The influence of star clusters on galactic disks: new insights on star-formation in galaxies

Stars form in embedded star clusters which play a key role in determining the properties of a galaxy's stellar population. Physical mechanisms discussed in this paper are runaway stars shot out from young clusters, binary-star disruption in clusters, gas blow-out from clusters and the origin of thick galactic disks. I emphasise that the SNIa rate per low-mass star depends on the star-clusters formed in a galaxy and I discuss the IGIMF theory. Based on the IGIMF theory, the re-calibrated Halpha-luminosity--SFR relation implies dwarf irregular galaxies to have the same gas-depletion time-scale as major disk galaxies, suggesting a major change in our understanding of dwarf-galaxy evolution. The IGIMF-theory also naturally leads to the observed radial Halpha cutoff in disk galaxies without a radial star-formation cutoff. It emerges that the thorough understanding of the physics and distribution of star clusters may be leading to a major paradigm shift in our understanding of galaxy evolution.Comment: 12 papges, to appear in The Galactic disk in a cosmological context, IAUS254, eds J. Andersen, J. Bland-Hawthorn and B. Nordstro

On String Tunneling in Power Law Inflationary Universes

We consider the evolution of circular string loops in power law expanding universes represented by a spatially flat Friedman-Robertson-Walker metric with scale factor $a(t)\propto t^p$ where $t$ is the cosmic time and $p\geq 0$. Our main result is the existence of a "magic" power $p_m=3+2\sqrt{2}$. In spacetimes with $p<p_m$ a circular string expands either forever or to a maximal radius and then contracts until it collapses into a point (black hole). For $p>p_m$, however, we find additional types of solutions. They include configurations which contract from a positive initial radius to a minimal one and then expand forever. Their existence we interpret as an indication for the presence of a finite potential barrier. Equivalently the new solutions signal string nucleation and tunneling, phenomena recently shown to occur in de Sitter space.Comment: Latex, 9 pages + 4 figures (not included), NBI-HE-93-5

Rings Reconcile Genotypic and Phenotypic Evolution within the Proteobacteria.

Although prokaryotes are usually classified using molecular phylogenies instead of phenotypes after the advent of gene sequencing, neither of these methods is satisfactory because the phenotypes cannot explain the molecular trees and the trees do not fit the phenotypes. This scientific crisis still exists and the profound disconnection between these two pillars of evolutionary biology--genotypes and phenotypes--grows larger. We use rings and a genomic form of goods thinking to resolve this conundrum (McInerney JO, Cummins C, Haggerty L. 2011. Goods thinking vs. tree thinking. Mobile Genet Elements. 1:304-308; Nelson-Sathi S, et al. 2015. Origins of major archaeal clades correspond to gene acquisitions from bacteria. Nature 517:77-80). The Proteobacteria is the most speciose prokaryotic phylum known. It is an ideal phylogenetic model for reconstructing Earth's evolutionary history. It contains diverse free living, pathogenic, photosynthetic, sulfur metabolizing, and symbiotic species. Due to its large number of species (Whitman WB, Coleman DC, Wiebe WJ. 1998. Prokaryotes: the unseen majority. Proc Nat Acad Sci U S A. 95:6578-6583) it was initially expected to provide strong phylogenetic support for a proteobacterial tree of life. But despite its many species, sequence-based tree analyses are unable to resolve its topology. Here we develop new rooted ring analyses and study proteobacterial evolution. Using protein family data and new genome-based outgroup rooting procedures, we reconstruct the complex evolutionary history of the proteobacterial rings (combinations of tree-like divergences and endosymbiotic-like convergences). We identify and map the origins of major gene flows within the rooted proteobacterial rings (P &lt; 3.6 × 10(-6)) and find that the evolution of the "Alpha-," "Beta-," and "Gammaproteobacteria" is represented by a unique set of rings. Using new techniques presented here we also root these rings using outgroups. We also map the independent flows of genes involved in DNA-, RNA-, ATP-, and membrane- related processes within the Proteobacteria and thereby demonstrate that these large gene flows are consistent with endosymbioses (P &lt; 3.6 × 10(-9)). Our analyses illustrate what it means to find that a gene is present, or absent, within a gene flow, and thereby clarify the origin of the apparent conflicts between genotypes and phenotypes. Here we identify the gene flows that introduced photosynthesis into the Alpha-, Beta-, and Gammaproteobacteria from the common ancestor of the Actinobacteria and the Firmicutes. Our results also explain why rooted rings, unlike trees, are consistent with the observed genotypic and phenotypic relationships observed among the various proteobacterial classes. We find that ring phylogenies can explain the genotypes and the phenotypes of biological processes within large and complex groups like the Proteobacteria

The Nature and Nurture of Star Clusters

Star clusters have hierarchical patterns in space and time, suggesting formation processes in the densest regions of a turbulent interstellar medium. Clusters also have hierarchical substructure when they are young, which makes them all look like the inner mixed parts of a pervasive stellar hierarchy. Young field stars share this distribution, presumably because some of them came from dissolved clusters and others formed in a dispersed fashion in the same gas. The fraction of star formation that ends up in clusters is apparently not constant, but may increase with interstellar pressure. Hierarchical structure explains why stars form in clusters and why many of these clusters are self-bound. It also explains the cluster mass function. Halo globular clusters share many properties of disk clusters, including what appears to be an upper cluster cutoff mass. However, halo globulars are self-enriched and often connected with dwarf galaxy streams. The mass function of halo globulars could have initially been like the power law mass function of disk clusters, but the halo globulars have lost their low mass members. The reasons for this loss are not understood. It could have happened slowly over time as a result of cluster evaporation, or it could have happened early after cluster formation as a result of gas loss. The latter model explains best the observation that the globular cluster mass function has no radial gradient in galaxies.Comment: to be published in IAUS266: Star Clusters Basic Galactic Building Blocks Throughout Time And Space, eds. Richard de Grijs and Jacques Lepine, Cambridge University Press, 11 page

Optimal infinite scheduling for multi-priced timed automata

This paper is concerned with the derivation of infinite schedules for timed automata that are in some sense optimal. To cover a wide class of optimality criteria we start out by introducing an extension of the (priced) timed automata model that includes both costs and rewards as separate modelling features. A precise definition is then given of what constitutes optimal infinite behaviours for this class of models. We subsequently show that the derivation of optimal non-terminating schedules for such double-priced timed automata is computable. This is done by a reduction of the problem to the determination of optimal mean-cycles in finite graphs with weighted edges. This reduction is obtained by introducing the so-called corner-point abstraction, a powerful abstraction technique of which we show that it preserves optimal schedules

Cell Cycle-Dependent Differentiation Dynamics Balances Growth and Endocrine Differentiation in the Pancreas

Organogenesis relies on the spatiotemporal balancing of differentiation and proliferation driven by an expanding pool of progenitor cells. In the mouse pancreas, lineage tracing at the population level has shown that the expanding pancreas progenitors can initially give rise to all endocrine, ductal, and acinar cells but become bipotent by embryonic day 13.5, giving rise to endocrine cells and ductal cells. However, the dynamics of individual progenitors balancing self-renewal and lineage-specific differentiation has never been described. Using three-dimensional live imaging and in vivo clonal analysis, we reveal the contribution of individual cells to the global behaviour and demonstrate three modes of progenitor divisions: symmetric renewing, symmetric endocrinogenic, and asymmetric generating a progenitor and an endocrine progenitor. Quantitative analysis shows that the endocrine differentiation process is consistent with a simple model of cell cycle-dependent stochastic priming of progenitors to endocrine fate. The findings provide insights to define control parameters to optimize the generation of β-cells in vitro

BDM Dark Matter: CDM with a core profile and a free streaming scale

We present a new dark matter model BDM which is an hybrid between hot dark matter HDM and cold dark matter CDM, in which the BDM particles behave as HDM above the energy scale E_c and as CDM below this scale. Evolution of structure formation is similar to that of CDM model but BDM predicts a nonvanishing free streaming l_fs scale and a inner galaxy core radius r_core, both quantities determined in terms of a single parameter E_c, which corresponds to the phase transition energy scale of the subjacent elementary particle model. For energies above E_c or for a scale factor a smaller then a_c, with a<a_c<a_{eq}, the particles are massless and rho redshifts as radiation. However, once the energy becomes E\leq E_c or a>a_c then the BDM particles acquire a large mass through a non perturbative mechanism, as baryons do, and rho redshifts as matter with the particles having a vanishing velocity. Typical energies are E_c=O(10-100) eV giving a l_fs \propto E_c^{-4/3}\lesssim Mpc and m_fs\propto E_c^{-4}\lesssim 10^9 M\odot. A l_fs\neq 0, r_core\neq 0 help to resolve some of the shortcomings of CDM such as overabundance substructure in CDM halos and numerical fit to rotation curves in dwarf spheroidal and LSB galaxies. Finally, our BDM model and the phase transition scale E_c can be derived from particle physics.Comment: 7 pages, 8 figure

Thickening of galactic disks through clustered star formation

(Abridged) The building blocks of galaxies are star clusters. These form with low-star formation efficiencies and, consequently, loose a large part of their stars that expand outwards once the residual gas is expelled by the action of the massive stars. Massive star clusters may thus add kinematically hot components to galactic field populations. This kinematical imprint on the stellar distribution function is estimated here by calculating the velocity distribution function for ensembles of star-clusters distributed as power-law or log-normal initial cluster mass functions (ICMFs). The resulting stellar velocity distribution function is non-Gaussian and may be interpreted as being composed of multiple kinematical sub-populations. The notion that the formation of star-clusters may add hot kinematical components to a galaxy is applied to the age--velocity-dispersion relation of the Milky Way disk to study the implied history of clustered star formation, with an emphasis on the possible origin of the thick disk.Comment: MNRAS, accepted, 27 pages, 9 figure

Interplay between telecommunications and face-to-face interactions - a study using mobile phone data

In this study we analyze one year of anonymized telecommunications data for over one million customers from a large European cellphone operator, and we investigate the relationship between people's calls and their physical location. We discover that more than 90% of users who have called each other have also shared the same space (cell tower), even if they live far apart. Moreover, we find that close to 70% of users who call each other frequently (at least once per month on average) have shared the same space at the same time - an instance that we call co-location. Co-locations appear indicative of coordination calls, which occur just before face-to-face meetings. Their number is highly predictable based on the amount of calls between two users and the distance between their home locations - suggesting a new way to quantify the interplay between telecommunications and face-to-face interactions