Large Fluctuations in the High-Redshift Metagalactic Ionizing Background

Recent observations have shown that the scatter in opacities among coeval segments of the Lyman-alpha forest increases rapidly at z > 5. In this paper, we assess whether the large scatter can be explained by fluctuations in the ionizing background in the post-reionization intergalactic medium. We find that matching the observed scatter at z ~ 5.5 requires a short spatially averaged mean free path of 3 shorter than direct measurements at z ~ 5.2. We argue that such rapid evolution in the mean free path is difficult to reconcile with our measurements of the global H I photoionization rate, which stay approximately constant over the interval z ~ 4.8 - 5.5. However, we also show that measurements of the mean free path at z > 5 are likely biased towards higher values by the quasar proximity effect. This bias can reconcile the short values of the mean free path that are required to explain the large scatter in opacities. We discuss the implications of this scenario for cosmological reionization. Finally, we investigate whether other statistics applied to the z > 5 Lyman-alpha forest can shed light on the origin of the scatter. Compared to a model with a uniform ionizing background, models that successfully account for the scatter lead to enhanced power in the line-of-sight flux power spectrum on scales k < 0.1 h/Mpc. We find tentative evidence for this enhancement in observations of the high-redshift Lyman-alpha forest.Comment: Matches version published by MNRAS with clarifications and expanded discussio

Using the Tip of the Red Giant Branch as a Distance Indicator in the Near Infrared

The tip of the red giant branch (TRGB) is a well-established standard candle used to measure distances to nearby galaxies. The TRGB luminosity is typically measured in the I-band, where the luminosity has little dependency on stellar age or stellar metallicity. As the TRGB is brighter at wavelengths redder than the I-band, observational gains can be made if the TRGB luminosity can be robustly calibrated at longer wavelengths. This is of particular interest given the infrared capabilities that will be available with the James Webb Space Telescope and an important calibration consideration for using TRGB distances as part of an independent measurement of the Hubble constant. Here, we use simulated photometry to investigate the dependency of the TRGB luminosity on stellar age and metallicity as a function of wavelength (475 nm - 4.5 micron). We find intrinsic variations in the TRGB magnitude to increase from a few hundredths of a magnitude at 800-900 nm to ~0.6 mag by 1.5 micron. We show that variations at the longer infrared wavelengths can be reduced to 0.02-0.05 mag (1-2% accuracy in distance) with careful calibrations that account for changes in age and metal content. These represent the minimum uncertainties; observational uncertainties will be higher. Such calibration efforts may also provide independent constraints of the age and metallicity of stellar halos where TRGB distances are best measured. At 3.6 and 4.5 micron, the TRGB magnitude is predicted to vary up to ~0.15 mag even after corrections for stellar age and metallicity, making these wavelengths less suitable for precision distances.Comment: 11 pages, 7 figures, 1 table, Accepted to the Astrophysical Journa

Observational Constraints on Red and Blue Helium Burning Sequences

We derive the optical luminosity, colors, and ratios of the blue and red helium burning (HeB) stellar populations from archival Hubble Space Telescope observations of nineteen starburst dwarf galaxies and compare them with theoretical isochrones from Padova stellar evolution models across metallicities from Z=0.001 to 0.009. We find that the observational data and the theoretical isochrones for both blue and red HeB populations overlap in optical luminosities and colors and the observed and predicted blue to red HeB ratios agree for stars older than 50 Myr over the time bins studied. These findings confirm the usefulness of applying isochrones to interpret observations of HeB populations. However, there are significant differences, especially for the red HeB population. Specifically we find: (1) offsets in color between the observations and theoretical isochrones of order 0.15 mag (0.5 mag) for the blue (red) HeB populations brighter than M_V ~ -4 mag, which cannot be solely due to differential extinction; (2) blue HeB stars fainter than M_V ~ -3 mag are bluer than predicted; (3) the slope of the red HeB sequence is shallower than predicted by a factor of ~3; and (4) the models overpredict the ratio of the most luminous blue to red HeB stars corresponding to ages <50 Myr. Additionally, we find that for the more metal-rich galaxies in our sample (Z> 0.5 Zsolar) the red HeB stars overlap with the red giant branch stars in the color magnitude diagrams, thus reducing their usefulness as indicators of star formation for ages >100 Myr.Comment: 18 pages, 11 figures, 3 table

Constraining reionization using 21 cm observations in combination with CMB and Lyman-alpha forest data

In this paper, we explore the constraints on the reionization history that are provided by current observations of the Lyman-alpha forest and the CMB. Rather than using a particular semi-analytic model, we take the novel approach of parametrizing the ionizing sources with arbitrary functions, and perform likelihood analyses to constrain possible reionization histories. We find model independent conclusions that reionization is likely to be mostly complete by z=8 and that the IGM was 50% ionized at z=9-10. Upcoming low-frequency observations of the redshifted 21 cm line of neutral hydrogen are expected to place significantly better constraints on the hydrogen neutral fraction at 6<z<12. We use our constraints on the reionization history to predict the likely amplitude of the 21 cm power spectrum and show that observations with the highest signal-to-noise ratio will most likely be made at frequencies corresponding to z=9-10. This result provides an important guide to the upcoming 21 cm observations. Finally, we assess the impact that measurement of the neutral fraction will have on our knowledge of reionization and the early source population. Our results show that a single measurement of the neutral fraction mid-way through the reionization era will significantly enhance our knowledge of the entire reionization history.Comment: 15 pages, 16 figures, submitted to MNRA