Revisiting The First Galaxies: The effects of Population III stars on their host galaxies

We revisit the formation and evolution of the first galaxies using new hydrodynamic cosmological simulations with the ART code. Our simulations feature a recently developed model for H2 formation and dissociation, and a star formation recipe that is based on molecular rather than atomic gas. Here, we develop and implement a recipe for the formation of metal-free Population III stars in galaxy-scale simulations that resolve primordial clouds with sufficiently high density. We base our recipe on the results of prior zoom-in simulations that resolved the protostellar collapse in pre-galactic objects. We find the epoch during which Pop III stars dominated the energy and metal budget of the first galaxies to be short-lived. Galaxies which host Pop III stars do not retain dynamical signatures of their thermal and radiative feedback for more than 10^8 yr after the lives of the stars end in pair-instability supernovae, even when we consider the maximum reasonable efficiency of the feedback. Though metals ejected by the supernovae can travel well beyond the virial radius of the host galaxy, they typically begin to fall back quickly, and do not enrich a large fraction of the intergalactic medium. Galaxies with total mass in excess of 3 x 10^6 Msun re-accrete most of their baryons and transition to metal-enriched Pop II star formation.Comment: 13 pages, 9 figures, published in Ap

Revisiting The First Galaxies: The epoch of Population III stars

We investigate the transition from primordial Population III (Pop III) star formation to normal Pop II star formation in the first galaxies using new cosmological hydrodynamic simulations. We find that while the first stars seed their host galaxies with metals, they cannot sustain significant outflows to enrich the intergalactic medium, even assuming a top-heavy initial mass function. This means that Pop III star formation could potentially continue until z~6 in different unenriched regions of the universe, before being ultimately shut off by cosmic reionization. Within an individual galaxy, the metal production and stellar feedback from Pop II stars overtake Pop III stars in 20-200 Myr, depending on galaxy mass.Comment: 9 pages, 7 figures, published in Ap

Data Set Modelability by QSAR

We introduce a simple MODelability Index (MODI) that estimates the feasibility of obtaining predictive QSAR models (Correct Classification Rate above 0.7) for a binary dataset of bioactive compounds. MODI is defined as an activity class-weighted ratio of the number of the nearest neighbor pairs of compounds with the same activity class versus the total number of pairs. The MODI values were calculated for more than 100 datasets and the threshold of 0.65 was found to separate non-modelable from the modelable datasets

Modelling of self-organizaton of microtubules in plant cells

Microtubules are ubiquitous elements of any eucaryotic cell, serving many functions at different stages of its life. In plant cells they form so-called plant cell cortex, where they are organized into parallel arrays. These arrays serve as a matrix of synthesis of a plant cell wall, defining the direction of growth. Microtubule arrays are sensible to tropic stimuli. However, the nature of such sensibility is still not well established. We provide a computational analysis of this phenomenon. Using both kinetic Monte-Carlo simulations and theoretical investigation, we show that compression due to mechanical stress may cause orientation of microtubules along major stress lines. We also show that anisotropic distribution of chemical agents interacting with microtubule-associated proteins may also cause orientation of microtubules. Such mechanisms are primarily connected with gravitropism but similar reorientations of microtubules in response to light may suggest that these mechanisms can also be relevant for other tropisms.Los microtúbulos son elementos ubicuos de cualquier célula eucariota, que poseen distintas funciones en diferentes etapas de su vida. En células de plantas ellos forman la estructura llamada corteza célula vegetal, donde se organizan en conjuntos paralelos. Estos conjuntos sirven como una matriz de síntesis de una pared celular vegetal, definiendo la dirección del crecimiento de la célula. Los conjuntos de microtúbulos son sensibles a estímulos trópicos. Sin embargo, la naturaleza de dicha sensibilidad todavía no está bien establecida. Les ofrecemos un análisis computacional de este fenómeno. Usando simulaciones cinéticas Monte-Carlo y la investigación teórica, nos mostramos que la compresión debida al estrés mecánico puede causar la orientación de los microtúbulos a lo largo de las principales líneas de tensión. También se muestra que la distribución anisotrópica de agentes químicos que interactúan con las proteínas asociadas a los microtúbulos también puede provocar orientación de los microtúbulos. Estos mecanismos están conectados principalmente con gravitropismo pero reorientaciones similares de microtúbulos en respuesta a la luz puede sugerir que estos mecanismos también pueden ser relevantes para otros tropismos

Cosmological Small-Scale Structure: The Formation of The First Stars, Galaxies, and Globular Clusters.

Though the majority of stars now live in large, massive galaxies, understanding the origins of all galaxies ab initio requires fully comprehensive modeling of cosmological small-scale structure. In this thesis, I present a theoretical study of galaxy formation that focuses on low-mass halos. These halos are the sites for the formation of the first stars and galaxies at high redshift, and they also they play a role in forming massive globular clusters in the outskirts of the Milky Way. I develop a physical model for Population III star formation and feedback, and implemented it into the Eulerian hydrodynamic Adaptive Refinement Tree (ART) code. With this code, I designed, performed, and analyzed a suite of cosmological simulations that resolve the formation of the first stars and galaxies. I quantify the extent of the dynamical signatures Population III stars can impart on their host galaxies, and derive a characteristic mass threshold, 3 million solar masses, above which Population III stellar feedback is no longer dynamically significant over significant cosmic timescales. I measure the duration of time for which Population III stars are the dominant drivers of feedback in the universe. Due to the inhomogeneous and patchy enrichment of the intergalactic medium, I find Population III stars can continue forming in some environments well after the end of the cosmic dark ages. However, in individual galaxies that are sufficiently massive, Population II star formation takes over soon after the efficient enrichment by a single pair-instability supernova. Globally, Population II is dominant at cosmic epochs later than redshift (z ~ 15). Finally, I construct a semi-analytical model for globular cluster formation in hierarchical cosmology, and use it to demonstrate a plausible scenario for the formation of the Milky Way’s globular cluster system. My model is successful in matching both the metallicity and mass distributions of galactic globular clusters. In particular, the bimodal nature of the metallicity distribution is for the first time explained by the single mechanism of the merging of protogalaxies.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99887/1/muratov_1.pd

The Origin and Evolution of the Galaxy Mass-Metallicity Relation

We use high-resolution cosmological zoom-in simulations from the Feedback in Realistic Environment (FIRE) project to study the galaxy mass-metallicity relations (MZR) from z=0-6. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback. The simulations cover halo masses Mhalo=10^9-10^13 Msun and stellar mass Mstar=10^4-10^11 Msun at z=0 and have been shown to produce many observed galaxy properties from z=0-6. For the first time, our simulations agree reasonably well with the observed mass-metallicity relations at z=0-3 for a broad range of galaxy masses. We predict the evolution of the MZR from z=0-6 as log(Zgas/Zsun)=12+log(O/H)-9.0=0.35[log(Mstar/Msun)-10]+0.93 exp(-0.43 z)-1.05 and log(Zstar/Zsun)=[Fe/H]-0.2=0.40[log(Mstar/Msun)-10]+0.67 exp(-0.50 z)-1.04, for gas-phase and stellar metallicity, respectively. Our simulations suggest that the evolution of MZR is associated with the evolution of stellar/gas mass fractions at different redshifts, indicating the existence of a universal metallicity relation between stellar mass, gas mass, and metallicities. In our simulations, galaxies above Mstar=10^6 Msun are able to retain a large fraction of their metals inside the halo, because metal-rich winds fail to escape completely and are recycled into the galaxy. This resolves a long-standing discrepancy between "sub-grid" wind models (and semi-analytic models) and observations, where common sub-grid models cannot simultaneously reproduce the MZR and the stellar mass functions.Comment: 17 pages, 14 figures, re-submitted to MNRAS after revisions on referee comment

Computer-Assisted Decision Support for Student Admissions Based on Their Predicted Academic Performance

Objective. To develop predictive computational models forecasting the academic performance of students in the didactic-rich portion of a doctor of pharmacy (PharmD) curriculum as admission-assisting tools

Trust, But Verify: On the Importance of Chemical Structure Curation in Cheminformatics and QSAR Modeling Research

Molecular modelers and cheminformaticians typically analyze experimental data generated by other scientists. Consequently, when it comes to data accuracy, cheminformaticians are always at the mercy of data providers who may inadvertently publish (partially) erroneous data. Thus, dataset curation is crucial for any cheminformatics analysis such as similarity searching, clustering, QSAR modeling, virtual screening, etc., especially nowadays when the availability of chemical datasets in public domain has skyrocketed in recent years. Despite the obvious importance of this preliminary step in the computational analysis of any dataset, there appears to be no commonly accepted guidance or set of procedures for chemical data curation. The main objective of this paper is to emphasize the need for a standardized chemical data curation strategy that should be followed at the onset of any molecular modeling investigation. Herein, we discuss several simple but important steps for cleaning chemical records in a database including the removal of a fraction of the data that cannot be appropriately handled by conventional cheminformatics techniques. Such steps include the removal of inorganic and organometallic compounds, counterions, salts and mixtures; structure validation; ring aromatization; normalization of specific chemotypes; curation of tautomeric forms; and the deletion of duplicates. To emphasize the importance of data curation as a mandatory step in data analysis, we discuss several case studies where chemical curation of the original “raw” database enabled the successful modeling study (specifically, QSAR analysis) or resulted in a significant improvement of model's prediction accuracy. We also demonstrate that in some cases rigorously developed QSAR models could be even used to correct erroneous biological data associated with chemical compounds. We believe that good practices for curation of chemical records outlined in this paper will be of value to all scientists working in the fields of molecular modeling, cheminformatics, and QSAR studies