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

Cooling Radiation and the Lyman-alpha Luminosity of Forming Galaxies

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Ly-alpha line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z ~= 2. However, in contrast to the classical picture of gas cooling from the \sim 10^6 K virial temperature of a typical dark matter halo, we find that most of the cooling radiation is emitted by gas with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation emerges in the Ly-alpha line. While a galaxy's cooling luminosity is usually smaller than the ionizing continuum luminosity of its young stars, the two are comparable in the most massive systems, and the cooling radiation is produced at larger radii, where the Ly-alpha photons are less likely to be extinguished by dust. We suggest, in particular, that cooling radiation could explain the two large (\sim 100 kpc), luminous (L_{Ly-alpha} \sim 10^{44} erg s^{-1}) ``blobs'' of Ly-alpha emission found in Steidel et al.'s (1999) narrow band survey of a z = 3 proto-cluster. Our simulations predict objects of the observed luminosity at about the right space density, and radiative transfer effects can account for the observed sizes and line widths. We discuss observable tests of this hypothesis for the nature of the Ly-alpha blobs, and we present predictions for the contribution of cooling radiation to the Ly-alpha luminosity function of galaxies as a function of redshift.Comment: Submitted to ApJ. 28 pages including 9 PS figures. Version with color figures available at http://donald.astro.umass.edu/~fardal/papers/cooling/cooling.htm

Learners reconceptualising education: Widening participation through creative engagement?

This paper argues that engaging imaginatively with ways in which statutory and further education is provided and expanding the repertoire of possible transitions into higher education, is necessary for providers both in higher education and in the contexts and phases which precede study at this level. Fostering dispositions for creativity in dynamic engagement with educational technology together with the consideration of pedagogy, learning objects, inclusion, policy and the management of change, requires innovative provision to span the spaces between school, home, work and higher education learning. Reporting on The Aspire Pilot, a NESTA-funded initiative at The Open University, the paper offers the beginning of a theoretical frame for considering learning, learners and learning systems in the information age prioritizing learner agency. It will report emergent empirical findings from this inter-disciplinary project, with a significant e-dimension, which seeks to foster the creativity of 13-19 year olds in considering future learning systems, developing provocations for others to explore creative but grounded possibilities. It explores implications arising from this project for approaches that may facilitate widening participation in higher education

Testing Cosmological Models Against the Abundance of Damped Lyman-Alpha Absorbers

We calculate the number of damped Lyman-alpha absorbers expected in various popular cosmological models as a function of redshift and compare our predictions with observed abundances. The Press-Schechter formalism is used to obtain the distribution of halos with circular velocity in different cosmologies, and we calibrate the relation between circular velocity and absorption cross-section using detailed gas dynamical simulations of a ``standard'' cold dark matter (CDM) model. Because of this calibration, our approach makes more realistic assumptions about the absorption properties of collapsed objects than previous, analytic calculations of the damped Lyman-alpha abundance. CDM models with Omega_0=1, H_0=50, baryon density Omega_b=0.05, and scale-invariant primeval fluctuations reproduce the observed incidence and redshift evolution of damped Lyman-alpha absorption to within observational uncertainty, for both COBE normalization (sigma_8=1.2) and a lower normalization (sigma_8=0.7) that better matches the observed cluster abundance at z=0. A tilted (n=0.8, sigma_8=0.7) CDM model tends to underproduce absorption, especially at z=4. With COBE normalization, a CDM model with Omega_0=0.4, Omega_{Lambda}=0.6 gives an acceptable fit to the observed absorption; an open CDM model is marginally acceptable if Omega_0 is at least 0.4 and strongly inconsistent with the z=4 data if Omega_0=0.3. Mixed dark matter models tend not to produce sufficient absorption, being roughly comparable to tilted CDM models if Omega_{nu} = 0.2 and failing drastically if Omega_{nu} = 0.3.Comment: AASlatex, 13 pages w/ 2 embedded ps figures. To be published in ApJ, Sept. 1, 199

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

Oppositional defiant disorder

Oppositional defiant disorder (ODD) is a disruptive behaviour disorder involving an ongoing pattern of angry/irritable mood, argumentative/defiant behaviour and vindictiveness. Onset is typically before 8 years of age, although ODD can be diagnosed in both children and adults. This disorder is associated with substantial social and economic burden, and childhood ODD is one of the most common precursors of other mental health problems that can arise across the lifespan. The population prevalence of ODD is ~3 to 5%. A higher prevalence in males than females has been reported, particularly before adolescence. No single risk factor accounts for ODD. The development of this disorder seems to arise from the interaction of genetic and environmental factors, and mechanisms embedded in social relationships are understood to contribute to its maintenance. The treatment of ODD is often successful, and relatively brief parenting interventions produce large sized treatment effects in early childhood. Accordingly, ODD represents an important focus for research, practice and policy concerning early intervention and prevention in mental health.</p

Oppositional defiant disorder

Oppositional defiant disorder (ODD) is a disruptive behaviour disorder involving an ongoing pattern of angry/irritable mood, argumentative/defiant behaviour and vindictiveness. Onset is typically before 8 years of age, although ODD can be diagnosed in both children and adults. This disorder is associated with substantial social and economic burden, and childhood ODD is one of the most common precursors of other mental health problems that can arise across the lifespan. The population prevalence of ODD is ~3 to 5%. A higher prevalence in males than females has been reported, particularly before adolescence. No single risk factor accounts for ODD. The development of this disorder seems to arise from the interaction of genetic and environmental factors, and mechanisms embedded in social relationships are understood to contribute to its maintenance. The treatment of ODD is often successful, and relatively brief parenting interventions produce large sized treatment effects in early childhood. Accordingly, ODD represents an important focus for research, practice and policy concerning early intervention and prevention in mental health.</p

Mechanisms Mediating the Biologic Activity of Synthetic Proline, Glycine, and Hydroxyproline Polypeptides in Human Neutrophils

The accumulation of neutrophils at sites of tissue injury or infection is mediated by chemotactic factors released as part of the inflammatory process. Some of these factors are generated as a direct consequence of tissue injury or infection, including degradation fragments of connective tissue collagen and bacterial- or viral-derived peptides containing collagen-related structural motifs. In these studies, we examined biochemical mechanisms mediating the biologic activity of synthetic polypeptides consisting of repeated units of proline (Pro), glycine (Gly), and hydroxyproline (Hyp), major amino acids found within mammalian and bacterial collagens. We found that the peptides were chemoattractants for neutrophils. Moreover, their chemotactic potency was directly related to their size and composition. Thus, the pentameric peptides (Pro-Pro-Gly)(5) and (Pro-Hyp-Gly)(5) were more active in inducing chemotaxis than the corresponding decameric peptides (Pro-Pro-Gly)(10) and (Pro-Hyp-Gly)(10). In addition, the presence of Hyp in peptides reduced chemotactic activity. The synthetic peptides were also found to reduce neutrophil apoptosis. In contrast to chemotaxis, this activity was independent of peptide size or composition. The effects of the peptides on both chemotaxis and apoptosis were blocked by inhibitors of phosphatidylinositol 3-kinase (PI3-K) and p38 mitogen-activated protein (MAP) kinase. However, only (Pro-Pro-Gly)(5) and (Pro-Pro-Gly)(10) induced expression of PI3-K and phosphorylation of p38 MAP kinase, suggesting a potential mechanism underlying reduced chemotactic activity of Hyp-containing peptides. Although none of the synthetic peptides tested had any effect on intracellular calcium mobilization, each induced nuclear binding activity of the transcription factor NF-κB. These findings indicate that polymeric polypeptides containing Gly-X-Y collagen-related structural motifs promote inflammation by inducing chemotaxis and blocking apoptosis. However, distinct calcium-independent signaling pathways appear to be involved in these activities

Simulations of Damped Lyman-Alpha and Lyman Limit Absorbers in Different Cosmologies: Implications for Structure Formation at High Redshift

We use hydrodynamic cosmological simulations to study damped Lyman-alpha (DLA) and Lyman limit (LL) absorption at redshifts z=2-4 in five variants of the cold dark matter scenario. Our standard simulations resolve the formation of dense concentrations of neutral gas in halos with circular velocity v_c roughly 140 km/s for Omega_m=1 and 90 km/s for Omega_m=0.4, at z=2; an additional LCDM simulation resolves halos down to v_c approximately 50 km/s at z=3. We find a clear relation between HI column density and projected distance to the center of the nearest galaxy, with DLA absorption usually confined to galactocentric radii less than 10-15 kpc and LL absorption arising out to projected separations of 30 kpc or more. Detailed examination provides evidence of non-equilibrium effects on absorption cross-section. If we consider only absorption in the halos resolved by our standard simulations, then all five models fall short of reproducing the observed abundance of DLA and LL systems at these redshifts. If we extrapolate to lower halo masses, we find all four models are consistent with the observed abundance of DLA systems if the the extrapolated behavior extends to circular velocities roughly 50-80 km/s, and they may produce too much absorption if the relation continues to 40 km/s. Our results suggest that LL absorption is closely akin to DLA absorption, arising in less massive halos or at larger galactocentric radii but not caused by processes acting on a radically different mass scale.Comment: 33 pages with 10 embedded EPS figures. Substantially revised and updated from original version. Includes new high-resolution simulations. Accepted for publication in the Ap