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

The Collision Between The Milky Way And Andromeda

We use a N--body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given current observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years - within the Sun's lifetime. During the the interaction, there is a chance that the Sun will be pulled away from its present orbital radius and reside in an extended tidal tail. The likelihood for this outcome increases as the merger progresses, and there is a remote possibility that our Sun will be more tightly bound to Andromeda than to the Milky Way before the final merger. Eventually, after the merger has completed, the Sun is most likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late--forming elliptical galaxies.Comment: accepted to MNRA

Discriminating Between the Physical Processes that Drive Spheroid Size Evolution

Massive galaxies at high-z have smaller effective radii than those today, but similar central densities. Their size growth therefore relates primarily to the evolving abundance of low-density material. Various models have been proposed to explain this evolution, which have different implications for galaxy, star, and BH formation. We compile observations of spheroid properties as a function of redshift and use them to test proposed models. Evolution in progenitor gas-richness with redshift gives rise to initial formation of smaller spheroids at high-z. These systems can then evolve in apparent or physical size via several channels: (1) equal-density 'dry' mergers, (2) later major or minor 'dry' mergers with less-dense galaxies, (3) adiabatic expansion, (4) evolution in stellar populations & mass-to-light-ratio gradients, (5) age-dependent bias in stellar mass estimators, (6) observational fitting/selection effects. If any one of these is tuned to explain observed size evolution, they make distinct predictions for evolution in other galaxy properties. Only model (2) is consistent with observations as a dominant effect. It is the only model which allows for an increase in M_BH/M_bulge with redshift. Still, the amount of merging needed is larger than that observed or predicted. We therefore compare cosmologically motivated simulations, in which all these effects occur, & show they are consistent with all the observational constraints. Effect (2), which builds up an extended low-density envelope, does dominate the evolution, but effects 1,3,4, & 6 each contribute ~20% to the size evolution (a net factor ~2). This naturally also predicts evolution in M_BH-sigma similar to that observed.Comment: 19 pages, 7 figures. accepted to MNRAS (matches accepted version

T cell immunity in the female genital tract

Thesis (Ph.D.)--University of Washington, 2020Many pathogens of global health significance are sexually transmitted, highlighting the importance of studying immune cells in genital barrier tissues. T cells are a critical component of the immune response to viruses, yet their role in protecting the female genital tract against viral infection is incompletely understood. Here, we combined studies of the mouse and human female genital tract to characterize T cells isolated from female genital barrier tissues. In human cervicovaginal tissue, CD8 T cells resembled effector T cells, raising the question of whether they were recently activated. By employing mouse models, we found that systemic immunization resulted in a cervicovaginal CD8 T cell compartment that was poorly maintained and underwent progressive differentiation that was not observed in spleen, lymph nodes, or small intestine lamina propria. This progressive differentiation occurred over five months, after which CD8 T cells gained an effector-like phenotype, ultimately resembling human cervicovaginal CD8 T cells. Vaginal inflammation combined with vaginal antigen exposure accelerated this differentiation process. The CD8 T cell compartment induced by systemic immunization mediated partial protection against vaginal infection with herpes simplex virus type 2 (HSV-2). This protection waned over time as the loss and differentiation of the CD8 T cell compartment progressed. Together, these results demonstrate that tissue-intrinsic factors interact with environmental cues to dictate the final longevity and phenotype of the cervicovaginal CD8 T cell compartment.Women living with HSV-2 often experience recurring genital herpes lesions. We explored the T cell compartment in human genital skin during these localized HSV-2 reactivation events. Upon HSV-2 lesion formation, we found that CD4 and CD8 T cells expanded in the infected skin site and upregulated markers of proliferation and activation. Some of the expanded T cells resembled circulating memory T cells, suggesting influx of immune cells from the blood. Cell-intrinsic and cell-extrinsic regulatory mechanisms were also upregulated in HSV-2 lesions, indicating that the immune system acted to control the virus as well as limit immunopathology. Meanwhile, T cells isolated from a nearby unaffected region distinct from the HSV-2 lesion were quiescent, instead resembling the T cell isolate from genital biopsies from HSV seronegative participants. Despite the frequency of HSV-2 reactivation in people with chronic genital herpes, we did not observe evidence of T cell exhaustion or loss of functionality. After lesion resolution, the T cell infiltrate in HSV-2 lesions returned to quiescence, again resembling the T cell compartment in contralateral and seronegative control biopsies. Given these results, we conclude that T cell responses to HSV-2 reactivation events in genital skin are highly localized, include both resident T cells and circulating memory T cells, and are rapidly constrained by regulatory T cells and intrinsic regulatory mechanisms. Taken together, our data highlight unique features of T cells in the female genital tract and their roles in vaccination and viral immunity