| 1. |
- Ahn, Kyungjin, et al.
(författare)
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DETECTING THE RISE AND FALL OF THE FIRST STARS BY THEIR IMPACT ON COSMIC REIONIZATION
- 2012
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Ingår i: The Astrophysical Journal Letters. - 2041-8205. ; 756:1, s. L16-
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Tidskriftsartikel (refereegranskat)abstract
- The intergalactic medium was reionized before redshift z similar to 6, most likely by starlight which escaped from early galaxies. The very first stars formed when hydrogen molecules (H-2) cooled gas inside the smallest galaxies, minihalos (MHs) of mass between 10(5) and 10(8) M-circle dot. Although the very first stars began forming inside these MHs before redshift z similar to 40, their contribution has, to date, been ignored in large-scale simulations of this cosmic reionization. Here we report results from the first reionization simulations to include these first stars and the radiative feedback that limited their formation, in a volume large enough to follow the crucial spatial variations that influenced the process and its observability. We show that, while MH stars stopped far short of fully ionizing the universe, reionization began much earlier with MH sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background significantly. This boost should be readily detectable by Planck, although within current Wilkinson Microwave Anisotropy Probe uncertainties. If reionization ended as late as z(ov) less than or similar to 7, as suggested by other observations, Planck will thereby see the signature of the first stars at high redshift, currently undetectable by other probes.
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| 2. |
- 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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| 3. |
- Ciardi, B., et al.
(författare)
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Prospects for detecting the 21 cm forest from the diffuse intergalactic medium with LOFAR
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 428:2, s. 1755-1765
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the feasibility of the detection of the 21 cm forest in the diffuse intergalactic medium (IGM) with the radio telescope LOFAR. The optical depth to the 21 cm line has been derived using simulations of reionization which include detailed radiative transfer of ionizing photons. We find that the spectra from reionization models with similar total comoving hydrogen ionizing emissivity but different frequency distribution look remarkably similar. Thus, unless the reionization histories are very different from each other (e.g. a predominance of UV versus X-ray heating) we do not expect to distinguish them by means of observations of the 21 cm forest. Because the presence of a strong X-ray background would make the detection of the 21 cm line absorption impossible, the lack of absorption could be used as a probe of the presence/intensity of the X-ray background and the thermal history of the Universe. Along a random line of sight LOFAR could detect a global suppression of the spectrum from z greater than or similar to 12, when the IGM is still mostly neutral and cold, in contrast with the more well-defined, albeit broad, absorption features visible at lower redshift. Sharp, strong absorption features associated with rare, high-density pockets of gas could also be detected at z similar to 7 along preferential lines of sight.
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| 4. |
- 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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| 5. |
- Datta, Kanan K., et al.
(författare)
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Light cone effect on the reionization 21-cm signal - II. Evolution, anisotropies and observational implications
- 2014
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 442:2, s. 1491-1506
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of the H i 21-cm power spectra from the reionization epoch will be influenced by the evolution of the signal along the line-of-sight direction of any observed volume. We use numerical as well as seminumerical simulations of reionization in a cubic volume of 607 Mpc across to study this so-called light-cone effect on the H i 21-cm power spectrum. We find that the light-cone effect has the largest impact at two different stages of reionization: one when reionization is similar to 20 per cent and other when it is similar to 80 per cent completed. We find a factor of similar to 4 amplification of the power spectrum at the largest scale available in our simulations. We do not find any significant anisotropy in the 21-cm power spectrum due to the light-cone effect. We argue that for the power spectrum to become anisotropic, the light-cone effect would have to make the ionized bubbles significantly elongated or compressed along the line of sight, which would require extreme reionization scenarios. We also calculate the two-point correlation functions parallel and perpendicular to the line of sight and find them to differ. Finally, we calculate an optimum frequency bandwidth below which the light-cone effect can be neglected when extracting power spectra from observations. We find that if one is willing to accept a 10 per cent error due to the light-cone effect, the optimum frequency bandwidth for k = 0.056 Mpc(-1) is similar to 7.5 MHz. For k = 0.15 and 0.41 Mpc(-1), the optimum bandwidth is similar to 11 and similar to 16 MHz, respectively.
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| 6. |
- 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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| 7. |
- Fernandez, Elizabeth R., et al.
(författare)
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Stars and reionization : the cross-correlation of the 21 cm line and the near-infrared background
- 2014
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 440:1, s. 298-306
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Tidskriftsartikel (refereegranskat)abstract
- With improving telescopes, it may now be possible to observe the Epoch of Reionization in multiple ways. We examine two of these observables - the excess light in the near-infrared background that may be due to high-redshift stars and ionized Hii bubbles, and the 21 cm emission from neutral hydrogen. Because these two forms of emission should result from different, mutually exclusive regions, an anticorrelation should exist between them. We discuss the strengths of using cross-correlations between these observations to learn more about high-redshift star formation and reionization history. In particular, we create simulated maps of emission from both the near-infrared background and 21 cm emission. We find that these observations are anticorrelated, with the strongest anticorrelation originating from times when the universe is half ionized. This result is robust and does not depend on the properties of the stars themselves. Rather, it depends on the ionization history. Cross-correlations can provide redshift information, which the near-infrared background cannot provide alone. In addition, cross-correlations can help separate foreground emission from the true high-redshift component, making it possible to say with greater certainty that we are indeed witnessing the Epoch of Reionization.
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| 8. |
- 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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| 9. |
- Friedrich, Martina M., 1982-
(författare)
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Simulations of Cosmic Reionization : Shapes & Sizes of H II regions around Galaxies and Quasars
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- After the era of recombination, roughly 360 000 years after the big bang (redshift 1100), the universe was neutral, continued to expand and eventually the first gravitationally collapsed structures capable of forming stars, formed. Observations show that approximately 1 billion years later (redshift 6), the Universe had become highly ionized. The transition from a neutral intergalactic medium to a highly ionized one, is called the epoch of Reionization (EoR). Although quasar spectra and polarization power-spectra from cosmic microwave background experiments set some time-constrains on this epoch, the details of this process are currently not known. New radio telescopes operating at low frequencies aim at measuring directly the neutral hydrogen content between redshifts 6 - 10 via the HI spin-flip line at 21cm. The interpretation of these first measurements is not going to be trivial. Therefore, simulations of the EoR are useful to test the many ill-constrained parameters such as the properties of the sources responsible for reionization. This thesis contributes to such simulations. It addresses different source models and discusses different measures to quantify their effect on the shapes and sizes of the emerging H II regions. It also presents a new version of the widely used radiative transfer code C2-Ray which is capable of handling the ionizing radiation produced by energetic sources such as quasars. Using this new version we study whether 21cm experiments could detect the signature of a quasar. We find that different size measures of ionized regions can distinguish between different source models in the simulations and that a topological measure of the ionized fraction field confirms the inside-out (i.e. overdense regions ionize first) reionization scenario. We find that the HII regions from luminous quasars may be detectable in 21cm, but that it might not be possible to distinguish them from the largest HII regions produced by clustered galaxies.
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| 10. |
- 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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