arXiv subjects

Britton Smith

Publications and source records attributed to Britton Smith.

22 records · Page 2Linked to original sources

arXiv2011

Critical Star-Formation Rates for Reionization: Full Reionization occurs at z = 7

We assess the probable redshift (z_rei ~ 7) for full reionization of the intergalactic medium (IGM) using a prescription for the co-moving star-formation-rate (SFR) density (rho_SFR) required to maintain photoionization against recombination. Our newly developed on-line reionization simulator allows users to assess the required SFR and ionization histories, using a variety of assumptions for galactic and stellar populations, IGM clumping factor and temperature, and LyC escape fraction. The decline in high-redshift galaxy candidates and Lya emitters at z = 6-8 suggests a rising neutral fraction, with reionization at z > 7 increasingly difficult owing to increased recombination rates and constraints from the ionizing background and LyC mean free path. The required rate is rho_SFR = (0.018 M_sun/yr/Mpc^3) [(1+z)/8]^3 (C_H/3)(0.2/f_esc) T_4^(-0.845) scaled to fiducial values of clumping factor C_H = 3, escape fraction f_esc = 0.2, electron temperature T_e = 10^4 K, and low-metallicity initial mass functions (IMF) and stellar atmospheres. Our hydrodynamical + N-body simulations find a mean clumping factor C_H = (2.9)[(1+z)/6]^-1.1 in the photoionized, photoheated filaments at z = 5-9. The critical SFR could be reduced by increasing the minimum stellar mass, invoking a top-heavy IMF, or systematically increasing f_esc at high z. The CMB optical depth, tau_e = 0.088 +/- 0.015, can be explained by full reionization, producing tau_e = 0.050 back to z_rei = 7, augmented by Delta-tau_e = 0.01-0.04 in a UV/X-ray partially ionized IGM at z > 7. In this scenario, the strongest 21-cm signal should occur at redshifted frequencies 124-167 MHz owing to IGM heating over an interval Delta z ~ 3 from z = 7.5-10.5.

astro-ph.CO↗

arXiv2011

He II Lyβ Gunn-Peterson Absorption: New HST Observations, and Theoretical Expectations

Observations of He II Lyα Gunn-Peterson troughs have proved to be a valuable probe of the epoch of helium reionization at z~3. Since this optical depth can become unmeasurably large even for modest He II fractions, various alternate techniques have been proposed to push to higher redshift, and among the more promising is looking at higher order Lyman-series troughs. We here report four new observations of the He II Lyβ trough, including new data on the only sightline with a prior Lyβ observation. However, the effective optical depth ratio τ_eff,β/τ_eff,α is not simply predicted by (f_β λ_β)/(f_α λ_α)=0.16, and we analyze cosmological simulations to find that the correct ratio for helium at z~3 is ~0.35. In one case we infer τ_eff,α > 8.8, strong evidence that helium was not fully reionized at z=3.2--3.5, in agreement with previous measurements suggesting a later completion of reionization.

astro-ph.CO↗

arXiv2010

Hubble/COS Observations of the Quasar HE 2347-4342: Probing the Epoch of He II Patchy Reionization at Redshifts z = 2.4-2.9

We report ultraviolet spectra of the high-redshift (z_em = 2.9) quasar, HE 2347-4342, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). Spectra in the G130M (medium-resolution, 1135-1440 A) and G140L (low-resolution, 1030-2000 A) gratings exhibit patchy Gunn-Peterson absorption in the 303.78 A (Ly-alpha) line of He II between z = 2.39-2.87 (G140L) and z = 2.74-2.90 (G130M). With COS, we obtain better spectral resolution, higher-S/N, and better determined backgrounds than previous studies, with sensitivity to abundance fractions x_HeII = 0.01 in filaments of the cosmic web. The He II optical depths from COS are higher than those with the Far Ultraviolet Spectroscopic Explorer (FUSE) and range from tau_HeII < 0.02 to tau_HeII > 5, with a slow recovery in mean optical depth, tau < 2 at z < 2.7. The He II/H I optical-depth ratio varies (eta = 10-100 for 2.4 < z < 2.73 and eta = 5-500 for 2.75 < z < 2.89) on scales Delta z < 0.01 (10.8 Mpc in comoving radial distance at z = 2.8), with numerous flux-transmission windows between 1135-1186 A. The He II absorption extends to 1186.26 A (z = 2.905), including associated absorbers with z_abs ~ z_QSO and minimal "proximity effect" of flux transmission at the He II edge. We propose a QSO systemic redshift z_QSO = 2.904 +/- 0.002, some Delta z = 0.019 higher than that derived from O I (1302 A) emission. Three long troughs (4-10 A or 25-60 Mpc comoving distance) of strong He II absorption between z = 2.75-2.90 are uncharacteristic of the intergalactic medium if He II reionized at z_r ~ 3. Contrary to recent indirect estimates (z_r = 3.2 +/- 0.2) from H I optical depths, the epoch of HeII reionization may extend to z ~ 2.7.

astro-ph.CO↗

arXiv2009

First Light Sources at the End of the Dark Ages: Direct Observations of Population III Stars, Proto-Galaxies, and Supernovae During the Reionization Epoch

The cosmic dark ages are the mysterious epoch during which the pristine gas began to condense and ultimately form the first stars. Although these beginnings have long been a topic of theoretical interest, technology has only recently allowed the beginnings of observational insight into this epoch. Many questions surround the formation of stars in metal-free gas and the history of the build-up of metals in the intergalactic medium: (1) What were the properties of the first stellar and galactic sources to form in pristine (metal-free) gas? (2) When did the epoch of Population III (metal-free) star formation take place and how long did it last? (3) Was the stellar initial mass function dramatically different for the first stars and galaxies? These questions are all active areas of theoretical research. However, new observational constraints via the direct detection of Population III star formation are vital to making progress in answering the broader questions surrounding how galaxies formed and how the cosmological properties of the universe have affected the objects it contains.

astro-ph.CO↗