Development of Realistic Simulations of the Interactions Between Stars and the Interstellar Medium in Disk Galaxies

fWe have developed GALAXY, a two dimensional, self-consistent N-body plus hydro-dynamic computer program to model and study the behavior of disk galaxies like our Milky Way. Our goal is to better understand how such galaxies maintain their spiral structure for billions of years. The program utilizes two independent gravitating N-body components. One simulates the collision-less star particles in the galaxy, and the other simulates colliding clouds. A gravitating hydrodynamic code simulates the interstellar medium. Additionally, a static spherical halo and a central black hole interact gravitationally with the other components of the model galaxy. All components interact with each other through various evolution processes and gravity, and they exchange mass, energy, and momentum. For this dissertation I have primarily studied these interactions between the galaxy’s individual components. These interactions include cloud formation from gas by Jeans instability, star formation through cloud collisions, star formation due to the snow-plow effect, gas infall from the halo, and supernovae. Each process is controlled by one or several parameters. These parameters are specified at the start of a simulation, along with other parameters which define the initial setup of the model galaxy. The influences of these evolutionary processes, and of different initial conditions of the model galaxy, were probed through series of simulations, during which all parameters were systematically varied. The simulations showed that the stellar evolution processes of the program have the ability to trigger the formation of spiral features in galaxies. Through simulations involving galaxies with counter-rotating components we con-firmed analytically derived predictions about such galaxies, like the formation of a leading one-armed spiral, the weakening of this spiral as Toomre’s Q of the system exceeds 1.8, and the strengthening of this spiral as the fraction of counter-rotating components increases. In another series of simulations we explored the properties and the origin of lopsided galaxies as a result of the high speed passage of a companion galaxy. We found that the disk’s center of mass spiraled inward, creating observed or observable phenomena, including one-armed spirals, clumps of particles, and the possible suppression of a central bar

One-Armed Spiral Waves in Galaxy Simulations with Counter-Rotating Stars

Motivated by observations of disk galaxies with counter-rotating stars, we have run two-dimensional, collisionless N-body simulations of disk galaxies with significant counter-rotating components. For all our simulations the initial value of Toomre's stability parameter was Q = 1.1. The percentage of counter-rotating particles ranges from 25% to 50%. A stationary one-arm spiral wave is observed to form in each run, persisting from a few to five rotation periods, measured at the half-mass radius. In one run, the spiral wave was initially a leading arm which subsequently transformed into a trailing arm. We also observed a change in spiral direction in the run initially containing equal numbers of particles orbiting in both directions. The results of our simulations support an interpretation of the one armed waves as due to the two stream instability.Comment: 13 pages, 4 figure

Effects of altering dietary roughage and concentrate proportions on fermentation and performance in beef cattle

University of Minnesota M.S. thesis. July 2020 . Major: Animal Sciences. Advisor: Alredo DiCostanzo. 1 computer file (PDF); vii, 239100 pages.Alterations of beef cattle diets can lead to changes in both rumen fermentation and growth of animals. Proportions of roughage and concentrate in the diet have long been the focus of nutritionists and researchers as key components in altering rumen function, post-ruminal flow of nutrients, animal performance, and carcass characteristics. Two experiments were conducted to evaluate effects of varying dietary roughage and concentrate concentrations on feedlot performance, carcass characteristics, and rumen fermentation variables. Experiment 1 investigated effects of feeding a moderate-energy diet, as a result of increased roughage, on feedlot performance and carcass characteristics of steers receiving a two-stage, terminal implant. Steers received either a moderate-energy diet or a high-energy diet for 63 d prior to finishing to coincide with initial release of a growth-promoting implant. Growing phase dry matter intake (DMI) were greater (P 0.05) in average daily gain (ADG), feed conversion efficiency, final weight, and days on feed were detected at any point during the feeding period. Marbling score was decreased (P < 0.05) by feeding a moderate-energy diet, this was further evident by an increase in number of carcasses from cattle consuming the moderate-energy ration that graded select (P < 0.05). Experiment 2 utilized dual-flow, continuous culture fermenters to evaluate three different dietary roughage and concentrate concentrations supplemented with corn or flax oil on fermenter pH, digestion, nutrient metabolism, and fermentation products. Fermenter pH was lowered as dietary concentrate increased (P < 0.05); dry matter and organic matter digestion also increased (P < 0.05) with greater proportions of concentrate. Factors associated with nitrogen metabolism were decreased (P < 0.05) as dietary roughage decreased; this may have resulted from lower pH values as well as differences in substrate. Total amount of volatile fatty acids was increased (P < 0.05) as more concentrate was included in the diet. Biohydrogenation of polyunsaturated fatty acids (PUFA) was reduced as dietary concentrate increased. This was likely more a result of decreased pH rather than dietary concentrate concentration. From these experiments it is concluded that altering roughage and concentrate concentrations increased DMI without impacting feedlot performance. Given performance of cattle was similar, marbling and carcass quality may have been impacted as a result of substrate rather than metabolizable energy intake. Rumen digestion and fermentation were improved by increasing dietary concentrate, while fermenter pH was decreased, and nitrogen metabolism was suppressed. Enzymatic breakdown of PUFA was limited as roughage concentrations were lowered in the diet. These results provide further evidence that altering dietary roughage and concentrate proportions can subsequently impact final products available to consumers.Zeltwanger, Joshua. (2020). Effects of altering dietary roughage and concentrate proportions on fermentation and performance in beef cattle. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/224920

Sloshing in High Speed Galaxy Interactions

Observations of lopsided spiral galaxies motivated us to explore whether the rapid passage of a companion galaxy could cause them. We examine whether the center of mass of the visible matter becomes displaced from the center of mass of the dark halo during the intruder's passage, thereby causing an asymmetric response and asymmetric structure. Two dimensional $N$-body simulations indicate that this can happen. We also explore some consequences of this offset. These include the center of mass of the visible disk following a decaying orbit around the halo center of mass and the development of transient one-armed spirals that persist for up to six rotation periods. We then study the results of a variety of initial conditions based on such offsets. We report on the results of several runs in which we initially offset a disk from its halo's center of mass by an amount typical of the above interaction. In some runs the halo is free to move, while in others it is held fixed. We used three different mass distributions for the halo in these runs. We find that the disk's center of mass spiraled inward creating a variety of observed or observable phenomena including one-armed spirals, massive clumps of particles, and counter-rotating waves. The systems settle into relatively axisymmetric configurations. Whether or not the end states included a bar depended on a variety of initial conditions.Comment: 20 text pages, 3 tables, 24 figures. A gzipped postscripped version with higher resolution figures can be downloaded from http://butch.umephy.maine.edu/kickers/Research/Sloshing/ . Accepted for publication in The Astrophysical Journa

CFTR gating: Invisible transitions made visible

Csanády discusses a new study that provides insight into the unique conductance properties of the CFTR chloride channel

Timing of CFTR Pore Opening and Structure of Its Transition State

In CFTR, the chloride ion channel mutated in cystic fibrosis (CF) patients, pore opening is coupled to ATP-binding-induced dimerization of two cytosolic nucleotide binding domains (NBDs) and closure to dimer disruption following ATP hydrolysis. CFTR opening rate, unusually slow because of its high-energy transition state, is further slowed by CF mutation DeltaF508. Here, we exploit equilibrium gating of hydrolysis-deficient CFTR mutant D1370N and apply rate-equilibrium free-energy relationship analysis to estimate relative timing of opening movements in distinct protein regions. We find clear directionality of motion along the longitudinal protein axis and identify an opening transition-state structure with the NBD dimer formed but the pore still closed. Thus, strain at the NBD/pore-domain interface, the DeltaF508 mutation locus, underlies the energetic barrier for opening. Our findings suggest a therapeutic opportunity to stabilize this transition-state structure pharmacologically in DeltaF508-CFTR to correct its opening defect, an essential step toward restoring CFTR function

Negative Energy and Angular Momentum Modes of Thin Accretion Disks

This work derives the linearized equations of motion, the Lagrangian density, the Hamiltonian density, and the canonical angular momentum density for general perturbations [$\propto \exp(im\phi)$ with $m=0,\pm 1,..$] of a geometrically thin self-gravitating, homentropic fluid disk including the pressure. The theory is applied to ``eccentric,'' $m=\pm 1$ perturbations of a geometrically thin Keplerian disk. We find $m=1$ modes at low frequencies relative to the Keplerian frequency. Further, it shown that these modes can have negative energy and negative angular momentum. The radial propagation of these low frequency $m=1$ modes can transport angular momentum away from the inner region of a disk and thus increase the rate of mass accretion. Depending on the radial boundary conditions there can be discrete low-frequency, negative-energy, $m=1$ modes.Comment: 24 pages, 8 figure

Are interactions the primary triggers of star formation in dwarf galaxies?

We investigate the assumption that the trigger of star formation in dwarf galaxies are interactions with other galaxies, in the context of a search for a "primary" trigger of a first generation of stars. This is cosmologically relevant because the galaxy formation process consists not only of the accumulation of gas in a gravitational potential well, but also of the triggering of star formation in this gas mass and also because some high-z potentially primeval galaxy blocks look like nearby star-forming dwarf galaxies. We review theoretical ideas proposed to account for the tidal interaction triggering mechanism and present a series of observational tests of this assumption using published data. We show also results of a search in the vicinity of a composite sample of 96 dwarf late-type galaxies for interaction candidates showing star formation. The small number of possible perturbing galaxies indentified in the neighborhood of our sample galaxies, along with similar findings from other studies, supports the view that tidal interactions may not be relevant as primary triggers of star formation. We conclude that interactions between galaxies may explain some forms of star formation triggering, perhaps in central regions of large galaxies, but they do not seem to be significant for dwarf galaxies and, by inference, for first-time galaxies forming at high redshifts. Intuitive reasoning, based on an analogy with stellar dynamics, shows that conditions for primary star formation triggering may occur in gas masses oscillating in a dark matter potential. We propose this mechanism as a plausible primary trigger scenario, worth investigating theoretically.Comment: Accepted for publication in MNRA

Stable ATP binding mediated by a partial NBD dimer of the CFTR chloride channel

Cystic fibrosis transmembrane conductance regulator (CFTR), a member of the adenosine triphosphate (ATP) binding cassette (ABC) superfamily, is an ATP-gated chloride channel. Like other ABC proteins, CFTR encompasses two nucleotide binding domains (NBDs), NBD1 and NBD2, each accommodating an ATP binding site. It is generally accepted that CFTR’s opening–closing cycles, each completed within 1 s, are driven by rapid ATP binding and hydrolysis events in NBD2. Here, by recording CFTR currents in real time with a ligand exchange protocol, we demonstrated that during many of these gating cycles, NBD1 is constantly occupied by a stably bound ATP or 8-N3-ATP molecule for tens of seconds. We provided evidence that this tightly bound ATP or 8-N3-ATP also interacts with residues in the signature sequence of NBD2, a telltale sign for an event occurring at the NBD1–NBD2 interface. The open state of CFTR has been shown to represent a two-ATP–bound NBD dimer. Our results indicate that upon ATP hydrolysis in NBD2, the channel closes into a “partial NBD dimer” state where the NBD interface remains partially closed, preventing ATP dissociation from NBD1 but allowing the release of hydrolytic products and binding of the next ATP to occur in NBD2. Opening and closing of CFTR can then be coupled to the formation and “partial” separation of the NBD dimer. The tightly bound ATP molecule in NBD1 can occasionally dissociate from the partial dimer state, resulting in a nucleotide-free monomeric state of NBDs. Our data, together with other structural/functional studies of CFTR’s NBDs, suggest that this process is poorly reversible, implying that the channel in the partial dimer state or monomeric state enters the open state through different pathways. We therefore proposed a gating model for CFTR with two distinct cycles. The structural and functional significance of our results to other ABC proteins is discussed