9 research outputs found
Constraining the Quasar Contribution to the Reionisation of Cosmic Hydrogen
Absorption spectra of high redshift quasars suggest that the reionisation of
cosmic hydrogen was complete near z~6. The dominant sources of ionising photons
responsible for this reionisation are generally thought to be stars and
quasars. In this paper we make a quantitative estimate of the relative
contributions made by these sources. Our approach is to compute the evolution
of the post overlap ionising background radiation by combining semi-analytic
descriptions of reionisation in a clumpy medium with a model for the quasar
luminosity function. Our overall model has two free parameters, the star
formation efficiency and the minimum quasar luminosity. By adjusting these
parameters, we constrain the relative contributions made by stars and quasars
through comparison with reported observations (Fan et al. 2005). We find that
the relative quasar contribution (at z=5.7) to the ionising background was
between 1.4% and 14.5%. The range of uncertainty is dominated by the unknown
minimum quasar luminosity.Comment: 8 pages, 2 figures. Accepted for publication in MNRA
The escape fraction of ionizing photons from high redshift galaxies
The fraction of ionizing photons which escape their host galaxy and so are
able to ionize hydrogen in the inter-galactic medium (IGM) is a critical
parameter in studies of the reionization era and early galaxy formation. In
this paper we combine observations of Lyman-alpha absorption towards high
redshift quasars with the measured UV luminosity function of high redshift
galaxies to constrain the escape fraction (f_esc) of ionizing photons from
galaxies at z ~ 5.5-6. The observed Lyman-alpha transmission constrains the
escape fraction to lie in the range f_esc ~ 10-25 % (at z ~ 5.5-6). Excluding
halos with M< 10^10 M_sun (as might be expected if galaxy formation is
suppressed due to the reionization of the IGM) implies a larger escape fraction
of f_esc ~ 20-45 %. Using the numerical results to calibrate an analytic
relation between the escape fraction and minimum galaxy halo mass we also
extrapolate our results to a mass (M~10^8 M_sun) corresponding to the hydrogen
cooling threshold. In this case we find f_esc ~ 5-10 %, consistent with
observed estimates at lower redshift. We find that the escape fraction of high
redshift galaxies must be greater than 5 % irrespepctive of galaxy mass. Based
on these results we use a semi-analytic description to model the reionization
history of the IGM, assuming ionizing sources with escape fractions suggested
by our numerical simulations. We find that the IBG observed at z ~ 5.5-6
implies a sufficient number of ionizing photons to have reionized the Universe
by z ~ 6. However, if the minimum mass for star-formation were greater than
10^9 M_sun, the IBG would be over-produced at redshifts less than z ~ 5. In
summary, our results support a scenario in which the IGM was reionized by low
mass galaxies.Comment: 14 pages, 9 figure
Redshifted 21cm Observations of High Redshift Quasar Proximity Zones
The introduction of low-frequency radio arrays is expected to revolutionize
the study of the reionization epoch. Observation of the contrast in redshifted
21cm emission between a large HII region and the surrounding neutral
inter-galactic medium (IGM) will be the simplest and most easily interpreted
signature. However the highest redshift quasars known are thought to reside in
an ionized IGM. Using a semi-analytic model we describe the redshifted 21cm
signal from the IGM surrounding quasars discovered using the i-drop out
technique (i.e. quasars at z~6). We argue that while quasars at z<6.5 seem to
reside in the post overlap IGM, they will still provide valuable probes of the
late stages of the overlap era because the light-travel time across a quasar
proximity zone should be comparable to the duration of overlap. For redshifted
21cm observations within a 32MHz bandpass, we find that the subtraction of a
spectrally smooth foreground will not remove spectral features due to the
proximity zone. These features could be used to measure the neutral hydrogen
content of the IGM during the late stages of reionization. The density of
quasars at z~6 is now well constrained. We use the measured quasar luminosity
function to estimate the prospects for discovery of high redshift quasars in
fields that will be observed by the Murchison Widefield Array.Comment: 14 pages, 9 figures. Accepted for publication in MNRA
Evidence for inhomogeneous reionization in the local Universe from metal-poor globular cluster systems
Exploiting a fundamental characteristic of galaxy assembly in the
{\Lambda}CDM paradigm, the observed spatial biasing and kinematics of
metal-poor globular star clusters are used to constrain the local reionization
epoch around individual galaxies. Selecting three galaxies located in different
environments, the first attempt at constraining the environmental propagation
of reionization in the local Universe is carried out. The joint constraint from
the three galaxies (z_reion = 10.5^{+1.0}_{-0.9}) agrees remarkably well with
the latest WMAP constraint on z_reion for a simple instantaneous reionization
model. More importantly, the range of z_reion values found here are consistent
with the global range of z_reion estimates from other observations. We
furthermore find a 1.7{\sigma} indication that reionization completed in
low-density environments before the intergalactic medium in high-density
environments was reionized. This is consistent with certain theoretical models
that predict that reionization was globally prolonged in duration, with neutral
hydrogen pockets surviving in high-density environments, even after the
surrounding regions were reionized. More generally, this work provides a useful
constraint on the formation history of galaxy stellar halos.Comment: MNRAS in pres
Observations of Ly Emitters at High Redshift
In this series of lectures, I review our observational understanding of
high- Ly emitters (LAEs) and relevant scientific topics. Since the
discovery of LAEs in the late 1990s, more than ten (one) thousand(s) of LAEs
have been identified photometrically (spectroscopically) at to . These large samples of LAEs are useful to address two major astrophysical
issues, galaxy formation and cosmic reionization. Statistical studies have
revealed the general picture of LAEs' physical properties: young stellar
populations, remarkable luminosity function evolutions, compact morphologies,
highly ionized inter-stellar media (ISM) with low metal/dust contents, low
masses of dark-matter halos. Typical LAEs represent low-mass high- galaxies,
high- analogs of dwarf galaxies, some of which are thought to be candidates
of population III galaxies. These observational studies have also pinpointed
rare bright Ly sources extended over kpc, dubbed
Ly blobs, whose physical origins are under debate. LAEs are used as
probes of cosmic reionization history through the Ly damping wing
absorption given by the neutral hydrogen of the inter-galactic medium (IGM),
which complement the cosmic microwave background radiation and 21cm
observations. The low-mass and highly-ionized population of LAEs can be major
sources of cosmic reionization. The budget of ionizing photons for cosmic
reionization has been constrained, although there remain large observational
uncertainties in the parameters. Beyond galaxy formation and cosmic
reionization, several new usages of LAEs for science frontiers have been
suggested such as the distribution of {\sc Hi} gas in the circum-galactic
medium and filaments of large-scale structures. On-going programs and future
telescope projects, such as JWST, ELTs, and SKA, will push the horizons of the
science frontiers.Comment: Lecture notes for `Lyman-alpha as an Astrophysical and Cosmological
Tool', Saas-Fee Advanced Course 46. Verhamme, A., North, P., Cantalupo, S., &
Atek, H. (eds.) --- 147 pages, 103 figures. Abstract abridged. Link to the
lecture program including the video recording and ppt files :
https://obswww.unige.ch/Courses/saas-fee-2016/program.cg
Simulating nano-carbon materials
The structure of carbon based nano-scale materials is extremely diverse at the molecular level and is very important in determining the properties and potential uses of these materials. Ab initio quantum mechanical methods have proved to be successful in giving insight into this structure. This is especially so, for nano-diamond clusters, -wires and -tubes. Here, we apply ab initio DFT based simulations to study some aspects of the structure and properties of graphene layers and graphene tubes to compare and contrast some of their properties such as energy, interconversion and X-ray spectra
ACCESS datasets for CMIP6: methodology and idealised experiments
The Australian Community Climate and Earth System Simulator (ACCESS) has contributed to the World Climate Research Programme’s Coupled Model Intercomparison Project Phase 6 (CMIP6) using two fully coupled model versions (ACCESS-CM2 and ACCESS-ESM1.5) and two ocean–sea-ice model versions (1° and 0.25° resolution versions of ACCESS-OM2). The fully coupled models differ primarily in the configuration and version of their atmosphere components (including the aerosol scheme), with smaller differences in their sea-ice and land model versions. Additionally, ACCESS-ESM1.5 includes biogeochemistry in the land and ocean components and can be run with an interactive carbon cycle. CMIP6 comprises core experiments and associated thematic Model Intercomparison Projects (MIPs). This paper provides an overview of the CMIP6 submission, including the methods used for the preparation of input forcing datasets and the post-processing of model output, along with a comprehensive list of experiments performed, detailing their initialisation, duration, ensemble number and computational cost. A small selection of model output is presented, focusing on idealised experiments and their variants at global scale. Differences in the climate simulation of the two coupled models are highlighted. ACCESS-CM2 produces a larger equilibrium climate sensitivity (4.7°C) than ACCESS-ESM1.5 (3.9°C), likely a result of updated atmospheric parameterisation in recent versions of the atmospheric component of ACCESS-CM2. The idealised experiments run with ACCESS-ESM1.5 show that land and ocean carbon fluxes respond to both changing atmospheric CO2 and to changing temperature. ACCESS data submitted to CMIP6 are available from the Earth System Grid Federation (https://doi.org/10.22033/ESGF/CMIP6.2281 and https://doi.org/10.22033/ESGF/CMIP6.2288). The information provided in this paper should facilitate easier use of these significant datasets by the broader climate community