440 research outputs found
Mechanical properties of polyethylene terephthalate under selected conditions and methods of preparation Semiannual progress report, period ending 31 Oct. 1967
Mechanical properties of polyethylene terephthalate under selected conditions and methods of preparatio
Dark ages reionization & galaxy formation simulation XII: Bubbles at dawn
Direct detection of regions of ionized hydrogen (HII) has been suggested as a
promising probe of cosmic reionization. Observing the redshifted 21-cm signal
of hydrogen from the epoch of reionization (EoR) is a key scientific driver
behind new-generation, low-frequency radio interferometers. We investigate the
feasibility of combining low-frequency observations with the Square Kilometre
Array and near infra-red survey data of the Wide-Field Infrared Survey
Telescope to detect cosmic reionization by imaging HII bubbles surrounding
massive galaxies during the cosmic dawn. While individual bubbles will be too
small to be detected, we find that by stacking redshifted 21-cm spectra centred
on known galaxies, it will be possible to directly detect the EoR at , and to place qualitative constraints on the evolution of the spin
temperature of the intergalactic medium (IGM) at . In particular,
given a detection of ionized bubbles using this technique, it is possible to
determine if the IGM surrounding them is typically in absorption or emission.
Determining the globally-averaged neutral fraction of the IGM using this method
will prove more difficult due to degeneracy with the average size of HII
regions.Comment: 14 pages, 11 figures, 2 tables, published in MNRAS. Updated to match
published version. Additional results and comments added from previous
version. All other results and conclusions remain unchange
Polarised foreground removal at low radio frequencies using rotation measure synthesis: uncovering the signature of hydrogen reionisation
Measurement of redshifted 21-cm emission from neutral hydrogen promises to be
the most effective method for studying the reionisation history of hydrogen
and, indirectly, the first galaxies. These studies will be limited not by raw
sensitivity to the signal, but rather, by bright foreground radiation from
Galactic and extragalactic radio sources and the Galactic continuum. In
addition, leakage due to gain errors and non-ideal feeds conspire to further
contaminate low-frequency radio obsevations. This leakage leads to a portion of
the complex linear polarisation signal finding its way into Stokes I, and
inhibits the detection of the non-polarised cosmological signal from the epoch
of reionisation. In this work, we show that rotation measure synthesis can be
used to recover the signature of cosmic hydrogen reionisation in the presence
of contamination by polarised foregrounds. To achieve this, we apply the
rotation measure synthesis technique to the Stokes I component of a synthetic
data cube containing Galactic foreground emission, the effect of instrumental
polarisation leakage, and redshifted 21-cm emission by neutral hydrogen from
the epoch of reionisation. This produces an effective Stokes I Faraday
dispersion function for each line of sight, from which instrumental
polarisation leakage can be fitted and subtracted. Our results show that it is
possible to recover the signature of reionisation in its late stages (z ~ 7) by
way of the 21-cm power spectrum, as well as through tomographic imaging of
ionised cavities in the intergalactic medium.Comment: 22 pages including 11 figures. Minor revisions following referee's
report. MNRAS, in pres
Dark-ages Reionization and Galaxy Formation Simulation - XIV. Gas accretion, cooling and star formation in dwarf galaxies at high redshift
We study dwarf galaxy formation at high redshift () using a suite of
high- resolution, cosmological hydrodynamic simulations and a semi-analytic
model (SAM). We focus on gas accretion, cooling and star formation in this work
by isolating the relevant process from reionization and supernova feedback,
which will be further discussed in a companion paper. We apply the SAM to halo
merger trees constructed from a collisionless N-body simulation sharing
identical initial conditions to the hydrodynamic suite, and calibrate the free
parameters against the stellar mass function predicted by the hydrodynamic
simulations at z = 5. By making comparisons of the star formation history and
gas components calculated by the two modelling techniques, we find that
semi-analytic prescriptions that are commonly adopted in the literature of
low-redshift galaxy formation do not accurately represent dwarf galaxy
properties in the hydrodynamic simulation at earlier times. We propose 3
modifications to SAMs that will provide more accurate high-redshift
simulations. These include 1) the halo mass and baryon fraction which are
overestimated by collisionless N-body simulations; 2) the star formation
efficiency which follows a different cosmic evolutionary path from the
hydrodynamic simulation; and 3) the cooling rate which is not well defined for
dwarf galaxies at high redshift. Accurate semi-analytic modelling of dwarf
galaxy formation informed by detailed hydrodynamical modelling will facilitate
reliable semi-analytic predictions over the large volumes needed for the study
of reionization.Comment: 20 pages, 10 figures; Updated to match the published version. All
results and conclusions remain unchange
Dark-ages reionization and galaxy formation simulation - IX. Economics of reionizing galaxies
Using a series of high-resolution hydrodynamical simulations we show that
during the rapid growth of high-redshift (z > 5) galaxies, reserves of
molecular gas are consumed over a time-scale of 300Myr, almost independent of
feedback scheme. We find that there exists no such simple relation for the
total gas fractions of these galaxies, with little correlation between gas
fractions and specific star formation rates. The bottleneck or limiting factor
in the growth of early galaxies is in converting infalling gas to cold
star-forming gas. Thus, we find that the majority of high redshift dwarf
galaxies are effectively in recession, with demand (of star formation) never
rising to meet supply (of gas), irrespective of the baryonic feedback physics
modelled. We conclude that the basic assumption of self-regulation in galaxies
- that they can adjust total gas consumption within a Hubble time - does not
apply for the dwarf galaxies thought to be responsible for providing most UV
photons to reionize the high redshift Universe. We demonstrate how this rapid
molecular time-scale improves agreement between semi-analytic model predictions
of the early Universe and observed stellar mass functions.Comment: 17 pages, 27 figures, accepted for publication in MNRAS, minor
updates to align with final published versio
Modification of the 21-cm power spectrum by X-rays during the epoch of reionisation
We incorporate a contribution to reionization from X-rays within analytic and
semi-numerical simulations of the 21-cm signal arising from neutral hydrogen
during the epoch of reionization. We explore the impact that X-ray ionizations
have on the power spectrum (PS) of 21-cm fluctuations by varying both the
average X-ray MFP and the fractional contribution of X-rays to reionization. In
general, prior to the epoch when the intergalactic medium is dominated by
ionized regions (H {\sevensize II} regions), X-ray-induced ionization enhances
fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray
heating does not strongly supress galaxy formation. Conversely, at later times
when \H2 regions dominate, small-scale fluctuations in the 21-cm signal are
suppressed by X-ray ionization. Our modelling also shows that the modification
of the 21-cm signal due to the presence of X-rays is sensitive to the relative
scales of the X-ray MFP, and the characteristic size of \H2 regions. We
therefore find that X-rays imprint an epoch and scale-dependent signature on
the 21-cm PS, whose prominence depends on fractional X-ray contribution. The
degree of X-ray heating of the IGM also determines the extent to which these
features can be discerned. We show that the MWA will have sufficient
sensitivity to detect this modification of the PS, so long as the X-ray photon
MFP falls within the range of scales over which the array is most sensitive
( Mpc). In cases in which this MFP takes a much smaller value,
an array with larger collecting area would be required.Comment: 15 pages, 6 figures, Accepted for publication in MNRAS X-ray heating
contribution now adde
Dark-ages Reionization & Galaxy Formation Simulation VIII. Suppressed growth of dark matter halos during the Epoch of Reionization
We investigate how the hydrostatic suppression of baryonic accretion affects
the growth rate of dark matter halos during the Epoch of Reionization. By
comparing halo properties in a simplistic hydrodynamic simulation in which gas
only cools adiabatically, with its collisionless equivalent, we find that halo
growth is slowed as hydrostatic forces prevent gas from collapsing. In our
simulations, at the high redshifts relevant for reionization (between
and ), halos that host dwarf galaxies () can be reduced by up to a factor of 2 in mass due to the
hydrostatic pressure of baryons. Consequently, the inclusion of baryonic
effects reduces the amplitude of the low mass tail of the halo mass function by
factors of 2 to 4. In addition, we find that the fraction of baryons in dark
matter halos hosting dwarf galaxies at high redshift never exceeds
of the cosmic baryon fraction. When implementing baryonic processes, including
cooling, star formation, supernova feedback and reionization, the suppression
effects become more significant with further reductions of to
60\%. Although convergence tests suggest that the suppression may become weaker
in higher resolution simulations, this suppressed growth will be important for
semi-analytic models of galaxy formation, in which the halo mass inherited from
an underlying N-body simulation directly determines galaxy properties. Based on
the adiabatic simulation, we provide tables to account for these effects in
N-body simulations, and present a modification of the halo mass function along
with explanatory analytic calculations.Comment: 17 pages, 11 figures; Updated to match the published version. Two
changes in Figures 1 and 3 in order to 1) correct bin sizes of the 10^8 and
10^8.5 Msol bins for NOSN_NOZCOOL_NoRe (was 0.5, should be 0.25); 2) include
stellar mass in baryon fraction (was missed in Fig. 3). Quantitative
description of Fig. 3 changed slightly in Section 2.2. All other results and
conclusions remain unchange
Dark-ages reionization and galaxy formation simulation--VII. The sizes of high-redshift galaxies
We investigate high-redshift galaxy sizes using a semi-analytic model
constructed for the Dark-ages Reionization And Galaxy-formation Observables
from Numerical Simulation project. Our fiducial model, including strong
feedback from supernovae and photoionization background, accurately reproduces
the evolution of the stellar mass function and UV luminosity function. Using
this model, we study the size--luminosity relation of galaxies and find that
the effective radius scales with UV luminosity as at --. We show that recently discovered very luminous
galaxies at (Bowler et al. 2016) and (Oesch et al. 2016)
lie on our predicted size--luminosity relations. We find that a significant
fraction of galaxies at will not be resolved by JWST, but GMT will have
the ability to resolve all galaxies in haloes above the atomic cooling limit.
We show that our fiducial model successfully reproduces the redshift evolution
of average galaxy sizes at . We also explore galaxy sizes in models
without supernova feedback. The no-supernova feedback models produce galaxy
sizes that are smaller than observations. We therefore confirm that supernova
feedback plays an important role in determining the size--luminosity relation
of galaxies and its redshift evolution during reionization.Comment: 10 pages, 4 figures, Accepted for publication in MNRA
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