| 1. |
- Arthur, S. J., et al.
(författare)
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Radiation-magnetohydrodynamic simulations of H II regions and their associated PDRs in turbulent molecular clouds
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 414:2, s. 1747-1768
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of radiation-magnetohydrodynamic simulations of the formation and expansion of H II regions and their surrounding photodissociation regions (PDRs) in turbulent, magnetized, molecular clouds on scales of up to 4 pc. We include the effects of ionizing and non-ionizing ultraviolet radiation and X-rays from population synthesis models of young star clusters. For all our simulations we find that the H II region expansion reduces the disordered component of the magnetic field, imposing a large-scale order on the field around its border, with the field in the neutral gas tending to lie along the ionization front, while the field in the ionized gas tends to be perpendicular to the front. The highest pressure-compressed neutral and molecular gas is driven towards approximate equipartition between thermal, magnetic and turbulent energy densities, whereas lower pressure neutral/molecular gas bifurcates into, on the one hand, quiescent, magnetically dominated regions and, on the other hand, turbulent, demagnetized regions. The ionized gas shows approximate equipartition between thermal and turbulent energy densities, but with magnetic energy densities that are 1-3 orders of magnitude lower. A high velocity dispersion (similar to 8 km s(-1)) is maintained in the ionized gas throughout our simulations, despite the mean expansion velocity being significantly lower. The magnetic field does not significantly brake the large-scale H II region expansion on the length and time-scales accessible to our simulations, but it does tend to suppress the smallest scale fragmentation and radiation-driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the H II region. However, the relative luminosity of ionizing and non-ionizing radiation has a much larger influence than the presence or absence of the magnetic field. When the star cluster radiation field is relatively soft (as in the case of a lower mass cluster, containing an earliest spectral type of B0.5), then fragmentation is less vigorous and a thick, relatively smooth PDR forms.
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| 2. |
- Datta, Kanan K., et al.
(författare)
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Light-cone effect on the reionization 21-cm power spectrum
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 424:3, s. 1877-1891
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Tidskriftsartikel (refereegranskat)abstract
- Observations of redshifted 21-cm radiation from neutral hydrogen during the epoch of reionization are considered to constitute the most promising tool to probe that epoch. One of the major goals of the first generation of low-frequency radio telescopes is to measure the 3D 21-cm power spectrum. However, the 21-cm signal could evolve substantially along the line-of-sight (LOS) direction of an observed 3D volume, since the received signal from different planes transverses to the LOS originated from different look-back times and could therefore be statistically different. Using numerical simulations we investigate this so-called light-cone effect on the spherically averaged 3D 21-cm power spectrum. For this version of the power spectrum, we find that the effect mostly averages out and observe a smaller change in the power spectrum compared to the amount of evolution in the mean 21-cm signal and its rms variations along the LOS direction. Nevertheless, changes up to similar to 50?per cent at large scales are possible. In general, the power is enhanced/suppressed at large/small scales when the effect is included. The cross-over mode below/above which the power is enhanced/suppressed moves towards larger scales as reionization proceeds. When considering the 3D power spectrum we find it to be anisotropic at the late stages of reionization and on large scales. The effect is dominated by the evolution of the ionized fraction of hydrogen during reionization and including peculiar velocities hardly changes these conclusions. We present simple analytical models which explain qualitatively all the features we see in the simulations.
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| 3. |
- Datta, Kanan K., et al.
(författare)
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Prospects of observing a quasar H ii region during the epoch of reionization with the redshifted 21-cm signal
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 424:1, s. 762-778
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of the impact of a bright quasar on the redshifted 21-cm signal during the epoch of reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H ii regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H ii region substantially beyond those of large H ii regions produced by clustered stellar sources alone. However, the quasar H ii region is found to be more spherical. We next investigate the prospects of detecting such H ii regions in the redshifted 21-cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H ii regions with radii similar to 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 h of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21-cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21-cm data can set new constraints on quasar lifetimes.
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| 4. |
- Friedrich, Martina M., 1982-, et al.
(författare)
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Radiative Transfer of energetic photons : X-rays and helium ionization in C2-Ray
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 421:3, s. 2232-2250
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Tidskriftsartikel (refereegranskat)abstract
- We present an extension to the short-characteristic ray-tracing and non-equilibrium photoionization code C2-RAY. The new version includes the effects of helium and improved multifrequency heating. The motivation for this work is to be able to deal with harder ionizing spectra, such as from quasar-like sources during cosmic reionization. We review the basic algorithmic ingredients of C2-RAY before describing the changes implemented, which include a treatment of the full on-the-spot (OTS) approximation, secondary ionization, and multifrequency photoionization and heating. We performed a series of tests against equilibrium solutions from CLOUDY as well as comparisons to the hydrogen-only solutions by C2-RAY in the extensive cosmological radiative transfer code comparison project. We show that the full, coupled OTS approximation is more accurate than the simplified, uncoupled one. We find that also with helium and a multifrequency setup, long time-steps (up to ~10 per cent of the recombination time) still give accurate results for the ionization fractions. On the other hand, accurate results for the temperature set strong constraints on the time-step. The details of these constraints depend, however, on the optical depth of the cells. We use the new version of the code to confirm that the assumption made in many reionization simulations, namely that helium is singly ionized everywhere where hydrogen is, is indeed valid when the sources have stellar-like spectra.
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| 5. |
- Friedrich, Martina M., et al.
(författare)
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Topology and sizes of H II regions during cosmic reionization
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 413:2, s. 1353-1372
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Tidskriftsartikel (refereegranskat)abstract
- We use the results of large-scale simulations of reionization to explore methods for characterizing the topology and sizes of H ii regions during reionization. We use four independent methods for characterizing the sizes of ionized regions. Three of them give us a full size distribution: the friends-of-friends (FoF) method, the spherical average (SPA) method and the power spectrum (PS) of the ionized fraction. The latter three methods are complementary: while the FoF method captures the size distribution of the small-scale H ii regions, which contribute only a small amount to the total ionization fraction, the SPA method provides a smoothed measure for the average size of the H ii regions constituting the main contribution to the ionized fraction, and the PS does the same while retaining more details on the size distribution. Our fourth method for characterizing the sizes of the H ii regions is the average size which results if we divide the total volume of the H ii regions by their total surface area (i.e. 3V/A), computed in terms of the ratio of the corresponding Minkowski functionals of the ionized fraction field. To characterize the topology of the ionized regions, we calculate the evolution of the Euler characteristic. We find that the evolution of the topology during the first half of reionization is consistent with inside-out reionization of a Gaussian density field. We use these techniques to investigate the dependence of the size and topology on some basic source properties, such as the halo mass-to-light ratio, susceptibility of haloes to negative feedback from reionization and the minimum halo mass for sources to form. We find that the suppression of ionizing sources within ionized regions slows the growth of H ii regions and also changes their size distribution. Additionally, the topology of simulations including suppression is more complex, as indicated by the evolution of the Euler characteristic of the ionized regions. We find the density and ionized fraction to be correlated on large scales, in agreement with the inside-out picture of reionization.
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| 6. |
- Harker, Geraint J. A., et al.
(författare)
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Detection and extraction of signals from the epoch of reionization using higher-order one-point statistics
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 393, s. 1449-1458
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Tidskriftsartikel (refereegranskat)abstract
- Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features - such as a dip at the onset of reionization, followed by a rise towards its later stages - may be generic, and give us a promising route to a statistical detection of reionization.
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| 7. |
- Harker, Geraint, et al.
(författare)
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Non-parametric foreground subtraction for 21-cm epoch of reionization experiments
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 397, s. 1138-1152
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Tidskriftsartikel (refereegranskat)abstract
- One of the problems facing experiments designed to detect redshifted 21-cm emission from the epoch of reionization (EoR) is the presence of foregrounds which exceed the cosmological signal in intensity by orders of magnitude. While fitting them so that they can be removed, we must be careful to minimize `overfitting', in which we fit away some of the cosmological signal, and `underfitting', in which real features of the foregrounds cannot be captured by the fit, polluting the signal reconstruction. We argue that in principle it would be better to fit the foregrounds non-parametrically - allowing the data to determine their shape - rather than selecting some functional form in advance and then fitting its parameters. Non-parametric fits often suffer from other problems, however. We discuss these before suggesting a non-parametric method, Wp smoothing, which seems to avoid some of them. After outlining the principles of Wp smoothing, we describe an algorithm used to implement it. Some useful results for implementing an alternative algorithm are given in an appendix. We apply Wp smoothing to a synthetic data cube for the Low Frequency Array (LOFAR) EoR experiment. This cube includes realistic models for the signal, foregrounds, instrumental response and noise. The performance of Wp smoothing, measured by the extent to which it is able to recover the variance of the cosmological signal and to which it avoids the fitting residuals being polluted by leakage of power from the foregrounds, is compared to that of a parametric fit, and to another non-parametric method (smoothing splines). We find that Wp smoothing is superior to smoothing splines for our application, and is competitive with parametric methods even though in the latter case we may choose the functional form of the fit with advance knowledge of the simulated foregrounds. Finally, we discuss how the quality of the fit is affected by the frequency resolution and range, by the characteristics of the cosmological signal and by edge effects.
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| 8. |
- Harker, Geraint, et al.
(författare)
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Power spectrum extraction for redshifted 21-cm Epoch of Reionization experiments : the LOFAR case
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 405:4, s. 2492-2504
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Tidskriftsartikel (refereegranskat)abstract
- One of the aims of the Low Frequency Array (LOFAR) Epoch of Reionization (EoR) project is to measure the power spectrum of variations in the intensity of redshifted 21-cm radiation from the EoR. The sensitivity with which this power spectrum can be estimated depends on the level of thermal noise and sample variance, and also on the systematic errors arising from the extraction process, in particular from the subtraction of foreground contamination. We model the extraction process using realistic simulations of the cosmological signal, the foregrounds and noise, and so estimate the sensitivity of the LOFAR EoR experiment to the redshifted 21-cm power spectrum. Detection of emission from the EoR should be possible within 360 h of observation with a single station beam. Integrating for longer, and synthesizing multiple station beams within the primary (tile) beam, then enables us to extract progressively more accurate estimates of the power at a greater range of scales and redshifts. We discuss different observational strategies which compromise between depth of observation, sky coverage and frequency coverage. A plan in which lower frequencies receive a larger fraction of the time appears to be promising. We also study the nature of the bias which foreground fitting errors induce on the inferred power spectrum and discuss how to reduce and correct for this bias. The angular and line-of-sight power spectra have different merits in this respect, and we suggest considering them separately in the analysis of LOFAR data.
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| 9. |
- Henney, William J., et al.
(författare)
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Radiation-magnetohydrodynamic simulations of the photoionization of magnetized globules
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 398, s. 157-175
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Tidskriftsartikel (refereegranskat)abstract
- We present the first three-dimensional radiation-magnetohydrodynamic simulations of the photoionization of a dense, magnetized molecular globule by an external source of ultraviolet radiation. We find that, for the case of a strong ionizing field, significant deviations from the non-magnetic evolution are seen when the initial magnetic field threading the globule has an associated magnetic pressure that is greater than 100 times the gas pressure. In such a strong-field case, the photoevaporating globule will adopt a flattened or `curled up' shape, depending on the initial field orientation, and magnetic confinement of the ionized photoevaporation flow can lead to recombination and subsequent fragmentation during advanced stages of the globule evolution. We find suggestive evidence that such magnetic effects may be important in the formation of bright, bar-like emission features in HII regions. We include simple but realistic fits to heating and cooling rates in the neutral and molecular gas in the vicinity of a high-mass star cluster, and show that the frequently used isothermal approximation can lead to an overestimate of the importance of gravitational instability in the radiatively imploded globule. For globules within 2 pc of a high-mass star cluster, we find that heating by stellar X-rays prevents the molecular gas from cooling below 50 K. Based in part on numerical simulations carried out using the Kan Balam supercomputer, operated by the Departamento de Supercómputo, Dirección General de Servicios de Cómputo Académico, Universidad Nacional Autónoma de México. E-mail: w.henney@astrosmo.unam.mx
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| 10. |
- Ichikawa, Kazuhide, et al.
(författare)
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Measuring the history of cosmic reionization using the 21-cm probability distribution function from simulations
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 406:4, s. 2521-2532
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Tidskriftsartikel (refereegranskat)abstract
- The 21-cm probability distribution function (PDF; i.e. distribution of pixel brightness temperatures) is expected to be highly non-Gaussian during reionization and to provide important information on the distribution of density and ionization. We measure the 21-cm PDF as a function of redshift in a large simulation of cosmic reionization and propose a simple empirical fit. Guided by the simulated PDF, we then carry out a maximum likelihood analysis of the ability of upcoming experiments to measure the shape of the 21-cm PDF and derive from it the cosmic reionization history. Under the strongest assumptions, we find that upcoming experiments can measure the reionization history in the mid to late stages of reionization to 1-10 per cent accuracy. Under a more flexible approach that allows for four free parameters at each redshift, a similar accuracy requires the lower noise levels of second-generation 21-cm experiments.
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