and a masseteric foramen, located in the masseteric fossa on the buccal surface of the dentary. On the lingual surface, the mandibular foramen is offset from the Meckel’s sulcus, and positioned below the alveolar plane. These morphologies allow an updated analysis of the phylogenetic position of P. ooliticus, confirming a sister-taxa relationship between Palaeoxonodon and Amphitherium. The position of the mandibular foramen, and the slight extension of the masseteric fossa into the body of the dentary are new autapomorphies for Palaeoxonodon

Rise of Dinosaurs Reveals Major Body-Size Transitions Are Driven by Passive Processes of Trait Evolution

A major macroevolutionary question concerns how long-term patterns of body-size evolution are underpinned by smaller scale processes along lineages. One outstanding long-term transition is the replacement of basal therapsids (stem-group mammals) by archosauromorphs, including dinosaurs, as the dominant large-bodied terrestrial fauna during the Triassic (approx. 252–201 million years ago). This landmark event preceded more than 150 million years of archosauromorph dominance. We analyse a new body-size dataset of more than 400 therapsid and archosauromorph species spanning the Late Permian–Middle Jurassic. Maximum-likelihood analyses indicate that Cope's rule (an active within-lineage trend of body-size increase) is extremely rare, despite conspicuous patterns of body-size turnover, and contrary to proposals that Cope's rule is central to vertebrate evolution. Instead, passive processes predominate in taxonomically and ecomorphologically more inclusive clades, with stasis common in less inclusive clades. Body-size limits are clade-dependent, suggesting intrinsic, biological factors are more important than the external environment. This clade-dependence is exemplified by maximum size of Middle–early Late Triassic archosauromorph predators exceeding that of contemporary herbivores, breaking a widely-accepted ‘rule’ that herbivore maximum size greatly exceeds carnivore maximum size. Archosauromorph and dinosaur dominance occurred via opportunistic replacement of therapsids following extinction, but were facilitated by higher archosauromorph growth rates

Biology, not environment, drives major patterns in maximum tetrapod body size through time

Abiotic and biological factors have been hypothesized as controlling maximum body size of tetrapods and other animals through geological time. We analyse the effects of three abiotic factors—oxygen, temperature and land area—on maximum size of Permian–Jurassic archosauromorphs and therapsids, and Cenozoic mammals, using time series generalized least-squares regression models. We also examine maximum size growth curves for the Permian–Jurassic data by comparing fits of Gompertz and logistic models. When serial correlation is removed, we find no robust correlations, indicating that these environmental factors did not consistently control tetrapod maximum size. Gompertz models—i.e. exponentially decreasing rate of size increase at larger sizes—fit maximum size curves far better than logistic models. This suggests that biological limits such as reduced fecundity and niche space availability become increasingly limiting as larger sizes are reached. Environmental factors analysed may still have imposed an upper limit on tetrapod body size, but any environmentally imposed limit did not vary substantially during the intervals examined despite variation in these environmental factors

A temperate palaeodiversity peak in Mesozoic dinosaurs and evidence for Late Cretaceous geographical partitioning

Aim  Modern biodiversity peaks in the tropics and declines poleward, a pattern that is potentially driven by climate. Although this latitudinal biodiversity gradient (LBG) also characterizes the marine invertebrate fossil record, distributions of ancient terrestrial faunas are poorly understood. This study utilizes data on the dinosaur fossil record to examine spatial patterns in terrestrial biodiversity throughout the Mesozoic.\ud Location  We compiled data on fossil occurrences across the globe.\ud Methods  We compiled a comprehensive dataset of Mesozoic dinosaur genera (738), including birds. Following the utilization of sampling standardization techniques to mediate for the uneven sampling of the fossil record, we constructed latitudinal patterns of biodiversity from this dataset.\ud Results  The dominant group of Mesozoic terrestrial vertebrates did not conform to the modern LBG. Instead, dinosaur diversity was highest at temperate palaeolatitudes throughout the 160 million year span of dinosaurian evolutionary history. Latitudinal diversity correlates strongly with the distribution of land area. Late Cretaceous sauropods and ornithischians exhibit disparate LBGs.\ud Main conclusions  The continuity of the palaeotemperate peak in dinosaur diversity indicates a diminished role for climate on the Mesozoic LBG; instead, dinosaur diversity may have been driven by the amount of land area among latitudinal belts. There is no evidence that the tropics acted as a cradle for dinosaur diversity. Geographical partitioning among major clades of herbivorous dinosaurs in the Late Cretaceous may result from the advanced stages of continental fragmentation and/or differing responses to increasing latitudinal climatic zonation. Our results suggest that the modern-day LBG on land was only established 30 million years ago, following a significant post-Eocene recalibration, potentially related to increased seasonality