The Atomic-to-Molecular Transition in Galaxies. III. A New Method for Determining the Molecular Content of Primordial and Dusty Clouds

Understanding the molecular content of galaxies is a critical problem in star formation and galactic evolution. Here we present a new method, based on a Stromgren-type analysis, to calculate the amount of HI that surrounds a molecular cloud irradiated by an isotropic radiation field. We consider both planar and spherical clouds, and H_2 formation either in the gas phase or catalyzed by dust grains. Under the assumption that the transition from atomic to molecular gas is sharp, our method gives the solution without any reference to the photodissociation cross section. We test our results for the planar case against those of a PDR code, and find typical accuracies of about 10%. Our results are also consistent with the scaling relations found in Paper I of this series, but they apply to a wider range of physical conditions. We present simple, accurate analytic fits to our results that are suitable for comparison to observations and to implementation in numerical and semi-analytic models.Comment: 14 pages, 5 figures, accepted to Ap

The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities

Gas in galactic disks is collected by gravitational instabilities into giant atomic-molecular complexes, but only the inner, molecular parts of these structures are able to collapse to form stars. Determining what controls the ratio of atomic to molecular hydrogen in complexes is therefore a significant problem in star formation and galactic evolution. In this paper we use the model of H_2 formation, dissociation, and shielding developed in the previous paper in this series to make theoretical predictions for atomic to molecular ratios as a function of galactic properties. We find that the molecular fraction in a galaxy is determined primarily by its column density and secondarily by its metallicity, and is to good approximation independent of the strength of the interstellar radiation field. We show that the column of atomic hydrogen required to shield a molecular region against dissociation is ~10 Msun pc^-2 at solar metallicity. We compare our model to data from recent surveys of the Milky Way and of nearby galaxies, and show that the both the primary dependence of molecular fraction on column density and the secondary dependence on metallicity that we predict are in good agreement with observed galaxy properties.Comment: Accepted to ApJ. 22 pages, 13 figures, emulateapj format. This version corrects a minor error in the binning procedure in section 4.1.2. The remainder of the paper is unchange

Gravity-driven Lyman-alpha blobs from cold streams into galaxies

We use high-resolution cosmological hydrodynamical AMR simulations to predict the characteristics of La emission from the cold gas streams that fed galaxies in massive haloes at high redshift. The La luminosity in our simulations is powered by the release of gravitational energy as gas flows from the intergalactic medium into the halo potential wells. The UV background contributes only <20% to the gas heating. The La emissivity is due primarily to electron-impact excitation cooling radiation in gas ~2x10^4K. We calculate the La emissivities assuming collisional ionisation equilibrium (CIE) at all gas temperatures. The simulated streams are self-shielded against the UV background, so photoionisation and recombination contribute negligibly to the La line formation. We produce theoretical maps of the La surface brightnesses, assuming that ~85% of the La photons are directly observable. We find that typical haloes of mass Mv~10^12-13 Msun at z~3 emit as La blobs (LABs) with luminosities 10^43-44 erg/s. Most of the La comes from the extended narrow, partly clumpy, inflowing, cold streams that feed the growing galaxies. The predicted LAB morphology is therefore irregular, with dense clumps and elongated extensions. The linewidth is expected to range from 10^2 to more than 10^3 km/s with a large variance. The typical La surface brightness profile is proportional to r^-1.2 where r is the distance from the halo centre. Our simulated LABs are similar in luminosity, morphology and extent to the observed LABs, with distinct kinematic features. The predicted La luminosity function is consistent with observations, and the predicted areas and linewidths roughly recover the observed scaling relations. This mechanism for producing LABs appears inevitable in many high-z galaxies. Some of the LABs may thus be regarded as direct detections of the cold streams that drove galaxy evolution at high z.Comment: 21 pages, 20 figures, final version accepted for publication in MNRA

Exploring the solid state and solution structural chemistry of the utility amide potassium hexamethyldisilazide (KHMDS)

The structural chemistry of eleven donor complexes of the important Brønsted base potassium 1,1,1,3,3,3-hexamethyldisilazide (KHMDS) has been studied. Depending on the donor, each complex adopted one of four general structural motifs. Specifically, in this study the donors employed were toluene (to give polymeric 1 and dimeric 2), THF (dimeric 3), N,N,N',N'-tetramethylethylenediamine (TMEDA) (dimeric 4), (R,R)-N,N,N',N'-tetramethyl-1,2-diaminocyclohexane [(R,R)-TMCDA] (dimeric 5), 12-crown-4 (dimeric 6), N,N,N',N'-tetramethyldiaminoethyl ether (TMDAE) (tetranuclear dimeric 8 and monomeric 10), N,N,N',N',N''-pentamethyldiethylentriamine (PMDETA) (tetranuclear dimeric 7), tris[2-dimethyl(amino)ethyl]amine (Me6TREN) (tetranuclear dimeric 9) and tris{2-(2-methoxyethoxy)ethyl}amine (TMEEA) (monomeric 11). The complexes were also studied in solution by 1H and 13C NMR spectroscopy as well as DOSY NMR spectroscopy

What Phase of the Interstellar Medium Correlates with the Star Formation Rate?

Nearby spiral galaxies show an extremely tight correlation between tracers of molecular hydrogen (H_2) in the interstellar medium (ISM) and tracers of recent star formation, but it is unclear whether this correlation is fundamental or accidental. In the galaxies that have been surveyed to date, H_2 resides predominantly in gravitationally bound clouds cooled by carbon monoxide (CO) molecules, but in galaxies of low metal content the correlations between bound clouds, CO, and H_2 break down, and it is unclear if the star formation rate will then correlate with H_2 or with some other quantity. Here we show that star formation will continue to follow H_2 independent of metallicity. This is not because H_2 is directly important for cooling, but instead because the transition from predominantly atomic hydrogen (HI) to H_2 occurs under the same conditions as a dramatic drop in gas temperature and Bonnor-Ebert mass that destabilizes clouds and initiates collapse. We use this model to compute how star formation rate will correlate with total gas mass, with mass of gas where the hydrogen is H_2, and with mass of gas where the carbon is CO in galaxies of varying metallicity, and show that preliminary observations match the trend we predict.Comment: 13 pages, 7 figures, emulateapj format, accepted to ApJ; minor revisions to discussion, and a minor error in figures 4 and 5 fixed. No other change

An Empirical Study of the Relationship between Ly{\alpha} and UV selected Galaxies: Do Theorists and Observers `Select' the Same Objects?

Lyman Alpha Emitters (LAEs) are galaxies that have been selected on the basis of a strong Ly{\alpha} emission line in their spectra. Observational campaigns have dramatically increased the sample of known LAEs, which now extends out to z=7. These discoveries have motivated numerous theoretical studies on the subject, which usually define LAEs in their models based on sharp Ly{\alpha} luminosity and equivalent width (EW) cuts. While broadly representative, this procedure does not mimic the selection from observational programs in detail, which instead use cuts in various colour-spaces. We investigate what implications this disjoint may have for studies that aim to model LAEs. We construct an empirical model for the number density of star forming galaxies as a function of their UV and Ly{\alpha} luminosity, utilising measured constraints on the luminosity functions (LFs) of drop-out galaxies, and their luminosity dependent probability distribution function of Ly{\alpha} EW. In particular, we investigate whether the LAE LFs can be reproduced by defining LAEs using a (z-dependent) Ly{\alpha} luminosity and EW threshold. While we are able to reproduce the observed distribution of Ly{\alpha} EW among LAEs out to restframe EW 200 A, we find that our formalism over-predicts both the UV and Ly{\alpha} LFs of LAEs by a factor of 2-3, and is inconsistent with observations at the ~95% level. This tension is partially resolved if we assume the Ly{\alpha} EW-distribution of drop-out galaxies to be truncated at restframe EW>150 A. However the overprediction indicates that modeling LAEs with simple REW and luminosity cuts does not accurately mimic observed selection criteria, and can lead to uncertainties in the predicted number density of LAEs. On the other hand, the predicted z-evolution is not affected. We apply our formalism to drop-out galaxies at z>6, and predict the LFs of LAEs at z=7-9.Comment: MNRAS in press. Minor changes: expanded comparison with previous work, and fixed some typos in the equation

Modeling intracranial aneurysm stability and growth: An integrative mechanobiological framework for clinical cases

We present a novel patient-specific fluid-solid-growth framework to model the mechanobiological state of clinically detected intracranial aneurysms (IAs) and their evolution. The artery and IA sac are modeled as thick-walled, non-linear elastic fiber-reinforced composites. We represent the undulation distribution of collagen fibers: the adventitia of the healthy artery is modeled as a protective sheath whereas the aneurysm sac is modeled to bear load within physiological range of pressures. Initially, we assume the detected IA is stable and then consider two flow-related mechanisms to drive enlargement: (1) low wall shear stress; (2) dysfunctional endothelium which is associated with regions of high oscillatory flow. Localized collagen degradation and remodelling gives rise to formation of secondary blebs on the aneurysm dome. Restabilization of blebs is achieved by remodelling of the homeostatic collagen fiber stretch distribution. This integrative mechanobiological modelling workflow provides a step towards a personalized risk-assessment and treatment of clinically detected IAs

Spatial and temporal analogies in microbial communities in natural drinking water biofilms

Biofilms are ubiquitous throughout drinking water distribution systems (DWDS), playing central roles in system performance and delivery of safe clean drinking water. However, little is known about how the interaction of abiotic and biotic factors influence the microbial communities of these biofilms in real systems. Results are presented here from a one-year study using in situ sampling devices installed in two operational systems supplied with different source waters. Independently of the characteristics of the incoming water and marked differences in hydraulic conditions between sites and over time, a core bacterial community was observed in all samples suggesting that internal factors (autogenic) are central in shaping biofilm formation and composition. From this it is apparent that future research and management strategies need to consider the specific microorganisms found to be able to colonise pipe surfaces and form biofilms, such that it might be possible to exclude these and hence protect the supply of safe clean drinking water

International consensus on the most useful physical examination tests used by physiotherapists for patients with headache: A Delphi study

Background: A wide range of physical tests have been published for use in the assessment of musculoskeletal dysfunction in patients with headache. Which tests are used depends on a physiotherapist's clinical and scientific background as there is little guidance on the most clinically useful tests. Objectives: To identify which physical examination tests international experts in physiotherapy consider the most clinically useful for the assessment of patients with headache. Design/methods: Delphi survey with pre-specified procedures based on a systematic search of the literature for physical examination tests proposed for the assessment of musculoskeletal dysfunction in patients with headache. Results: Seventeen experts completed all three rounds of the survey. Fifteen tests were included in round one with eleven additional tests suggested by the experts. Finally eleven physical examination tests were considered clinically useful: manual joint palpation, the cranio-cervical flexion test, the cervical flexion-rotation test, active range of cervical movement, head forward position, trigger point palpation, muscle tests of the shoulder girdle, passive physiological intervertebral movements, reproduction and resolution of headache symptoms, screening of the thoracic spine, and combined movement tests. Conclusions: Eleven tests are suggested as a minimum standard for the physical examination of musculoskeletal dysfunctions in patients with headache