Lyman Edge Research Articles

Corona and XUV emission modelling of the Sun and Sun-like stars

The X-ray and extreme-ultraviolet (EUV) emissions from low-mass stars significantly affect the evolution of the planetary atmosphere. However, it is observationally difficult to constrain the stellar high-energy emission because of the strong interstellar extinction of EUV photons. In this study, we simulate the XUV (X-ray plus EUV) emission from Sun-like stars by extending the solar coronal heating model that self-consistently solves, with sufficiently high resolution, the surface-to-coronal energy transport, turbulent coronal heating, and coronal thermal response by conduction and radiation. The simulations are performed with a range of loop lengths and magnetic filling factors at the stellar surface. With the solar parameters, the model reproduces the observed solar XUV spectrum below the Lyman edge, thus validating its capability of predicting the XUV spectra of other Sun-like stars. The model also reproduces the observed nearly linear relation between the unsigned magnetic flux and the X-ray luminosity. From the simulation runs with various loop lengths and filling factors, we also find a scaling relation, namely logLEUV= 9.93 + 0.67 logLX, whereLEUVandLXare the luminosity in the EUV (100 Å <λ≤ 912 Å) and X-ray (5 Å <λ≤ 100 Å) range, respectively, in cgs. By assuming a power–law relation between the Rossby number and the magnetic filling factor, we reproduce the renowned relation between the Rossby number and the X-ray luminosity. We also propose an analytical description of the energy injected into the corona, which, in combination with the conventional Rosner–Tucker–Vaiana scaling law, semi-analytically explains the simulation results. This study refines the concepts of solar and stellar coronal heating and derives a theoretical relation for estimating the hidden stellar EUV luminosity from X-ray observations.

The Low-redshift Lyman-continuum Survey: [S ii] Deficiency and the Leakage of Ionizing Radiation

Abstract The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the [S ii] nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the [S ii] deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. We show that LyC leakers tend to be more [S ii] deficient and that the fraction of their detections increases as [S ii] deficiency becomes more prominent. Correlational studies suggest that [S ii] deficiency complements other LyC diagnostics (such as strong Lyα emission and high [O iii]/[O ii]). Our results verify an additional technique by which reionization-era galaxies could be studied.

A New Technique for Finding Galaxies Leaking Lyman-continuum Radiation: [S ii]-deficiency

The source responsible for the reionization of the Universe is believed to be the population of star-forming galaxies at $z\sim6$ to 12. The biggest uncertainty concerns the fraction of Lyman-continuum photons that actually escape from the galaxies. In recent years, several relatively small samples of "leaky" galaxies have been uncovered, and clues have begun to emerge as to both the indirect signposts of leakiness and of the conditions/processes that enable the escape of ionizing radiation. In this paper we present the results of a pilot program aimed to test a new technique for finding leaky galaxies---using the weakness of the [SII] nebular emission-lines relative to typical star-forming galaxies as evidence that the interstellar medium is optically-thin to the Lyman continuum. We use the Cosmic Origins Spectrograph on the Hubble Space Telescope to detect significant emerging flux below the Lyman edge in two out of three [SII]-weak star-forming galaxies at $z\sim0.3$. We show that these galaxies differ markedly in their properties from the class of leaky "Green-Pea" galaxies at similar redshifts: our sample galaxies are more massive, more metal-rich, and less extreme in terms of their stellar population and the ionization state of the interstellar medium. Like the Green Peas, they have exceptionally high star-formation rates per unit area. They also share some properties with the known leaky galaxies at $z\sim3$, but are significantly dustier. Our results validate a new way to identify local laboratories for exploring the processes that made it possible for galaxies to reionize the Universe.

DIRECT DETECTION OF LYMAN CONTINUUM ESCAPE FROM LOCAL STARBURST GALAXIES WITH THE COSMIC ORIGINS SPECTROGRAPH

ABSTRACT We report on the detection of Lyman continuum radiation in two nearby starburst galaxies. Tol 0440-381, Tol 1247-232, and Mrk 54 were observed with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. The three galaxies have radial velocities of ∼13,000 km s−1, permitting a ∼35 Å window on the restframe Lyman continuum shortward of the Milky Way Lyman edge at 912 Å. The chosen instrument configuration using the G140L grating covers the spectral range from 912 to 2000 Å. We developed a dedicated background subtraction method to account for the temporal and spatial background variations of the detector, which is crucial at the low flux levels around 912 Å. This modified pipeline allowed us to significantly improve the statistical and systematic detector noise and will be made available to the community. We detect Lyman continuum in all three galaxies. However, we conservatively interpret the emission in Tol 0440-381 as an upper limit due to possible contamination by geocoronal Lyman series lines. We determined the current star formation properties from the far-ultraviolet continuum and spectral lines and used synthesis models to predict the Lyman continuum radiation emitted by the current population of hot stars. We discuss various model uncertainties such as, among others, atmospheres and evolution models. Lyman continuum escape fractions were derived from a comparison between the observed and predicted Lyman continuum fluxes. Tol 1247-232, Mrk 54, and Tol 0440-381 have absolute escape fractions of (4.5 ± 1.2)%, (2.5 ± 0.72)%, and <(7.1 ± 1.1)%, respectively.

The peculiar optical-UV X-ray spectra of the X-ray weak quasar PG 0043+039

The object PG 0043+039 has been identified as a broad absorption line (BAL) quasar based on its UV spectra. However, this optical luminous quasar has not been detected before in deep X-ray observations, making it the most extreme X-ray weak quasar known today. This study aims to detect PG 0043+039 in a deep X-ray exposure. The question is what causes the extreme X-ray weakness of PG 0043+039? Does PG 0043+039 show other spectral or continuum peculiarities? We took simultaneous deep X-ray spectra with XMM-Newton, far-ultraviolet (FUV) spectra with the Hubble Space Telescope (HST) and optical spectra of PG 0043+039 with the Hobby-Eberly Telescope (HET) and Southern African Large Telescope (SALT) in July, 2013. We have detected PG 0043+039 in our X-ray exposure taken in 2013. We presented our first results in a separate paper (Kollatschny et al. 2015). PG 0043+039 shows an extreme {\alpha}_ox gradient ({\alpha}_ox =-2.37). Furthermore, we were able to verify an X-ray flux of this source in a reanalysis of the X-ray data taken in 2005. At that time, it was fainter by a factor of 3.8 +- 0.9 with {\alpha}_ox=-2.55. The X-ray spectrum is compatible with a normal quasar power-law spectrum ({\Gamma}=1.70_-0.45^+0.57) with moderate intrinsic absorption (N_H=5.5_-3.9^+6.9 +- 10^21cm^-2) and reflection. The UV/optical flux of PG 0043+039 has increased by a factor of 1.8 compared to spectra taken in the years 1990-1991. The FUV spectrum is highly peculiar and dominated by broad bumps besides Ly{\alpha}. There is no detectable Lyman edge associated with the BAL absorbing gas seen in the CIV line. PG 0043+039 shows a maximum in the overall continuum flux at around {\lambda} 2500{\AA} in contrast to most other AGN where the maximum is found at shorter wavelengths. All the above is compatible with an intrinsically X-ray weak quasar, rather than an absorbed X-ray emission.

THE EFFICIENCY OF STELLAR REIONIZATION: EFFECTS OF ROTATION, METALLICITY, AND INITIAL MASS FUNCTION

We compute the production rate of photons in the ionizing Lyman continua (LyC) of H I (lambda < 912 A), He I (lambda < 504 A), and He II (lambda < 228 A) using recent stellar evolutionary tracks coupled to a grid of non-LTE, line-blanketed (WM-basic) model atmospheres. The median LyC production efficiency is Q_LyC = (6+/-2)x10^60 LyC photons per Msun of star formation (range [3.1-9.4]x10^60) corresponding to a revised calibration of 10^{53.3+/-0.2} photons/s per Msun/yr. Efficiencies in the helium continua are Q_HeI ~ 10^60 photons/Msun and Q_HeII ~ 10^56 photons/Msun at solar metallicity and larger at low metallicity. The critical star formation rate needed to maintain reionization against recombinations at z = 7 is rho_SFR = (0.012 Msun/yr/Mpc^3) [(1+z)/8]^3 [(C_H /3) (0.2/ f_esc) for fiducial values of IGM clumping factor C_H = 3 and LyC escape fraction f_esc = 0.2. The boost in LyC production efficiency is an important ingredient, together with metallicity, C_H, and f_esc, in assessing whether IGM reionization was complete by z ~ 7. Monte-Carlo sampled spectra of coeval starbursts during the first 5 Myr have intrinsic flux ratios of F(1500)/F(900) = 0.4-0.5 and F(912^-)/F(912^+) = 0.4-0.7 in the far-UV (1500A), the LyC (900A), and at the Lyman edge (912A). These ratios can be used to calibrate the LyC escape fractions in starbursts.

Lyman edges in supermassive black hole binaries

Abstract We propose a new spectral signature for supermassive black hole binaries (SMBHBs) with circumbinary gas discs: a sharp drop in flux bluewards of the Lyman limit. A prominent edge is produced if the gas dominating the emission in the Lyman continuum region of the spectrum is sufficiently cold (T ≲ 20 000 K) to contain significant neutral hydrogen. Circumbinary discs may be in this regime if the binary torques open a central cavity in the disc and clear most of the hot gas from the inner region, and if any residual UV emission from the individual BHs is either dim or intermittent. We model the vertical structure and spectra of circumbinary discs using the radiative transfer code tlusty, and identify the range of BH masses and binary separations producing a Lyman edge. We find that compact supermassive (M ≳ 108 M⊙) binaries with orbital periods of ∼0.1–10 yr, whose gravitational waves are expected to be detectable by pulsar timing arrays, could have prominent Lyman edges. Such strong spectral edge features are not typically present in AGN spectra and could serve as corroborating evidence for the presence of an SMBHB.

The average absorption properties of broad absorption line quasars at 800 &lt; λrest &lt; 3000 Å, and the underlying physical parameters

Broad absorption line quasars (BALQs) present a large diversity in their broad absorption line (BAL) profiles. To investigate what physical parameters underlie this diversity, we construct a sample of BALQs which covers lambda_rest ~ 800-3000 {\AA}, based on the Sloan Digital Sky Survey DR7 quasar catalogue. The average BAL properties are evaluated by taking the ratios of average BALQ spectra to the average spectra of matched samples of non-BALQs, where the matching is based on various emission properties. We find the following properties. (i) There is no detectable Lyman edge associated with the BAL absorbing gas (tau<0.1). (ii) The known increase of average absorption depth with the ionization potential extends to the higher ionization N V and O VI BALs. We also find that the C IV BAL profile is controlled by two parameters. (i) The He II emission EW, which controls the typical velocity of the C IV BAL, but does not affect the absorption depth. (ii) The spectral slope in the 1700-3000 {\AA} range (alpha_UVl), which controls the C IV peak absorption depth, but does not affect the typical velocity. The He II EW and alpha_UVl also control the observed fraction of quasars that are BALQs. We suggest that a lower He II EW may indicate a weaker ionizing continuum, which allows the outflow to reach higher velocities before being overionized, possibly without a need to invoke a shielding gas. A redder continuum may indicate a more inclined system, and a larger covering factor and larger column of the outflow along the line of sight.

COSMIC DUST IN Mg II ABSORBERS

MgII absorbers induce reddening on background quasars. We measure this effect and infer the cosmic density of dust residing in these systems to be \Omega\ ~ 2e-6, in units of the critical density of the Universe, which is comparable to the amount of dust found in galactic disks or about half the amount inferred to exist outside galaxies. We also estimate the neutral hydrogen abundance in MgII clouds to be \Omega\ ~ 1.5e-4, which is approximately 5% of hydrogen in stars in galaxies. This implies a dust-to-gas mass ratio for MgII clouds of about 1/100, which is similar to the value for normal galaxies. This would support the hypothesis of the outflow origin of MgII clouds, which are intrinsically devoid of stars and hence have no sources of dust. Considerations of the dust abundance imply that the presence of MgII absorbers around galaxies lasts effectively for a few Gyr. High redshift absorbers allow us to measure the rest-frame extinction curve to 900 Angstroms at which the absorption by the Lyman edge dominates over scattering by dust in the extinction opacity.

HST-COS OBSERVATIONS OF AGNs. I. ULTRAVIOLET COMPOSITE SPECTRA OF THE IONIZING CONTINUUM AND EMISSION LINES

The ionizing fluxes from quasars and other active galactic nuclei (AGN) are critical for interpreting the emission-line spectra of AGN and for photoionization and heating of the intergalactic medium. Using ultraviolet spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we have directly measured the rest-frame ionizing continua and emission lines for 22 AGN. Over the redshift range 0.026 < z < 1.44, COS samples the Lyman continuum and many far-UV emission lines (Lya 1216, C IV 1549, Si IV/OIV] 1400, N V 1240, O VI 1035). Strong EUV emission lines with 14-22 eV excitation energies (Ne VIII 770,780, Ne V 569, O II 834, O III 833, 702, O IV 788,608,554, O V 630, N III 685) suggest the presence of hot gas in the broad emission-line region. The rest-frame continuum, F_nu ~ nu^{alpha_nu}, shows a break at wavelengths below 1000 A, with spectral index alpha_nu = -0.68 +/- 0.14 in the FUV (1200-2000 A) steepening to alpha_nu = -1.41 +/- 0.21 in the EUV (500-1000 A). The COS EUV index is similar to that of radio-quiet AGN in the 2002 HST/FOS survey (alpha_nu = -1.57 +/- 0.17). We see no Lyman edge (tau_HI < 0.03) or He I 584 emission in the AGN composite. Our 22 AGN exhibit a substantial range of FUV/EUV spectral indices and a correlation with AGN luminosity and redshift, likely due to observing below the 1000 A break.

Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption

We discuss expectations for the absorption of high-energy gamma-rays by gamma-gamma pair production with intergalactic radiation fields (IRFs) at very high redshifts (z~5-20), and the prospects thereof for probing the cosmic reionization era. For the evolving IRF, a semi-analytical model incorporating both Population II and Population III stars is employed, which is consistent with a wide variety of existing high-z observations including QSO spectral measurements, WMAP Thomson depth constraints, near-IR source count limits, etc. We find that the UV IRF below the Lyman edge energy with intensities in the range of a few times 10^{-19} erg cm^{-2} s^{-1} Hz^{-1} sr^{-1} can cause appreciable attenuation above ~12 GeV at z~5, down to ~6-8 GeV at z>~8-10. This may be observable in the spectra of blazars or gamma-ray bursts by the Fermi Gamma-ray Space Telescope or next generation facilities such as the Cherenkov Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable insight into early star formation and cosmic reionization.

FAR-ULTRAVIOLET SENSITIVITY OF THE COSMIC ORIGINS SPECTROGRAPH

We demonstrate that the G140L segment B channel of the Cosmic Origins Spectrograph (COS) recently installed on the {\it Hubble Space Telescope (HST)} has an effective area consistent with $\sim$ 10 cm$^2$ in the bandpass between the Lyman edge at 912 \AA and Lyman $\beta$, rising to a peak in excess of 1000 cm$^2$ longward of 1130 \AA. This is a new wavelength regime for {\it HST} and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale-length of the hardness variation in the metagalactic ionizing background over the redshift range 2 $< z \lesssim$ 2.8, measure the ratio of CO to H$_2$ in dense interstellar environments with $A_V >$ 3, or harness the high temperature diagnostic power of the \ion{O}{6} $\lambda\lambda$ 1032, 1038 doublet can now be carried out with unprecedented sensitivity.

OBSERVATIONS OF STARBURST GALAXIES WITH FAR-ULTRAVIOLET SPECTROGRAPHIC EXPLORER: GALACTIC FEEDBACK IN THE LOCAL UNIVERSE

We have analyzed FUSE (905-1187 A) spectra of a sample of 16 local starburst galaxies. These galaxies cover almost three orders of magnitude in star-formation rates and over two orders of magnitude in stellar mass. Absorption features from the stars and interstellar medium are observed in all the spectra. The strongest interstellar absorption features are generally blue-shifted by ~ 50-300 km s–1, implying the almost ubiquitous presence of starburst-driven galactic winds in this sample. The outflow velocites increase with both the star-formation rate and the star-formation rate per unit stellar mass, consistent with a galactic wind, driven by the population of massive stars. We find outflowing coronal-phase gas (T ~105.5 K) detected via the O VI absorption line in nearly every galaxy. The O VI absorption-line profile is optically thin, is generally weak near the galaxy-systemic velocity, and has a higher mean outflow velocity than seen in the lower ionization lines. The relationship between the line width and column density for the O VI absorbing gas is in good agreement with expectations for radiatively cooling and outflowing gas. Such gas will be created in the interaction of the hot out-rushing wind seen in X-ray emission and cool dense ambient material. O VI emission is not generally detected in our sample, suggesting that radiative cooling by the coronal gas is not dynamically significant in draining energy from galactic winds. We find that the measured outflow velocities in the H I and H II phases of the interstellar gas in a given galaxy increase with the strength (equivalent width) of the absorption feature and not with the ionization potential of the species. The strong lines often have profiles consisting of a broad and optically-thick component centered near the galaxy-systemic velocity and weaker but highly blue-shifted absorption. This suggests that the outflowing gas with high velocity has a lower column density than the more quiescent gas, and can only be readily detected in the strongest absorption lines. From direct observations below the Lyman edge in the galaxy rest frame, we find no evidence of Lyman continuum radiation escaping from any of the galaxies in the sample. Moreover, the small escape fraction of light in the center of the strong C II absorption feature confirms the high opacity below the Lyman limit in the neutral ISM. The absolute fraction of escaping Lyman continuum photons is typically <1%. This sample provides a unique window on the global properties of local star-forming galaxies as observed in the far-UV and also provides a useful comparison sample for understanding spectra of high-redshift star-forming galaxies.

Far Ultraviolet Spectroscopic ExplorerSpectroscopy of Absorption and Emission Lines from the Narrow-Line Seyfert 1 Galaxy NGC 4051

We present three Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the Narrow-Line Seyfert 1 galaxy NGC4051. The most prominent features in the far-ultraviolet (FUV) spectrum are the OVI emission and absorption lines and the HI Lyman series absorption lines which are detected up to the Lyman edge. We also identify weak emission from NIII, CIII, and HeII. The CIII line shows absorption while none is detected in the NIII and HeII lines. In HI and CIII we detect two main absorption systems at outflow velocities of -50+/-30 and -240+/-40 km/s, as well as a possible third one at ~ -450 km/s. These systems are consistent in velocity with the 10 absorption systems found previously in CIV, NV, and SiIV, though the individual systems are blended together in the FUV spectrum. We estimate column densities of the two main absorption systems and find that the HI column density is lower for systems with larger outflow velocity. We detect no flux or spectral variations of NGC4051 at FUV wavelengths during three epochs spanning one year. This is consistent with the optical light curve which shows no variations between the three epochs. It is also consistent with the X-ray light curve which shows consistent flux levels at the three epochs of the FUSE observations, although the X-ray light curve shows strong variations on much shorter timescales.

The effects of ultraviolet background correlations on Lyα forest flux statistics

We examine the possible effects of ultraviolet (UV) background fluctuations on the pixel flux power spectrum and autocorrelation function of the Lyα forest due to a finite number of sources in an attenuating medium. We consider scenarios in which quasi-stellar object (QSO) sources dominate the contribution to the UV background. To estimate their contribution, we use the QSO luminosity functions from the Two-Degree Field and the Sloan Digital Sky Survey. We estimate self-consistent values for the attenuation length at the Lyman edge through the intergalactic medium using particle mesh simulations of the Lya forest, normalized by the measured mean Lya flux. It is necessary to add the contribution from Lyman-limit systems based on their measured statistical properties because the simulations are unable to reproduce the measured abundances of these systems, suggesting that their formation may involve more complex hydrodynamical processes than simple collapse into dark matter haloes. We examine the convergence properties of the flux power spectrum and autocorrelation function under differing assumptions regarding pressure smoothing. On the basis of our tests, we recommend that simulations used to estimate the flux power spectrum at high redshifts (z > 3) be performed in a box of comoving size at least 25 h -1 Mpc on a side and a comoving force mesh as fine as 30-60 h -1 kpc. The requirements become more severe at lower redshifts. Even with these specifications, we find that the flux autocorrelation function does not converge to better than 10 per cent on scales exceeding 3 per cent of the box size without added pressure smoothing. Much better convergence properties are obtained for the relative effects of the UV background fluctuations on the flux power spectrum. The background fluctuations increase the large-scale power and suppress the power at intermediate scales. At z ≤ 4, the effect is only at the few per cent level. By z = 5.5, however, the large-scale power is boosted by more than 10 per cent, growing rapidly at higher redshifts, and suggesting that the flux power spectrum may serve as a useful tool for distinguishing QSO-dominated UV background scenarios from those in which more abundant sources like galaxies dominate.

Molecular Hydrogen Optical Depth Templates for FUSE Data Analysis

ABSTRACTThe calculation and use of molecular hydrogen optical depth templates to quickly identify and model molecular hydrogen absorption features longward of the Lyman edge at 912 Å are described. Such features are commonly encountered in spectra obtained by the Far Ultraviolet Spectroscopic Explorer and also in spectra obtained by the Space Telescope Imaging Spectrograph, albeit less commonly. Individual templates are calculated containing all the Lyman and Werner transitions originating from a single rotational state (J′′) of the zeroth vibrational level (v′′) of the ground electronic state. Templates are provided with 0.01 Å sampling for Doppler parameters ranging from 2≤b≤20 km s−1 and rotational states 0≤J′′≤15. Optical depth templates for excited vibrational states are also available for select Doppler parameters. Each template is calculated for a fiducial column density of log [N(cm -2)] = 21 and may be scaled to any column less than this value without loss of accuracy. These templates will facilitate the determination of the distribution of molecular hydrogen column density as a function of rotational level. The use of these templates will free the user from the computationally intensive task of calculating profiles for a large number of lines and allow concentration on line‐profile or curve‐of‐growth fitting to determine column densities and Doppler parameters. The templates may be downloaded freely from http://www.pha.jhu.edu/∼stephan/h2ools.html.

Non‐LTE, Relativistic Accretion Disk Fits to 3C 273 and the Origin of the Lyman Limit Spectral Break

We fit general relativistic, geometrically thin accretion disk models with non-LTE atmospheres to near-simultaneous multiwavelength data of 3C 273, extending from the optical to the far-ultraviolet. Our model fits show no flux discontinuity associated with a hydrogen Lyman edge, but they do exhibit a spectral break which qualitatively resembles that seen in the data. This break arises from relativistic smearing of Lyman emission edges which are produced locally at tens of gravitational radii in the disk. We discuss the possible effects of metal line blanketing on the model spectra, as well as the substantial Comptonization required to explain the observed soft X-ray excess. Our best-fit accretion disk model underpredicts the near-ultraviolet emission in this source and also has an optical spectrum which is too red. We discuss some of the remaining physical uncertainties and suggest in particular that an extension of our models to the slim disk regime and/or including nonzero magnetic torques across the innermost stable circular orbit may help resolve these discrepancies.

Probing High‐Redshift Radiation Fields with Gamma‐Ray Absorption

The next generation of gamma-ray telescopes may be able to observe gamma-ray blazars at high redshift, possibly out to the epoch of reionization. The spectrum of such sources should exhibit an absorption edge due to pair production against UV photons along the line of sight. One expects a sharp drop in the number density of UV photons at the Lyman edge L. This implies that the universe becomes transparent after gamma-ray photons redshift below E ~ (mec2)2/L ~ 18 GeV. Thus, there is only a limited redshift interval over which GeV photons can pair produce. This implies that any observed absorption will probe radiation fields in the very early universe, regardless of the subsequent star formation history of the universe. Furthermore, measurements of differential absorption between blazars at different redshifts can cleanly isolate the opacity due to UV emissivity at high redshift. An observable absorption edge should be present for most reasonable radiation fields with sufficient energy to reionize the universe. Ly? photons may provide an important component of the pair production opacity. Observations of a number of blazars at different redshifts will thus allow us to probe the rise in comoving UV emissivity with time.

The Halo White Dwarf WD 0346+246 Revisited

The extremely helium-rich atmospheric composition determined for the halo white dwarf WD 0346+246 is reexamined. This solution is shown to be improbable from an astrophysical point of view when accretion of hydrogen and metals from the interstellar medium is taken into account. An alternate solution is proposed in which hydrogen and helium are present in the atmospheric regions in equal amounts. The best fit, at Teff = 3780 K, log g = 8.34, and N(He)/N(H) = 1.3, is achieved by including in the model calculations a bound-free opacity from the Lyman edge associated with the so-called dissolved atomic levels of the hydrogen atom, or pseudocontinuum opacity.

On the Escape of Ionizing Radiation from Starbursts

Far-ultraviolet spectra obtained with FUSE show that the strong C II λ1036 interstellar absorption line is essentially black in five of the UV-brightest local starburst galaxies. Because the opacity of the neutral ISM below the Lyman edge will be significantly larger than in the C II line, these data provide strong constraints on the escape of ionizing radiation from these starbursts. Interpreted as a uniform, absorbing slab, the implied optical depth at the Lyman edge is huge (τ0 ≥ 102). Alternatively, the areal covering factor of opaque material is typically ≥94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield fesc ≤ 6%. Inclusion of extinction due to dust will further decrease fesc. An analogous analysis of the rest-UV spectrum of the star-forming galaxy MS 1512-cB58 at z = 2.7 leads to similar constraints on fesc. These new results agree with the constraints provided by direct observations below the Lyman edge in a few other local starbursts. However, they differ from the recently reported properties of star-forming galaxies at z ≥ 3. We assess the idea that the strong galactic winds seen in many powerful starbursts clear channels through their neutral ISM. We show empirically that such outflows may be a necessary—but not sufficient—part of the process of creating a relatively porous ISM. We note that observations will soon document the cosmic evolution in the contribution of star-forming galaxies to the metagalactic ionizing background, with important implications for the evolution of the IGM.