The Nature of Lyman Break Galaxies in Cosmological Hydrodynamic Simulations

What type of objects are being detected as $z\sim 3$ "Lyman break galaxies"? Are they predominantly the most massive galaxies at that epoch, or are many of them smaller galaxies undergoing a short-lived burst of merger-induced star formation? We attempt to address this question using high-resolution cosmological hydrodynamic simulations including star formation and feedback. Our $\Lambda$CDM simulation, together with Bruzual-Charlot population synthesis models, reproduces the observed number density and luminosity function of Lyman break galaxies when dust is incorporated. The inclusion of dust is crucial for this agreement. In our simulation, these galaxies are predominantly the most massive objects at this epoch, and have a significant population of older stars. Nevertheless, it is possible that our simulations lack the resolution and requisite physics to produce starbursts, despite having a physical resolution of \la 700 pc at z=3. Thus we cannot rule out merger-induced starburst galaxies also contributing to the observed population of high-redshift objects.Comment: 5 pages, contribution to the Proceedings of Rencontres Internationales de l'IGRAP, Clustering at High Redshift, Marseille 199

Theoretical Modeling of the High Redshift Galaxy Population

We review theoretical approaches to the study of galaxy formation, with emphasis on the role of hydrodynamic simulations in modeling the high redshift galaxy population. We present new predictions for the abundance of star-forming galaxies in the Lambda + cold dark matter model (Omega_m=0.4, Omega_L=0.6), combining results from several simulations to probe a wide range of redshift. At a threshold density of one object per arcmin^2 per unit z, these simulations predict galaxies with star formation rates of 2 msun/yr (z=10), 5 msun/yr (z=8), 20 msun/yr (z=6), 70-100 msun/yr (z=4-2), and 30 msun/yr (z=0.5). For galaxies selected at a fixed comoving space density n=0.003 h^3 Mpc^{-3], a (50 Mpc/h)^3 simulation predicts a galaxy correlation function (r/5 Mpc/h)^{-1.8} in comoving coordinates, essentially independent of redshift from z=4 to z=0.5. Different cosmological models predict global histories of star formation that reflect their overall histories of mass clustering, but robust numerical predictions of the comoving space density of star formation are difficult because the simulations miss the contribution from galaxies below their resolution limit. The LCDM model appears to predict a star formation history with roughly the shape inferred from observations, but it produces too many stars at low redshift, predicting Omega_* ~ 0.015 at z=0. We conclude with a brief discussion of this discrepancy and three others that suggest gaps in our current theory of galaxy formation: small disks, steep central halo profiles, and an excess of low mass dark halos. While these problems could fade as the simulations or observations improve, they could also guide us towards a new understanding of galactic scale star formation, the spectrum of primordial fluctuations, or the nature of dark matter.Comment: 12 pages, 3 figs. To be published in "Photometric Redshifts and High Redshift Galaxies", eds. R. Weymann, L. Storrie-Lombardi, M. Sawicki & R. Brunner, (San Francisco: ASP Conference Series

Integrated Computer Systems for Monitoring of the Critically Ill

Conference PaperBiomedical Informatic

Temperature Dependence Of Wood Surface Energy

A thorough understanding of the wood surface is required to engineer adhesive bonding in composite applications. A surface analysis technique, dynamic contact angle (DCA) analysis, was used to examine the effects of temperature on the wood surface as measured by the contact angle and surface energy. A hydrophobic surface transition was found on the wood surface at 60 C, which coincides with the glass transition of lignin as measured by differential scanning calorimetry. The change in the surface at the glass transition can be attributed to the diffusion of nonpolar molecular groups to the surface. This could be the result of the migration and deposition of extractives, reorientation of macromolecules, or a combination of the two. Similar behavior has been observed in synthetic amorphous polymers. Although the surface of wood is complex, the results indicate that it can be investigated and understood like synthetic polymer materials

Hypothalamic Vitamin D Improves Glucose Homeostasis and Reduces Weight

Despite clear associations between vitamin D deficiency and obesity and/or type 2 diabetes, a causal relationship is not established. Vitamin D receptors (VDRs) are found within multiple tissues, including the brain. Given the importance of the brain in controlling both glucose levels and body weight, we hypothesized that activation of central VDR links vitamin D to the regulation of glucose and energy homeostasis. Indeed, we found that small doses of active vitamin D, 1α,25-dihydroxyvitamin D3 (1,25D3) (calcitriol), into the third ventricle of the brain improved glucose tolerance and markedly increased hepatic insulin sensitivity, an effect that is dependent upon VDR within the paraventricular nucleus of the hypothalamus. In addition, chronic central administration of 1,25D3 dramatically decreased body weight by lowering food intake in obese rodents. Our data indicate that 1,25D3-mediated changes in food intake occur through action within the arcuate nucleus. We found that VDR colocalized with and activated key appetite-regulating neurons in the arcuate, namely proopiomelanocortin neurons. Together, these findings define a novel pathway for vitamin D regulation of metabolism with unique and divergent roles for central nervous system VDR signaling. Specifically, our data suggest that vitamin D regulates glucose homeostasis via the paraventricular nuclei and energy homeostasis via the arcuate nuclei

Management factors associated with seropositivity to Lawsonia intracellularis in US swine herds.

abstract: This study was conducted to determine risk factors for Lawsonia intracellularis seropositivity in the breeding and grower-finisher units of US farrowing-to-finishing swine herds. Serum was collected from 15 breeding females and 15 grower-finisher pigs per herd in 184 farrow-to-finish herds, a subset of 405 herds in the National Animal Health Monitoring System (NAHMS) Swine 1995 Study that examined management, health and productivity in herds with at least 300 finisher pigs. Sera were tested by indirect fluorescent antibody test for L. intracellularis. Test results were linked with NAHMS questionnaire data and a logistic regression model of management factors associated with L. intracellularis serological status was developed. Separate models were used for breeding and grower-finisher units. Risk factors for seropositive breeding units were L intracellularis-seropositive status of the grower-finisher unit, use of a continuous system of management for the farrowing unit and a young parity structure (<75% multiparous sows). Risk factors for seropositive grower-finisher units were L. intracellularis-seropositive status of the breeding unit, the number of pigs entering the grower-finisher stage, raising pigs on concrete slats, and intensive management compared with raising pigs on outdoor lots. Use of all in-all out management in the farrowing house and an older parity structure in the sow herd were associated with a lower risk of L. intracellularis seropositivity in the breeding unit, and slatted concrete flooring in grower-finisher houses was associated with a greater risk. Alteration of these management factors might improve control of L. intracellularis infection in farrowing-to-finishing herds

The Population of Damped Lyman-alpha and Lyman Limit Systems in the Cold Dark Matter Model

Lyman limit and damped Lyman-alpha absorption systems probe the distribution of collapsed, cold gas at high redshift. Numerical simulations that incorporate gravity and gas dynamics can predict the abundance of such absorbers in cosmological models. We develop a semi-analytical method to correct the numerical predictions for the contribution of unresolved low mass halos, and we apply this method to the Katz et al. (1996) simulation of the standard cold dark matter model ($\Omega=1$, $h=0.5$, $\Omega_b=0.05$, $\sigma_8=0.7$). Using this simulation and higher resolution simulations of individual low mass systems, we determine the relation between a halo's circular velocity $v_c$ and its cross section for producing Lyman limit or damped absorption. We combine this relation with the Press-Schechter formula for the abundance of halos to compute the number of absorbers per unit redshift. The resolution correction increases the predicted abundances by about a factor of two at z=2, 3, and 4, bringing the predicted number of damped absorbers into quite good agreement with observations. Roughly half of the systems reside in halos with circular velocities v_c>100\kms and half in halos with 35\kms. Halos with v_c>150\kms typically harbor two or more systems capable of producing damped absorption. Even with the resolution correction, the predicted abundance of Lyman limit systems is a factor of three below observational estimates, signifying either a failure of standard CDM or a failure of these simulations to resolve the systems responsible for most Lyman limit absorption. By comparing simulations with and without star formation, we find that depletion of the gas supply by star formation affects absorption line statistics at $z>=2$ only for column densities exceeding $N_{HI}=10^{22} cm^{-2}$.Comment: AASlatex, 17 pages w/ 3 embedded ps figures. Submitted to Ap

Scintillation in the Circinus Galaxy water megamasers

We present observations of the 22 GHz water vapor megamasers in the Circinus galaxy made with the Tidbinbilla 70m telescope. These observations confirm the rapid variability seen earlier by Greenhill et al (1997). We show that this rapid variability can be explained by interstellar scintillation, based on what is now known of the interstellar scintillation seen in a significant number of flat spectrum AGN. The observed variability cannot be fully described by a simple model of either weak or diffractive scintillation.Comment: 10 pages, 5 figures. AJ accepte

The Infuence of Omega_baryon on High-Redshift Structure

We analyze high-redshift structure in three hydrodynamic simulations that have identical initial conditions and cosmological parameters and differ only in the value of the baryon density parameter, Omega_b=0.02, 0.05, 0.125. Increasing Omega_b does not change the fraction of baryons in the diffuse (unshocked) phase of the intergalactic medium, but it increases cooling rates and therefore transfers some baryons from the shocked intergalactic phase to the condensed phase associated with galaxies. Predictions of Lyman-alpha forest absorption are almost unaffected by changes of Omega_b provided that the UV background intensity is adjusted so that the mean opacity of the forest matches the observed value. The required UV background intensity scales as Omega_b^1.7, and the higher photoionization rate increases the gas temperature in low density regions. Damped Lyman-alpha absorption and Lyman limit absorption both increase with increasing Omega_b, though the impact is stronger for damped absorption and is weaker at z=4 than at z=2-3. The mass of cold gas and stars in high-redshift galaxies increases faster than Omega_b but slower than Omega_b^2, and the global star formation rate scales approximately as Omega_b^1.5. In the higher Omega_b models, the fraction of baryonic material within the virial radius of dark matter halos is usually higher than the universal fraction, indicating that gas dynamics and cooling can lead to over-representation of baryons in virialized systems. On the whole, our results imply a fairly intuitive picture of the influence of Omega_b on high-redshift structure, and we provide scalings that can be used to estimate the impact of Omega_b uncertainties on the predictions of hydrodynamic simulations.Comment: 22 pages, 9 figures, submitted to ApJ, high-resolution version availible at http://lahmu.phyast.pitt.edu/~gardner/publications/omegab.ps.g