Building the Stellar Halo Through Feedback in Dwarf Galaxies

We present a new model for the formation of stellar halos in dwarf galaxies. We demonstrate that the stars and star clusters that form naturally in the inner regions of dwarfs are expected to migrate from the gas rich, star forming centre to join the stellar spheroid. For dwarf galaxies, this process could be the dominant source of halo stars. The effect is caused by stellar feedback-driven bulk motions of dense gas which, by causing potential fluctuations in the inner regions of the halo, couple to all collisionless components. This effect has been demonstrated to generate cores in otherwise cuspy cold dark matter profiles and is particularly effective in dwarf galaxy haloes. It can build a stellar spheroid with larger ages and lower metallicities at greater radii without requiring an outside-in formation model. Globular cluster-type star clusters can be created in the galactic ISM and then migrate to the spheroid on 100\thinspace Myr timescales. Once outside the inner regions they are less susceptible to tidal disruption and are thus long lived; clusters on wider orbits may be easily unbound from the dwarf to join the halo of a larger galaxy during a merger. A simulated dwarf galaxy ($\text{M}_{vir}\simeq10^{9}\text{M}_{\odot}$ at $z=5$) is used to examine this gravitational coupling to dark matter and stars.Comment: 12 pages, 3 figures, accepted for publication in ApJ

Finite size effects on the Poynting-Robertson effect: a fully general relativistic treatment

Ever since the first discovery of Poynting and Robertson, the radiation source has been treated as merely a point. Even in a very few studies where the size of the source has been taken into account, the treatment of the problem remained largely non-relativistic. In the present work, we address the issue of the finite size effects on the Poynting-Robertson effect in a fully relativistic manner for the first time. As a result, the emergence and the characteristic of the critical point/suspension orbit can be studied in a systematic and detailed manner.Comment: 11pages, 3figure

Using chlorhexidine varnish to prevent early childhood caries in American Indian children

Objectives: To test the efficacy of 10% chlorhexidine (CHX) dental varnish applied to the mothers' dentition in preventing caries in American Indian children. Methods: This was a placebo‐controlled, double‐blind, randomized clinical trial. Mother–child pairs were enrolled when the child was 4.5‐6.0 months. Mothers received 4 weekly applications of the study treatment (CHX or placebo) followed by single applications when her child was age 12 and 18 months. Children received caries examinations at enrollment, 12, 18 and 24 months. Analyses were limited to the intent‐to‐treat (ITT) group: children whose mothers received the first study treatment and who received at least one post‐baseline exam. The outcome variable was the number of new carious surfaces (NNCS) at the child's last visit. Wilcoxon nonparametric and Fisher's exact tests were used to test differences between the active and placebo groups. Results: We randomized 414 mother–child pairs, with 367 (88.6%) included in the ITT group (active = 188, placebo = 179). The proportion of children caries‐free at their final exam was 51.1% and 50.8% for the active and placebo groups ( P  > 0.99). The mean NNCS for the active and placebo groups was 3.82 (standard deviation [SD] = 8.18) and 3.80 (SD = 6.08), respectively ( P  = 0.54). The proportion with NNCS > 6 was 18.1% for active children versus 27.9% for placebo (relative risk [RR] = 0.65, P  = 0.03). The number needed to treat to shift one child from NNCS > 6 to a lower severity was 10.2. Conclusions: In this population CHX varnish did not reduce the mean NNCS or proportion of children with caries, but did reduce the proportion with severe caries.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96744/1/jphd348.pd

The central slope of dark matter cores in dwarf galaxies: Simulations vs. THINGS

We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a LCDM cosmology and the observed dwarf galaxies sample from the THINGS survey in terms of (1) the rotation curve shape and (2) the logarithmic inner density slope alpha of mass density profiles. The simulations, which include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating and most importantly physically motivated gas outflows driven by supernovae (SNe), form bulgeless galaxies with DM cores. We show that the stellar and baryonic mass is similar to that inferred from photometric and kinematic methods for galaxies of similar circular velocity. Analyzing the simulations in exactly the same way as the observational sample allows us to address directly the so-called "cusp/core" problem in the LCDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than CDM rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes alpha of the simulated galaxies' dark matter density profiles is ~ -0.4 +- 0.1, which shows good agreement with \alpha = -0.29 +- 0.07 of the THINGS dwarf galaxies. The effect of non-circular motions is not significant enough to affect the results. This confirms that the baryonic feedback processes included in the simulations are efficiently able to make the initial cusps with \alpha ~ -1.0 to -1.5 predicted by dark-matter-only simulations shallower, and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.Comment: 13 pages, 7 figures; Accepted for publication in AJ; minor correction

Formation and evolution of dwarf galaxies in the CDM Universe

We first review the results of the tidal stirring model for the transformation of gas-rich dwarf irregulars into dwarf spheroidals, which turns rotationally supported stellar systems into pressure supported ones. We emphasize the importance of the combined effect of ram pressure stripping and heating from the cosmic ultraviolet background in removing the gas and converting the object into a gas poor system as dSphs. We discuss how the timing of infall of dwarfs into the primary halo determines the final mass-to-light ratio and star formation history. Secondly we review the results of recent cosmological simulations of the formation of gas-rich dwarfs. These simulations are finally capable to produce a realistic object with no bulge, an exponential profile and a slowly rising rotation curve. The result owes to the inclusion of an inhomogeneous ISM and a star formation scheme based on regions having the typical density of molecular cloud complexes. Supernovae-driven winds become more effective in such mode, driving low angular momentum baryons outside the virial radius at high redshift and turning the dark matter cusp into a core. Finally we show the first tidal stirring experiments adopting dwarfs formed in cosmological simulations as initial conditions. The latter are gas dominated and have have turbulent thick gaseous and stellar disks disks that cannot develop strong bars, yet they are efficiently heated into spheroids by tidal shocks.Comment: 14 pages, 4 Figures, o appear in the proceedings of the CRAL conference, Lyon, June 2010, "A Universe of Dwarf Galaxies", eds. Philippe Prugniel & Mina Koleva; EDP Sciences in the European Astronomical Society Publications Series. (invited talk

The First Galaxies: Assembly of Disks and Prospects for Direct Detection

The James Webb Space Telescope (JWST) will enable observations of galaxies at redshifts z > 10 and hence allow to test our current understanding of structure formation at very early times. Previous work has shown that the very first galaxies inside halos with virial temperatures T < 10^4 K and masses M < 10^8 M_sun at z > 10 are probably too faint, by at least one order of magnitude, to be detected even in deep exposures with JWST. The light collected with JWST may therefore be dominated by radiation from galaxies inside ten times more massive halos. We use cosmological zoomed smoothed particle hydrodynamics simulations to investigate the assembly of such galaxies and assess their observability with JWST. We compare two simulations that are identical except for the inclusion of non-equilibrium H/D chemistry and radiative cooling by molecular hydrogen. In both simulations a large fraction of the halo gas settles in two nested, extended gas disks which surround a compact massive gas core. The presence of molecular hydrogen allows the disk gas to reach low temperatures and to develop marked spiral structure but does not qualitatively change its stability against fragmentation. We post-process the simulated galaxies by combining idealized models for star formation with stellar population synthesis models to estimate the luminosities in nebular recombination lines as well as in the ultraviolet continuum. We demonstrate that JWST will be able to constrain the nature of the stellar populations in galaxies such as simulated here based on the detection of the He1640 recombination line. Extrapolation of our results to halos with masses both lower and higher than those simulated shows that JWST may find up to a thousand star-bursting galaxies in future deep exposures of the z > 10 universe.Comment: 19 pages, 13 figures. Accepted for publication in ApJ. Revised version with improved presentations and discussion

Interplay between Stellar Spirals and the ISM in Galactic Disks

We propose a new dynamical picture of galactic stellar and gas spirals, based on hydrodynamic simulations in a `live' stellar disk. We focus especially on spiral structures excited in a isolated galactic disk without a stellar bar. Using high-resolution, 3-dimensional N-body/SPH simulations, we found that the spiral features of the gas in galactic disks are formed by essentially different mechanisms from the galactic shock in stellar density waves. The stellar spiral arms and the interstellar matter on average corotate in a galactic potential at any radii. Unlike the stream motions in the galactic shock, the interstellar matter flows into the local potential minima with irregular motions. The flows converge to form dense gas clouds/filaments near the bottom of the stellar spirals, whose global structures resemble dust-lanes seen in late-type spiral galaxies. The stellar arms are non-steady; they are wound and stretched by the galactic shear, and thus local densities of the arm change on a time scale of ~ 100 Myrs, due to bifurcating or merging with other arms. This makes the gas spirals associated with the stellar arms non-steady. The association of dense gas clouds are eventually dissolved into inter-arm regions with non-cirucular motions. Star clusters are formed from the cold, dense gases, whose ages are less than ~30 Myrs, and they are roughly associated with the background stellar arms without a clear spatial offset between gas spiral arms and distribution of young stars.Comment: 13 pages, 12 figures, accepted by ApJ. Higher resolution of ms.pdf is available at http://d.pr/Nvjk A targzipped Supplementary movies is available at http://d.pr/TV6

Energy spectra of quasiperiodic systems via information entropy

We study the relationship between the electronic spectrum structure and the configurational order of one-dimensional quasiperiodic systems. We take the Fibonacci case as an specific example, but the ideas outlined here may be useful to accurately describe the energy spectra of general quasiperiodic systems of technological interest. Our main result concerns the {\em minimization} of the information entropy as a characteristic feature associated to quasiperiodic arrangements. This feature is shown to be related to the ability of quasiperiodic systems to encode more information, in the Shannon sense, than periodic ones. In the conclusion we comment on interesting implications of these results on further developments on the issue of quasiperiodic order.Comment: REVTeX 3.0, 8 pages, 3 figures available on request from FD-A ([email protected]), Phys Rev E submitted, MA/UC3M/02/9

Exercise Training Increases Parietal Lobe Cerebral Blood Flow in Chronic Stroke: An Observational Study

Exercise is increasingly recommended as an essential component of stroke rehabilitation, yet uncertainty remains with respect to its direct effect on the cerebral vasculature. The current study first demonstrated the repeatability of pseudo-continuous arterial spin labeling (ASL) magnetic resonance imaging (MRI) in older adults with stroke, and then investigated the change in cerebrovascular function following a 6-month cardiovascular rehabilitation program. In the repeatability study, 12 participants at least 3 months post-stroke underwent two ASL imaging scans 1 month apart. In the prospective observational study, eight individuals underwent ASL imaging and aerobic fitness testing before and after a 6-month cardiovascular rehabilitation program. Cerebral blood flow (CBF) and the spatial coefficient of variation of CBF (sCoV) were quantified to characterize tissue-level perfusion and large cerebral artery transit time properties, respectively. In repeat scanning, intraclass correlation (ICC) indicated moderate test-retest reliability for global gray matter CBF (ICC = 0.73) and excellent reliability for sCoV (ICC = 0.94). In the observational study, gray matter CBF increased after training (baseline: 40 ± 13 vs. 6-month: 46 ± 12 ml·100 g−1·min−1, P = 0.036). The greatest change occurred in the parietal lobe (+18 ± 12%). Gray matter sCoV, however, did not change following training (P = 0.31). This study provides preliminary evidence that exercise-based rehabilitation in chronic stroke enhances tissue-level perfusion, without changing the relative hemodynamic properties of the large cerebral arteries

Evidence for carbon sequestration by agricultural liming

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95356/1/gbc1382.pd