Showing papers by "Garrelt Mellema published in 2011"

Radiation-magnetohydrodynamic simulations of H ii regions and their associated PDRs in turbulent molecular clouds

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.

Radiation-magnetohydrodynamic simulations of HII regions and their associated PDRs in turbulent molecular clouds

Abstract: We present the results of radiation-magnetohydrodynamic simulations of the expansion of HII regions and surrounding photodissociation regions in turbulent, magnetised, molecular clouds on scales of up to 4 parsecs, including the effects of ionising and non-ionising ultraviolet radiation and x rays from young star clusters. We find that HII region expansion reduces the disordered component of the B field, imposing a large-scale order on the field around its border. The field in the neutral gas lies along the ionisation front, while the field in the ionised gas tends to be perpendicular to this. The highest pressure compressed neutral/molecular gas is driven towards approximate equipartition between thermal/magnetic/turbulent energy densities, whereas lower pressure neutral/molecular gas divides into quiescent, magnetically dominated regions, and, on the other hand, turbulent, demagnetised regions. The ionised gas shows approximate thermal/turbulent equipartition, but with magnetic energy densities 1 to 3 orders of magnitude lower. A high velocity dispersion (approx 8 km/s) is maintained in the ionised gas throughout our simulations, despite the mean expansion velocity being significantly lower. The B field does not significantly brake the HII region expansion on the length and timescales accessible to our simulations, but it does tend to suppress the small-scale fragmentation and radiation-driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the HII region. However, the relative luminosity of ionising and non-ionising radiation has a much larger influence than the presence or absence of the B field. When the radiation field is relatively soft (as in the case of a lower mass cluster, with earliest spectral type of B0.5), then fragmentation is less vigorous and a thick, relatively smooth PDR forms. Movies available at this http URL

Topology and sizes of H ii regions during cosmic reionization

Abstract: We use the results of large-scale simulations of reionization to explore methods for characterizing the topology and sizes of HII 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 method (SPA) and the power spectrum (PS) of the ionized fraction. These 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 spherical average method provides a smoothed measure for the average size of the H~II regions constituting the main contribution to the ionized fraction, and the power spectrum 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 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 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. (abridged)

Redshift Space Distortion of the 21cm Background from the Epoch of Reionization I: Methodology Re-examined

Abstract: The peculiar velocity of the intergalactic gas responsible for the cosmic 21cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. Measurement of this anisotropy by future 21cm surveys is a promising tool for separating cosmology from 21cm astrophysics. However, previous attempts to model the signal have often neglected peculiar velocity or only approximated it crudely. This paper re-examines the effects of peculiar velocity on the 21cm signal in detail, improving upon past treatment and addressing several issues for the first time. (1) We show that properly accounting for finite optical depth eliminates the unphysical divergence of 21cm brightness temperature in overdense regions of the IGM found by previous work that employed the usual optically-thin approximation. (2) The approximation made previously to circumvent the diverging brightness temperature problem by capping velocity gradient can misestimate the power spectrum on all scales. (3) The observed power spectrum in redshift-space remains finite even in the optically-thin approximation if one properly accounts for the redshift-space distortion. However, results that take full account of finite optical depth show that this approximation is only accurate in the limit of high spin temperature. (4) The linear theory for redshift-space distortion results in ~30% error in the observationally relevant wavenumber range, at the 50% ionized epoch. (5) We describe and test two numerical schemes to calculate the 21cm signal from reionization simulations to incorporate peculiar velocity effects in the optically-thin approximation accurately. One is particle-based, the other grid-based, and while the former is most accurate, we demonstrate that the latter is computationally more efficient and can achieve sufficient accuracy. [Abridged]

Reionization of the Local Group of galaxies

Abstract: We present the first detailed structure formation and radiative transfer simulations of the reionization history of our cosmic neighbourhood. To this end, we follow the formation of the Local Group of galaxies and nearby clusters by means of constrained simulations, which use the available observational constraints to construct a representation of those structures which reproduces their actual positions and properties at the present time. We find that the reionization history of the Local Group is strongly dependent on the assumed photon production efficiencies of the ionizing sources, which are still poorly constrained. If sources are relatively efficient, i.e. the process is 'photon-rich', the Local Group is primarily ionized externally by the nearby clusters. Alternatively, if the sources are inefficient, i.e. reionization is 'photon-poor' the Local Group evolves largely isolated and reionizes itself. The mode of reionization, external versus internal, has important implications for the evolution of our neighbourhood, in terms of e.g. its satellite galaxy populations and primordial stellar populations. This therefore provides an important avenue for understanding the young universe by detailed studies of our nearby structures.

Can 21-cm observations discriminate between high-mass and low-mass galaxies as reionization sources?

Abstract: The prospect of detecting the first galaxies by observing their impact on the intergalactic medium as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this by considering a set of large-scale radiative transfer simulations of reionization in sufficiently large volumes to make statistically meaningful predictions of observable signatures, while also directly resolving all atomically-cooling halos down to 10^8 M_solar. We focus here on predictions of the 21-cm background, to see if upcoming observations are capable of distinguishing a universe ionized primarily by high-mass halos from one in which both high-mass and low-mass halos are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21-cm fluctuation power spectra observed by the first generation EoR/21-cm radio interferometer arrays should be able to distinguish the case of reionization by high-mass halos alone from that by both high- and low-mass halos, together. Some reionization scenarios yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21-cm PDF distributions. We find that the skewness of the 21-cm PDF distribution smoothed over LOFAR-like window shows a clear feature correlated with the rise of the rms due to patchiness. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. (abridged)

Erratum: Observational constraints on supermassive dark stars

Abstract: The Letter ‘Observational constraints on supermassive dark stars’ was published in Mon. Not. R. Astron. Soc. 407, L74–L78 (2010). An error has been uncovered in the Letter. Owing to a numerical mistake, the formation rate of 1–2 × 108 M cold dark matter haloes used was too high by factors of ≈10–30. As a result, the observational constraints on f SMDS, the fraction of 1–2 × 108 M haloes that form 107 M supermassive dark stars (SMDS), should be relaxed accordingly. The corrected halo formation rate is presented as a function of redshift in Fig. 1. Because of the smaller number of haloes involved, the scatter between adjacent redshift bins is now considerably larger than in the original plot. By fitting a second-order polynomial (thick solid line) to the simulation data, we estimate that the formation rate of 1–2 × 108 M haloes is dn/dt ≈ 5 × 10−9 haloes per comoving Mpc3 and year at z = 10, and dn/dt ≈ 1 × 10−9 haloes per comoving Mpc3 and year at z = 15. This converts into Ṅ ≈ 580 haloes forming per unit redshift and arcmin2 at z = 10, and Ṅ ≈ 30 haloes forming per unit redshift and arcmin2 at z = 15. The resulting constraints on f SMDS, as a function of the SMDS lifetime τ , are plotted in Fig. 2 for our scenario A (where SMDS continue to form at z ≈ 10 rather than merely survive from previous

Light cone effect on the reionization 21-cm power spectrum

Abstract: Observations of redshifted 21-cm radiation from neutral hydrogen during the epoch of reionization (EoR) 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 transverse 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 50% 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 toward 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.