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We present measurements of the large-scale (≈40 comoving Mpc) effective optical depth of He ii Lyα absorption, ${\tau }_{\mathrm{eff}}$, at 2.54 < z < 3.86 toward 16 He ii-transparent quasars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope, to characterize the ionization state of helium in the intergalactic medium (IGM). We provide the first statistical sample of ${\tau }_{\mathrm{eff}}$ measurements in six signal-to-noise ratio gsim3 He ii sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of ${\tau }_{\mathrm{eff}}$ in 24 He ii sightlines. We confirm an increase of the median ${\tau }_{\mathrm{eff}}$ from sime2 at z = 2.7 to ${\tau }_{\mathrm{eff}}\gtrsim 5$ at z > 3, and a scatter in ${\tau }_{\mathrm{eff}}$ that increases with redshift. The z > 3.5 He ii absorption is predominantly saturated, but isolated narrow (Δv < 650 km s−1) transmission spikes indicate patches of reionized helium. We compare our measurements to predictions for a range of UV background models applied to outputs of a large-volume (146 comoving Mpc)3 hydrodynamical simulation by forward-modeling our sample's quality and size. At z > 2.74, the variance in ${\tau }_{\mathrm{eff}}$ significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median He ii photoionization rate ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}$ of a fluctuating UV background from the median ${\tau }_{\mathrm{eff}}$, finding a factor sime5 decrease in ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}$ between z sime 2.6 and z sime 3.1. At z sime 3.1, ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}=\left[{9.1}_{-1.2}^{+1.1}\,(\mathrm{stat}.){\,}_{-3.4}^{+2.4}\,(\mathrm{sys}.)\right]\times {10}^{-16}$ s−1 corresponds to a median He ii fraction of sime2.5%, indicating that our data probe the tail end of He ii reionization.
We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z(em) similar or equal to 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from <tau(eff, He II)> similar or equal to 1 at z similar or equal to 2.3 to <tau(eff, He II)> greater than or similar to 5 at z similar or equal to 3.2, but with a large scatter of 2 less than or similar to tau(eff, He II) less than or similar to 5 at 2.7 < z < 3 on scales of similar to 10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the coeval Hi forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z greater than or similar to 2.7, probably indicating He II reionization was incomplete at z(reion) greater than or similar to 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z(reion) similar or equal to 2.7 or even below, as suggested by a large tau(eff, He II) greater than or similar to 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 less than or similar to z less than or similar to 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized.
The He II transverse proximity effect-enhanced He II Ly alpha transmission in a background sightline caused by the ionizing radiation of a foreground quasar-offers a unique opportunity to probe the emission properties of quasars, in particular the emission geometry (obscuration, beaming) and the quasar lifetime. Building on the foreground quasar survey published in Schmidt et al., we present a detailed model of the He II transverse proximity effect, specifically designed to include light travel time effects, finite quasar ages, and quasar obscuration. We postprocess outputs from a cosmological hydrodynamical simulation with a fluctuating He II ultraviolet background model, with the added effect of the radiation from a single bright foreground quasar. We vary the age t(age) and obscured sky fractions Omega(obsc) of the foreground quasar, and explore the resulting effect on the He II transverse proximity effect signal. Fluctuations in intergalactic medium density and the ultraviolet background, as well as the unknown orientation of the foreground quasar, result in a large variance of the He II Ly alpha transmission along the background sightline. We develop a fully Bayesian statistical formalism to compare far-ultraviolet He II Ly alpha transmission spectra of the background quasars to our models, and extract joint constraints on t(age) and Omega(obsc) for the six Schmidt et al. foreground quasars with the highest implied He II photoionization rates. Our analysis suggests a bimodal distribution of quasar emission properties, whereby one foreground quasar, associated with a strong He II transmission spike, is relatively old (22 Myr) and unobscured (Omega(obsc) < 35%), whereas three others are either younger than 10 Myr or highly obscured (Omega(obsc) > 70%).
We report the detection of the Lyman continuum (LyC) radiation of the compact star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z = 0.3690, is characterized by a high emission-line flux ratio O-32 = [O III] lambda 5007/[O II] lambda 3727 = 11.5. The escape fraction of the LyC radiation f(esc)(LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Ly alpha emission line is detected in the spectrum of J1154+2443 with a separation between the peaks V-sep of 199 km s(-1), one of the lowest known for Ly alpha-emitting galaxies, implying a high f(esc)(Ly alpha). Comparing the extinction-corrected Ly alpha/H beta flux ratio with the case B value, we find f(esc)(Ly alpha) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O-32 with increasing f(esc)(LyC). We also find a tight anticorrelation between f(esc)(LyC) and V-sep. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length alpha = 1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+log O/H = 7.65 +/- 0.01, the lowest stellar mass M-star = 108.20 M-circle dot, a similar star formation rate SFR = 18.9 M-circle dot yr(-1), and a high specific SFR of 1.2 x 10(-7) yr(-1).
Intense C III] lambda lambda 1907,1909 emission from a strong Lyman continuum emitting galaxy
(2018)
We have obtained the first complete ultraviolet (UV) spectrum of a strong Lyman continuum (LyC) emitter at low redshift - the compact, low-metallicity, star-forming galaxy J1154+2443 - with a Lyman continuum escape fraction of 46% discovered recently. The Space Telescope Imaging Spectrograph spectrum shows strong Ly alpha and C III] lambda 1909 emission, as well as O III] lambda 1666. Our observations show that strong LyC emitters can have UV emission lines with a high equivalent width (e.g. EW(C III]) = 11.7 +/- 2.9 angstrom rest-frame), although their equivalent widths should be reduced due to the loss of ionizing photons. The intrinsic ionizing photon production efficiency of J1154+2443 is high, log(xi(0)(ion)) = 25.56 erg(-1) Hz, comparable to that of other recently discovered z similar to 0.3-0.4 LyC emitters. Combining our measurements and earlier determinations from the literature, we find a trend of increasing xi(0)(ion) with increasing C III] lambda 1909 equivalent width, which can be understood by a combination of decreasing stellar population age and metallicity. Simple ionization and density-bounded photoionization models can explain the main observational features including the UV spectrum of J1154+2443.
Determinations of the ultraviolet (UV) luminosity function of active galactic nuclei (AGN) at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from inconsistencies arising from inhomogeneous selection and analysis of data. We address this problem by constructing a sample of more than 80 000 colour-selected AGN from redshift z= 0 to 7.5 using multiple data sets homogenized to identical cosmologies, intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the AGN UV luminosity function from redshift z= 0 to 7.5. The luminosity function has a double power-law form at all redshifts. The break magnitude M-* shows a steep brightening from M-* similar to -24 at z = 0.7 to M-* similar to -29 at z = 6. The faint-end slope beta significantly steepens from -1.9 at z < 2.2 to -2.4 at z similar or equal to 6. In spite of this steepening, the contribution of AGN to the hydrogen photoionization rate at z similar to 6 is subdominant (< 3 per cent), although it can be non-negligible (similar to 10 per cent) if these luminosity functions hold down to M-1450 = -18. Under reasonable assumptions, AGN can reionize He II by redshift z = 2.9. At low redshifts (z < 0.5), AGN can produce about half of the hydrogen photoionization rate inferred from the statistics of HI absorption lines in the intergalactic medium. Our analysis also reveals important systematic errors in the data, which need to be addressed and incorporated in the AGN selection function in future in order to improve our results. We make various fitting functions, codes, and data publicly available.