775 research outputs found
Feedback-regulated Super Massive Black Hole Seed Formation
The nature of the seeds of high-redshift supermassive black holes (SMBHs) is
a key question in cosmology. Direct collapse black holes (DCBH) that form in
pristine, atomic-line cooling halos, illuminated by a Lyman-Werner (LW) UV flux
exceeding a critical threshold J_crit, represent an attractive possibility. We
investigate when and where these conditions are met during cosmic evolution.
For the LW intensity, J_LW, we account for departures from the background value
in close proximity to star forming galaxies. For the pristine halo fraction, we
account for both (i) supernova driven outflows, and (ii) the inherent pollution
from progenitor halos. We estimate the abundance of DCBH formation sites,
n_DCBH(z), and find that it increases with cosmic time from n_DCBH(z=20) ~
1e-12 -1e-7 cMpc^-3 to n_DCBH(z=10) ~ 1e-10 - 1e-5 cMpc^-3. Our analysis shows
the possible importance of galactic winds, which can suppress the predicted
n_DCBH by several orders of magnitude, and cause DCBH formation to
preferentially occur around the UV-brightest (M_UV ~ -22 to -20) star forming
galaxies. Our analysis further highlights the dependence of these predictions
on (i) the escape fraction of LW photons, (ii) J_crit, and (iii) the galactic
outflow prescription.Comment: 13 pages, 9 figures, accepted to MNRA
Gas-liquid phase separation in oppositely charged colloids: stability and interfacial tension
We study the phase behavior and the interfacial tension of the screened
Coulomb (Yukawa) restricted primitive model (YRPM) of oppositely charged hard
spheres with diameter s using Monte Carlo simulations. We determine the
gas-liquid and gas-solid phase transition using free energy calculations and
grand-canonical Monte Carlo simulations for varying inverse Debye screening
length k. We find that the gas-liquid phase separation is stable for k s <= 4,
and that the critical temperature decreases upon increasing the screening of
the interaction (decreasing the range of the interaction). In addition, we
determine the gas-liquid interfacial tension using grand-canonical Monte Carlo
simulations. The interfacial tension decreases upon increasing the range of the
interaction. In particular, we find that simple scaling can be used to relate
the interfacial tension of the YRPM to that of the restricted primitive model,
where particles interact with bare Coulomb interactions.Comment: 17 pages, 6 Figures, accepted for publication in J. Chem. Phy
Can the intergalactic medium cause a rapid drop in Lyman alpha emission at z>6?
The large cross-section of the Lyman alpha (Lya) line makes it a sensitive
probe of the ionization state of the intergalactic medium (IGM). Here we
present the most complete study to date of the IGM Lya opacity, and its
application to the redshift evolution of the 'Lya fraction', i.e. the fraction
of color-selected galaxies with a detectable Lya emission line. We use a tiered
approach, which combines large-scale semi-numeric simulations of reionization
with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us
to simultaneously account for evolution in both: (i) the opacity from an
incomplete (patchy) reionization, parameterized by the filling factor of
ionized regions, Q_HII; and (ii) the opacity from self-shielded systems in the
ionized IGM, parameterized by the average photo-ionization rate inside HII
regions, \Gamma. In contrast to recent empirical models, attenuation from
patchy reionization has a unimodal distribution along different sightlines,
while attenuation from self-shielded systems is more bimodal. We quantify the
average IGM transmission in our (Q_HII, \Gamma) parameter space, which can
easily be used to interpret new data sets. Using current observations, we
predict that the Lya fraction cannot drop by more than a factor of ~2 with IGM
attenuation alone, even for HII filling factors as low as Q_HII>0.1. Larger
changes in the Lya fraction could result from a co-evolution with galaxy
properties. Marginalizing over \Gamma, we find that current observations
constrain Q_HII < 0.6 at z=7 [68% confidence level (C.L.)]. However, all of our
parameter space is consistent with observations at 95% C.L., highlighting the
need for larger observational samples at z >= 6.Comment: 12 pages, 10 figures, MNRAS submitte
Crystallization and gelation in colloidal systems with short-ranged attractive interactions
We systematically study the relationship between equilibrium and
non-equilibrium phase diagrams of a system of short-ranged attractive colloids.
Using Monte Carlo and Brownian dynamics simulations we find a window of
enhanced crystallization that is limited at high interaction strength by a
slowing down of the dynamics and at low interaction strength by the high
nucleation barrier. We find that the crystallization is enhanced by the
metastable gas-liquid binodal by means of a two-stage crystallization process.
First, the formation of a dense liquid is observed and second the crystal
nucleates within the dense fluid. In addition, we find at low colloid packing
fractions a fluid of clusters, and at higher colloid packing fractions a
percolating network due to an arrested gas-liquid phase separation that we
identify with gelation. We find that this arrest is due to crystallization at
low interaction energy and it is caused by a slowing down of the dynamics at
high interaction strength. Likewise, we observe that the clusters which are
formed at low colloid packing fractions are crystalline at low interaction
energy, but glassy at high interaction energy. The clusters coalesce upon
encounter.Comment: 8 pages, 8 figure
Effects of tailoring ingredients in auditory persuasive health messages on fruit and vegetable intake
Objective: Health messages can be tailored by applying different tailoring ingredients, among which personalisation, feedback and adaptation. This experiment investigated the separate effects of these tailoring ingredients on behaviour in auditory health persuasion. Furthermore, the moderating effect of self-efficacy was assessed.Design: The between-participants design consisted of four conditions. A generic health message served as a control condition; personalisation was applied using the recipient's first name, feedback was given on the personal state, or the message was adapted to the recipient's value.Main outcome measures: The study consisted of a pre-test questionnaire (measuring fruit and vegetable intake and perceived difficulty of performing these behaviours, indicating self-efficacy), exposure to the auditory message and a follow-up questionnaire measuring fruit and vegetable intake two weeks after message exposure (n=112).Results: ANCOVAs showed no main effect of condition on either fruit or vegetable intake, but a moderation was found on vegetable intake: When self-efficacy was low, vegetable intake was higher after listening to the personalisation message. No significant differences between the conditions were found when self-efficacy was high.Conclusion: Individuals with low self-efficacy seemed to benefit from incorporating personalisation, but only regarding vegetable consumption. This finding warrants further investigation in tailoring research
Phase behavior of hard spheres confined between parallel hard plates: Manipulation of colloidal crystal structures by confinement
We study the phase behavior of hard spheres confined between two parallel
hard plates using extensive computer simulations. We determine the full
equilibrium phase diagram for arbitrary densities and plate separations from
one to five hard-sphere diameters using free energy calculations. We find a
first-order fluid-solid transition, which corresponds to either capillary
freezing or melting depending on the plate separation. The coexisting solid
phase consists of crystalline layers with either triangular or square symmetry.
Increasing the plate separation, we find a sequence of crystal structures from
n triangular to (n+1) square to (n+1) triangular, where n is the number of
crystal layers, in agreement with experiments on colloids. At high densities,
the transition between square to triangular phases are intervened by
intermediate structures, e.g., prism, buckled, and rhombic phases.Comment: 9 pages, 4 figures. Accepted for publication in J. Phys.: Condens.
Matte
Effect of excluded volume interactions on the interfacial properties of colloid-polymer mixtures
We report a numerical study of equilibrium phase-diagrams and interfacial
properties of bulk and confined colloid-polymer mixtures using grand canonical
Monte Carlo simulations. Colloidal particles are treated as hard spheres, while
the polymer chains are described as soft repulsive spheres. The
polymer-polymer, colloid-polymer, and wall-polymer interactions are described
by density-dependent potentials derived by Bolhuis and Louis [Macromolecules,
35 (2002), p.1860]. We compared our results with those of the
Asakura-Oosawa-Vrij model, that treats the polymers as ideal particles. We find
that the number of polymers needed to drive the demixing transition is larger
for the interacting polymers, and that the gas-liquid interfacial tension is
smaller. When the system is confined between two parallel hard plates, we find
capillary condensation. Compared with the AOV model, we find that the excluded
volume interactions between the polymers suppress capillary condensation. In
order to induce capillary condensation, smaller undersaturations and smaller
plate separations are needed in comparison with ideal polymers.Comment: 9 pages, 10 figures, accepted for publication in the J. Chem. Phy
Ancient giants: on the farthest galaxy at z=8.6
The observational frontiers for the detection of high-redshift galaxies have
recently been pushed to unimaginable distances with the record-holding Lyman
Alpha Emitter (LAE) UDFy-38135539 discovered at redshift z=8.6. However, the
physical nature and the implications of this discovery have yet to be assessed.
By selecting galaxies with observed luminosities similar to UDFy-38135539 in
state-of-the-art cosmological simulations tuned to reproduce the large scale
properties of LAEs, we bracket the physical nature of UDFy-38135539: it has a
star formation rate ~ 2.7-3.7 solar masses/yr, it contains ~ 10^{8.3-8.7} solar
mass of stars 50-80 Myr old, with stellar metallicity ~ 0.03-0.12 of the solar
value. For any of the simulated galaxies to be visible as a LAE in the observed
range, the intergalactic neutral hydrogen fraction at z=8.6 must be <= 0.2 and
extra ionizing radiation from sources clustered around UDFy-38135539 is
necessary. Finally, we predict that there is a 70% (15%) probability of
detecting at least 1 such source from JWST (HST/WFC3) observations in a
physical radius ~ 0.4 Mpc around UDFy-38135539.Comment: Accepted to MNRAS letter
The cool side of Lyman Alpha Emitters
We extend a previous study of Lyman Alpha Emitters (LAEs) based on
hydrodynamical cosmological simulations, by including two physical processes
important for LAEs: (a) Lyman Alpha and continuum luminosities produced by
cooling of collisionally excited HI in the galaxy, (b) dust formation and
evolution; we follow these processes on a galaxy-by-galaxy basis. HI cooling on
average contributes 16-18% of the Lyman Alpha radiation produced by stars, but
this value can be much higher in low mass LAEs and further increased if the HI
is clumpy. The continuum luminosity is instead almost completely dominated by
stellar sources. The dust content of galaxies scales with their stellar mass,
M_{dust} is proportional to M_*^0.7 and stellar metallicity, Z_*, such that
M_{dust} is proportional to Z_*^1.7. As a result, the massive galaxies have
Lyman Alpha escape fraction as low as f_alpha=0.1, with a LAE-averaged value
decreasing with redshift such that it is (0.33,0.23) at z =(5.7,6.6). The UV
continuum escape fraction shows the opposite trend with redshift, possibly
resulting from clumpiness evolution. The model successfully reproduces the
observed Lyman Alpha and UV luminosity functions at different redshifts and the
Lyman Alpha equivalent width scatter to a large degree, although the observed
distribution appears to be more more extended than the predicted one. We
discuss possible reasons for such tension.Comment: 9 pages, accepted for publication in MNRA
Phase behaviour of charged colloidal sphere dispersions with added polymer chains
We study the stability of mixtures of highly screened repulsive charged
spheres and non-adsorbing ideal polymer chains in a common solvent using free
volume theory. The effective interaction between charged colloids in an aqueous
salt solution is described by a screened-Coulomb pair potential, which
supplements the pure hard-sphere interaction. The ideal polymer chains are
treated as spheres that are excluded from the colloids by a hard-core
interaction, whereas the interaction between two ideal chains is set to zero.
In addition, we investigate the phase behaviour of charged colloid-polymer
mixtures in computer simulations, using the two-body (Asakura-Oosawa pair
potential) approximation to the effective one-component Hamiltonian of the
charged colloids. Both our results obtained from simulations and from free
volume theory show similar trends. We find that the screened-Coulomb repulsion
counteracts the effect of the effective polymer-mediated attraction. For
mixtures of small polymers and relatively large charged colloidal spheres, the
fluid-crystal transition shifts to significantly larger polymer concentrations
with increasing range of the screened-Coulomb repulsion. For relatively large
polymers, the effect of the screened-Coulomb repulsion is weaker. The resulting
fluid-fluid binodal is only slightly shifted towards larger polymer
concentrations upon increasing the range of the screened-Coulomb repulsion. In
conclusion, our results show that the miscibility of dispersions containing
charged colloids and neutral non-adsorbing polymers increases, upon increasing
the range of the screened-Coulomb repulsion, or upon lowering the salt
concentration, especially when the polymers are small compared to the colloids.Comment: 25 pages,13 figures, accepted for publication on J.Phys.:Condens.
Matte
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