The impact of helium reionization on the structure of the intergalactic medium

We examine the impact of helium reionization on the structure of the intergalactic medium (IGM). We model the reionization using a radiative transfer (RT) code coupled to the combined gravity hydrodynamics code Enzo. Neutral hydrogen and helium are initially ionized by a starburst spectrum, which is allowed to gradually evolve into a power law spectrum over the redshift interval 3 < z < 4. The temperature-density relation of the gas is found to fan out and flatten following HeII reionization, with an inversion for overdensities above 5. Peculiar velocities of up to 10 km/s are induced by the increased pressure, with the gas density field distorted over large coherent regions by 10-20%, and the dark matter by levels of 1%. The photoionization-induced flows may thus distort the matter power spectrum at comoving wavenumbers k > 0.5 h/Mpc by a few percent by z = 2. Absorption spectra for HI and HeII are drawn from the simulations, and absorption lines are fit to the spectra. A median Doppler parameter of 35 km/s is obtained for the HI absorption systems at z = 3. Dividing into subsamples optically thick and optically thin at line centre reveals that the optically thick systems undergo only mild evolution while the optically thin systems evolve rapidly following HeII reionization. A comparison between HeII and HI absorption features shows a broad distribution in the HeII and HI column density ratio, peaking near the measured value and only slightly narrower than measured. A comparison with approximate simulation methods shows moderately good agreement in the absorption line properties, but not to the precision to which they may be measured.Comment: 19 pages, 25 figures. Submitted to MNRA

Reionisation scenarios and the temperature of the IGM

We examine the temperature structure of the IGM due to the passage of individual ionisation fronts using a radiative transfer (RT) code coupled to a particle-mesh (PM) N-body code. Multiple simulations were performed with different spectra of ionising radiation: a power law (goes as nu^{-0.5}), miniquasar, starburst, and a time-varying spectrum that evolves from a starburst spectrum to a power law. The RT is sufficiently resolved in time and space to correctly model both the ionisation state and the temperature across the ionisation front. We find the post-ionisation temperature of the reionised intergalactic medium (IGM) is sensitive to the spectrum of the source of ionising radiation, which may be used to place strong constraints on the nature of the sources of reionisation. Radiative transfer effects also produce large fluctuations in the HeII to HI number density ratio eta. The spread in values is smaller than measured, except for the time-varying spectrum. For this case, the spread evolves as the spectral nature of the ionising background changes. Large values for eta are found in partially ionised HeII as the power-law spectrum begins to dominate the starburst, suggesting that the large eta values measured may be indicating the onset of the HeII reionisation epoch.Comment: Accepted to MNRAS. Version with high resolution colour figures available at http://www.roe.ac.uk/~ert/Publications/Tittley_Meiksin_07.pd

Helium reionization and the thermal proximity effect

We examine the temperature structure of the intergalactic medium IGM) surounding a hard radiation source, such as a Quasi-Stellar Object (QSO), as it responds to the onset of helium reionization by the source. We model the reionization using a radiative transfer (RT) code coupled to a particle-mesh (PM) N-body code. Neutral hydrogen and helium are initially ionized by a starburst spectrum, which is allowed to gradually evolve into a power law spectrum (fnu ~ nu^(-0.5)). Multiple simulations were performed with different times for the onset and dominance of the hard spectrum, with onset redshifts ranging from z = 3.5 to 5.5. The source is placed in a high-density region to mimic the expected local environment of a QSO. Simulations with the source placed in a low-density environment were also performed as control cases to explore the role of the environment on the properties of the surrounding IGM. We find in both cases that the IGM temperature within the HeIII region produced exceeds the IGM temperature before full helium reionization, resulting in a "thermal proximity effect", but that the temperature in the HeIII region increases systematically with distance from the source. With time the temperature relaxes with a reduced spread as a function of impact parameter along neighbouring lines of sight, although the trend continues to persist until z = 2. Such a trend could be detected using the widths of intervening metal absorption systems using high resolution, high signal-to-noise ratio spectra.Comment: 17 pages, 12 figures, for publication in MNRA

Lyman α\alpha forest clustering using cell counts statistics

We present a novel technique for calculating \Lya forest correlations based on cell counts. It is applied to the line lists from 7 QSOs observed at high resolution (\Delta v 3h^{-1}\Mpc.

Gas around galaxy haloes - II: hydrogen absorption signatures from the environments of galaxies at redshifts 2<z<3

We compare predictions of large-scale cosmological hydrodynamical simulations for neutral hydrogen absorption signatures in the vicinity of 1011–1012.5 M⊙ haloes with observational measurements. Two different hydrodynamical techniques and a variety of prescriptions for gas removal in high-density regions are examined. Star formation and wind feedback play only secondary roles in the H i absorption signatures outside the virial radius, but play important roles within. Accordingly, we identify three distinct gaseous regions around a halo: the virialized region, the mesogalactic medium outside the virial radius arising from the extended haloes of galaxies out to about two turnaround radii and the intergalactic medium beyond. Predictions for the amount of absorption from the mesogalactic and intergalactic media are robust across different methodologies, and the predictions agree with the amount of absorption observed around star-forming galaxies and quasi-stellar object host galaxies. Recovering the measured amount of absorption within the virialized region, however, requires either a higher dynamic range in the simulations, additional physics, or both

Gas around galaxy haloes: methodology comparisons using hydrodynamical simulations of the intergalactic medium

We perform cosmological simulations of the intergalactic medium (IGM) at redshift z ∼ 3 using the numerical gravity-hydrodynamics codes gadget-3 and enzo for the purpose of modelling the gaseous environments of galaxies. We identify haloes in the simulations using three different algorithms. Different rank orderings of the haloes by mass result, introducing a limiting factor, in identifying haloes with observed galaxies. We also compare the physical properties of the gas between the two codes, focusing primarily on the gas outside the virial radius, motivated by recent H i absorption measurements of the gas around z ∼ 2–3 galaxies. The internal dispersion velocities of the gas in the haloes have converged for a box size of 30 comoving Mpc, but the centre-of-mass peculiar velocities of the haloes have not up to a box size of 60 comoving Mpc. The density and temperature of the gas within the instantaneous turn-around radii of the haloes are adequately captured for box sizes of 30 Mpc on a side, but the results are highly sensitive to the treatment of unresolved, rapidly cooling gas, with the gas mass fraction within the virial radius severely depleted by star formation in the gadget-3 simulations. Convergence of the gas peculiar velocity field on large scales requires a box size of at least 60 Mpc. Outside the turn-around radius, the physical state of the gas agrees to 30 per cent or better both with box size and between simulation methods. We conclude that generic IGM simulations make accurate predictions for the intergalactic gas properties beyond the halo turn-around radii, but the gas properties on smaller scales are highly dependent on star formation and feedback implementations

Dilemmas of school-based Relationships and Sexuality Education for and about consent

In 2018, reflecting in this journal on the arrival of the ‘age of consent’ into sexuality education, Jen Gilbert questioned what would happen to a concept drawn in part from legal contexts, but partly also driven by the passion of feminist activists, when it met the demands and logics – the learning outcomes and lesson plans – of the classroom. This article offers one response, drawing on qualitative data from two whole-school sexual health programmes, Positive Choices and Project Respect, piloted in secondary schools in England between 2017 and 2019. It describes how each addressed the issue of consent and focuses on specific ‘moments’ that illuminate some of the challenges of doing so for both staff and students. Our analyses aim to contribute to the practice of relationships and sexuality education in schools by helping educators to anticipate, understand and therefore better address the dilemmas that teaching for and about consent might encounter. We argue that these dilemmas relate both to broader (and gendered) ideas of consent and entitlement, and to issues specific to schools. However, we also argue that a more theorised account of the school enables us to identify the minor achievements that are nonetheless possible

Is transactional sex exploitative? A social norms perspective, with implications for interventions with adolescent girls and young women in Tanzania

Although transactional sex is common in many sexual relationships, there has been little research into the degree to which the practice is considered exploitative in the settings in which it is practiced. We describe the social norms that influence transactional sex in two sites in Mwanza, Tanzania, and explore local understandings of whether and under what conditions it is considered exploitative. We then compare these "emic" understandings of exploitation to international definitions and norms around sexual exploitation. This study employed a qualitative research design involving 18 focus group discussions and 43 in-depth interviews with young people aged 14-24 years and parents with children aged 14-24 years in a rural area and an urban center within Mwanza, Tanzania. Thematic analysis was conducted with the aid of NVivo 10. The social norms influencing the practice of transactional sex included: reciprocity as a core cultural value that permeates the way exchange in sexual relationships is judged; gendered expectations that men should provide for women's material needs in sexual relationships and that women should reciprocate by means of sex; and peer pressure to be perceived as "fashionable". Adolescent girls and young women (AGYW) are under strong peer pressure to conform to a "modern lifestyle" as reflected in stylish clothing and other items of modernity such as cellphones. The emic conceptualization of exploitation is defined by circumstances surrounding the relationship or a sexual encounter. Important factors that characterize local notions of when transactional relationships are considered exploitative include: when the encounter or relationship involves an imbalance of power (based on age, male economic power and social status); when a man fails to reciprocate; and when sex is coerced. According to community perspectives, young women's behavior should be considered exploitative of men when they take gifts or money yet refuse sex or when they demand large sums of money. Interventions aimed at reducing AGYW's exploitation through transactional sex need to be cognizant of the variations in the understanding of what constitutes sexual exploitation as well as the social and gender norms influencing the practice of transactional sex. Interventions need to involve communities and families in critical thinking that helps them identify positive alternatives to current gendered social norms that shape the involvement of AGYW and men in transactional sex