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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 325 records · Page 18
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DOE OSTI2014

What next-generation 21 cm power spectrum measurements can teach us about the epoch of reionization

A number of experiments are currently working toward a measurement of the 21 cm signal from the epoch of reionization (EoR). Whether or not these experiments deliver a detection of cosmological emission, their limited sensitivity will prevent them from providing detailed information about the astrophysics of reionization. In this work, we consider what types of measurements will be enabled by the next generation of larger 21 cm EoR telescopes. To calculate the type of constraints that will be possible with such arrays, we use simple models for the instrument, foreground emission, and the reionization history. We focus primarily on an instrument modeled after the ∼0.1 km{sup 2} collecting area Hydrogen Epoch of Reionization Array concept design and parameterize the uncertainties with regard to foreground emission by considering different limits to the recently described 'wedge' footprint in k space. Uncertainties in the reionization history are accounted for using a series of simulations that vary the ionizing efficiency and minimum virial temperature of the galaxies responsible for reionization, as well as the mean free path of ionizing photons through the intergalactic medium. Given various combinations of models, we consider the significance of the possible power spectrum detections, the ability to trace the power spectrum evolution versus redshift, the detectability of salient power spectrum features, and the achievable level of quantitative constraints on astrophysical parameters. Ultimately, we find that 0.1 km{sup 2} of collecting area is enough to ensure a very high significance (≳ 30σ) detection of the reionization power spectrum in even the most pessimistic scenarios. This sensitivity should allow for meaningful constraints on the reionization history and astrophysical parameters, especially if foreground subtraction techniques can be improved and successfully implemented.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Magnified views of the ultrafast outflow of the z = 1.51 active galactic nucleus HS 0810+2554

We present results from an observation of the gravitationally lensed z = 1.51 narrow absorption line (NAL) active galactic nucleus (AGN) HS 0810+2554 performed with the Chandra X-Ray Observatory. The factor of ∼100 lensing magnification of HS 0810+2554 makes this source exceptionally bright. Absorption lines are detected at rest-frame energies of ∼7.7 keV and ∼11.0 keV at ≳97% significance. By interpreting these lines to arise from highly ionized iron the implied outflow velocities of the X-ray absorbing gas corresponding to these lines are 0.13c and 0.41c, respectively. The presence of these relativistic outflows and the absence of any significant low-energy X-ray absorption suggest that a shielding gas is not required for the generation of the relativistic X-ray absorbing winds in HS 0810+2554. UV spectroscopic observations with VLT/UVES indicate that the UV absorbing material is outflowing at v {sub UV} ∼0.065c. Our analysis indicates that the fraction of the total bolometric energy released by HS 0810+2554 into the intergalactic medium (IGM) in the form of kinetic energy is ε{sub k} = 1.0{sub −0.6}{sup +0.8}. An efficiency of greater than unity implies that magnetic driving is likely a significant contributor to the acceleration of this X-ray absorbing wind. We also estimate the mass-outflow rate of the strongest absorption component to be M-dot {sub abs} = 1.1{sub −0.7}{sup +0.9} M {sub ☉} yr{sup –1}. Assuming that the energetic outflow detected in the NAL AGN HS 0810+2554 is a common property of most AGN it would suggest that the X-ray absorbing wind may have a larger opening angle than previously thought. This has important consequences for estimating the feedback contribution of X-ray absorbing winds to the surrounding IGM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Plasma effects on fast pair beams. III. Oblique electrostatic growth rates for perpendicular Maxwellian pair beams

The distant universe is opaque to γ radiation from blazars due to gamma-gamma attenuation with extragalactic background light. This process produces electron-positron pair beams that interact with the intergalactic medium and are unstable to linear instabilities, particularly the electrostatic and Weibel instabilities. The electrostatic instability grows faster and so determines the dissipation of the free energy of the beam. Here, we generalize the calculation of the electrostatic growth rate to a beam plasma system with a Maxwellian perpendicular momentum spread and allow for oblique propagation directions. We show that the growth rate for the oblique electrostatic mode has a maximum value that is even higher than for a cold beam or for one with a constant perpendicular momentum spread.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

LyMAS: Predicting large-scale Lyα forest statistics from the dark matter density field

We describe Lyα Mass Association Scheme (LyMAS), a method of predicting clustering statistics in the Lyα forest on large scales from moderate-resolution simulations of the dark matter (DM) distribution, with calibration from high-resolution hydrodynamic simulations of smaller volumes. We use the 'Horizon-MareNostrum' simulation, a 50 h {sup –1} Mpc comoving volume evolved with the adaptive mesh hydrodynamic code RAMSES, to compute the conditional probability distribution P(F{sub s} |δ {sub s}) of the transmitted flux F{sub s} , smoothed (one-dimensionally, 1D) over the spectral resolution scale, on the DM density contrast δ {sub s}, smoothed (three-dimensionally, 3D) over a similar scale. In this study we adopt the spectral resolution of the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS) at z = 2.5, and we find optimal results for a DM smoothing length σ = 0.3 h {sup –1} Mpc (comoving). In its simplest form, LyMAS draws randomly from the hydro-calibrated P(F{sub s} |δ {sub s}) to convert DM skewers into Lyα forest pseudo-spectra, which are then used to compute cross-sightline flux statistics. In extended form, LyMAS exactly reproduces both the 1D power spectrum and one-point flux distribution of the hydro simulation spectra. Applied to the MareNostrum DM field, LyMAS accurately predicts the two-point conditional flux distribution and flux correlation function of the full hydro simulation for transverse sightline separations as small as 1 h {sup –1} Mpc, including redshift-space distortion effects. It is substantially more accurate than a deterministic density-flux mapping ({sup F}luctuating Gunn-Peterson Approximation{sup )}, often used for large-volume simulations of the forest. With the MareNostrum calibration, we apply LyMAS to 1024{sup 3} N-body simulations of a 300 h {sup –1} Mpc and 1.0 h {sup –1} Gpc cube to produce large, publicly available catalogs of mock BOSS spectra that probe a large comoving volume. LyMAS will be a powerful tool for interpreting 3D Lyα forest data, thereby transforming measurements from BOSS and other massive quasar absorption surveys into constraints on dark energy, DM, space geometry, and intergalactic medium physics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A physical model for the evolving ultraviolet luminosity function of high redshift galaxies and their contribution to the cosmic reionization

We present a physical model for the evolution of the ultraviolet (UV) luminosity function of high-redshift galaxies, taking into account in a self-consistent way their chemical evolution and the associated evolution of dust extinction. Dust extinction is found to increase fast with halo mass. A strong correlation between dust attenuation and halo/stellar mass for UV selected high-z galaxies is thus predicted. The model yields good fits of the UV and Lyman-α (Lyα) line luminosity functions at all redshifts at which they have been measured. The weak observed evolution of both luminosity functions between z = 2 and z = 6 is explained as the combined effect of the negative evolution of the halo mass function; of the increase with redshift of the star formation efficiency due to the faster gas cooling; and of dust extinction, differential with halo mass. The slope of the faint end of the UV luminosity function is found to steepen with increasing redshift, implying that low luminosity galaxies increasingly dominate the contribution to the UV background at higher and higher redshifts. The observed range of the UV luminosities at high z implies a minimum halo mass capable of hosting active star formation M {sub crit} ≲ 10{sup 9.8} M {sub ☉}, which is consistent with the constraints from hydrodynamical simulations. From fits of Lyα line luminosity functions, plus data on the luminosity dependence of extinction, and from the measured ratios of non-ionizing UV to Lyman-continuum flux density for samples of z ≅ 3 Lyman break galaxies and Lyα emitters, we derive a simple relationship between the escape fraction of ionizing photons and the star formation rate. It implies that the escape fraction is larger for low-mass galaxies, which are almost dust-free and have lower gas column densities. Galaxies already represented in the UV luminosity function (M {sub UV} ≲ –18) can keep the universe fully ionized up to z ≅ 6. This is consistent with (uncertain) data pointing to a rapid drop of the ionization degree above z ≅ 6, such as indications of a decrease of the comoving emission rate of ionizing photons at z ≅ 6, a decrease of sizes of quasar near zones, and a possible decline of the Lyα transmission through the intergalactic medium at z > 6. On the other hand, the electron scattering optical depth, τ{sub es}, inferred from cosmic microwave background (CMB) experiments favor an ionization degree close to unity up to z ≅ 9-10. Consistency with CMB data can be achieved if M {sub crit} ≅ 10{sup 8.5} M {sub ☉}, implying that the UV luminosity functions extend to M {sub UV} ≅ –13, although the corresponding τ{sub es} is still on the low side of CMB-based estimates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Shock waves and cosmic ray acceleration in the outskirts of galaxy clusters

The outskirts of galaxy clusters are continuously disturbed by mergers and gas infall along filaments, which in turn induce turbulent flow motions and shock waves. We examine the properties of shocks that form within r {sub 200} in sample galaxy clusters from structure formation simulations. While most of these shocks are weak and inefficient accelerators of cosmic rays (CRs), there are a number of strong, energetic shocks which can produce large amounts of CR protons via diffusive shock acceleration. We show that the energetic shocks reside mostly in the outskirts and a substantial fraction of them are induced by infall of the warm-hot intergalactic medium from filaments. As a result, the radial profile of the CR pressure in the intracluster medium is expected to be broad, dropping off more slowly than that of the gas pressure, and might be even temporarily inverted, peaking in the outskirts. The volume-integrated momentum spectrum of CR protons inside r {sub 200} has the power-law slope of 4.25-4.5, indicating that the average Mach number of the shocks of main CR production is in the range of {sub CR} ≈ 3-4. We suggest that some radio relics with relatively flat radio spectrum could be explained by primary electrons accelerated by energetic infall shocks with M{sub s} ≳ 3 induced in the cluster outskirts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Intensity mapping across cosmic times with the Lyα line

We present a quantitative model of Lyα emission throughout cosmic history and determine the prospects for intensity mapping spatial fluctuations in the Lyα signal. Since (1) our model assumes at z > 6 the minimum star formation required to sustain reionization and (2) is based at z < 6 on a luminosity function (LF) extrapolated from the few observed bright Lyα emitters, this should be considered a lower limit. Mapping the line emission allows probes of reionization, star formation, and large-scale structure (LSS) as a function of redshift. While Lyα emission during reionization has been studied, we also predict the postreionization signal to test predictions of the intensity and motivate future intensity mapping probes of reionization. We include emission from massive dark matter halos and the intergalactic medium (IGM) in our model. We find agreement with current, measured LFs of Lyα emitters at z < 8. However, diffuse IGM emission, not associated with Lyα emitters, dominates the intensity up to z ∼ 10. While our model is applicable for deep-optical or near-infrared observers like the James Webb Space Telescope, only intensity mapping will detect the diffuse IGM emission. We also construct a three-dimensional power spectrum model of the Lyα emission. Finally, we consider the prospects of an intensity mapper for measuring Lyα fluctuations while identifying interloper contamination for removal. Our results suggest that while the reionization signal is challenging, Lyα fluctuations can be an interesting new probe of LSS at late times when used in conjunction with other lines, e.g., Hα, to monitor low-redshift foreground confusion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Relativistic pair beams from TeV blazars: A source of reprocessed GeV emission rather than intergalactic heating

The interaction of TeV photons from blazars with the extragalactic background light produces a relativistic beam of electron-positron pairs streaming through the intergalactic medium (IGM). The fate of the beam energy is uncertain. By means of two- and three-dimensional particle-in-cell simulations, we study the nonlinear evolution of dilute ultra-relativistic pair beams propagating through the IGM. We explore a wide range of beam Lorentz factors γ {sub b} >> 1 and beam-to-plasma density ratios α << 1, so that our results can be extrapolated to the extreme parameters of blazar-induced beams (γ {sub b} ∼ 10{sup 6} and α ∼ 10{sup –15}, for powerful blazars). For cold beams, we show that the oblique instability governs the early stages of evolution, but its exponential growth terminates—due to self-heating of the beam in the transverse direction—when only a negligible fraction ∼(α/γ {sub b}){sup 1/3} ∼ 10{sup –7} of the beam energy has been transferred to the IGM plasma. Further relaxation of the beam proceeds through quasi-longitudinal modes, until the momentum dispersion in the direction of propagation saturates at Δp {sub b,} {sub ∥}/γ{sub b} m{sub e}c ∼ 0.2. This corresponds to a fraction ∼10% of the beam energy—irrespective of γ {sub b} or α—being ultimately transferred to the IGM plasma (as compared to the heating efficiency of ∼50% predicted by one-dimensional models, which cannot properly account for the transverse broadening of the beam). For the warm beams generated by TeV blazars, the development of the longitudinal relaxation is suppressed, since the initial dispersion in beam momentum is already Δp {sub b0,} {sub ∥}/γ {sub b} m{sub e}c ≳ 1. Here, the fraction of beam energy ultimately deposited into the IGM is only ∼α γ {sub b} ∼ 10{sup –9}. It follows that most of the beam energy is still available to power the GeV emission produced by inverse Compton up-scattering of the cosmic microwave background by the beam pairs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Chemical constraints on the contribution of population III stars to cosmic reionization

Recent studies have highlighted that galaxies at z = 6-8 fall short of producing enough ionizing photons to reionize the intergalactic medium, and suggest that Population III stars could resolve this tension, because their harder spectra can produce ∼10 × more ionizing photons than Population II. We use a semi-analytic model of galaxy formation, which tracks galactic chemical evolution, to gauge the impact of Population III stars on reionization. Population III supernovae produce distinct metal abundances, and we argue that the duration of the Population III era can be constrained by precise relative abundance measurements in high-z damped Lyα absorbers (DLAs), which provide a chemical record of past star formation. We find that a single generation of Population III stars can self-enrich galaxies above the critical metallicity Z {sub crit} = 10{sup –4} Z {sub ☉} for the Population III-to-II transition, on a very short timescale t {sub self-enrich} ∼ 10{sup 6} yr, owing to the large metal yields and short lifetimes of Population III stars. This subsequently terminates the Population III era, so they contribute ≳ 50% of the ionizing photons only for z ≳ 30, and at z = 10 contribute <1%. The Population III contribution can be increased by delaying metal mixing into the interstellar medium. However, comparing the resulting metal abundance pattern to existing measurements in z ≲ 6 DLAs, we show that the observed [O/Si] ratios of absorbers rule out Population III stars being a major contributor to reionization. Future abundance measurements of z ∼ 7-8 QSOs and gamma-ray bursts should probe the era when the chemical vestiges of Population III star formation become detectable.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Recombination clumping factor during cosmic reionization

We discuss the role of recombinations in the intergalactic medium, and the related concept of the clumping factor, during cosmic reionization. The clumping factor is, in general, a local quantity that depends on both the local overdensity and the scale below which the baryon density field can be assumed smooth. That scale, called the filtering scale, depends on over-density and local thermal history. We present a method for building a self-consistent analytical model of inhomogeneous reionization, assuming the linear growth rate of the density fluctuation, which simultaneously accounts for these effects. We show that taking into account the local clumping factor introduces significant corrections to the total recombination rate, compared to the model with a globally uniform clumping factor.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Spectroscopic observation of Lyα emitters at z ∼ 7.7 and implications on re-ionization

We present spectroscopic follow-up observations on two bright Lyα emitter (LAE) candidates originally found by Krug et al. at a redshift of z ∼ 7.7 using the Multi-Object Spectrometer for Infra-Red Exploration at Keck. We rule out any line emission at the >5σ level for both objects, putting on solid ground a previous null result for one of the objects. The limits inferred from the non-detections rule out the previous claim of no or even reversed evolution between 5.7 < z < 7.7 in the Lyα luminosity function (LF) and suggest a drop in the Lyα LF consistent with that seen in Lyman break galaxy (LBG) samples. We model the redshift evolution of the LAE LF using the LBG UV-continuum LF and the observed rest-frame equivalent width distribution. From the comparison of our empirical model with the observed LAE distribution, we estimate lower limits of the neutral hydrogen fraction to be 50%-70% at z ∼ 7.7. Together with this, we find a strong evolution in the Lyα optical depth characterized by (1 + z){sup 2.2} {sup ±} {sup 0.5} beyond z = 6, indicative of a strong evolution of the intergalactic medium. Finally, we extrapolate the LAE LF to z ∼ 9 using our model and show that it is unlikely that large area surveys, like UltraVISTA or Euclid, pick up LAEs at this redshift assuming the current depths and area.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Escape fraction of ionizing photons during reionization: Effects due to supernova feedback and runaway ob stars

The fraction of hydrogen ionizing photons escaping from galaxies into the intergalactic medium is a critical ingredient in the theory of reionization. We use two zoomed-in, high-resolution (4 pc), cosmological radiation hydrodynamic simulations with adaptive mesh refinement to investigate the impact of two physical mechanisms (supernova, SN, feedback, and runaway OB stars) on the escape fraction (f {sub esc}) at the epoch of reionization (z ≥ 7). We implement a new, physically motivated SN feedback model that can approximate the Sedov solutions at all (from the free expansion to snowplow) stages. We find that there is a significant time delay of about ten million years between the peak of star formation and that of escape fraction, due to the time required for the build-up and subsequent destruction of the star-forming cloud by SN feedback. Consequently, the photon number-weighted mean escape fraction for dwarf galaxies in halos of mass 10{sup 8}-10{sup 10.5} M {sub ☉} is found to be 〈f{sub esc}〉∼11%, although instantaneous values of f {sub esc} > 20% are common when star formation is strongly modulated by the SN explosions. We find that the inclusion of runaway OB stars increases the mean escape fraction by 22% to 〈f{sub esc}〉∼14%. As SNe resulting from runaway OB stars tend to occur in less dense environments, the feedback effect is enhanced and star formation is further suppressed in halos with M{sub vir}≳10{sup 9} M{sub ⊙} in the simulation with runaway OB stars compared with the model without them. While both our models produce enough ionizing photons to maintain a fully ionized universe at z ≤ 7 as observed, a still higher amount of ionizing photons at z ≥ 9 appears necessary to accommodate the high observed electron optical depth inferred from cosmic microwave background observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Plasma effects on extragalactic ultra-high-energy cosmic ray hadron beams in cosmic voids

The linear instability of an ultrarelativistic hadron beam (Γ {sub b} ≈ 10{sup 6}) in the unmagnetized intergalactic medium (IGM) is investigated with respect to the excitation of collective electrostatic and aperiodic electromagnetic fluctuations. This analysis is important for the propagation of extragalactic ultrarelativistic cosmic rays (E > 10{sup 15} eV) from their distant sources to Earth. We calculate minimum instability growth times that are orders of magnitude shorter than the cosmic ray propagation time in the IGM. Due to nonlinear effects, especially the modulation instability, the cosmic ray beam stabilizes and can propagate with nearly no energy loss through the IGM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Fast radio burst discovered in the Arecibo pulsar ALFA survey

Recent work has exploited pulsar survey data to identify temporally isolated, millisecond-duration radio bursts with large dispersion measures (DMs). These bursts have been interpreted as arising from a population of extragalactic sources, in which case they would provide unprecedented opportunities for probing the intergalactic medium; they may also be linked to new source classes. Until now, however, all so-called fast radio bursts (FRBs) have been detected with the Parkes radio telescope and its 13-beam receiver, casting some concern about the astrophysical nature of these signals. Here we present FRB 121102, the first FRB discovery from a geographic location other than Parkes. FRB 121102 was found in the Galactic anti-center region in the 1.4 GHz Pulsar Arecibo L-band Feed Array (ALFA) survey with the Arecibo Observatory with a DM = 557.4 ± 2.0 pc cm{sup –3}, pulse width of 3.0 ± 0.5 ms, and no evidence of interstellar scattering. The observed delay of the signal arrival time with frequency agrees precisely with the expectation of dispersion through an ionized medium. Despite its low Galactic latitude (b = –0.°2), the burst has three times the maximum Galactic DM expected along this particular line of sight, suggesting an extragalactic origin. A peculiar aspect of the signal is an inverted spectrum; we interpret this as a consequence of being detected in a sidelobe of the ALFA receiver. FRB 121102's brightness, duration, and the inferred event rate are all consistent with the properties of the previously detected Parkes bursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Probing reionization with the cross-power spectrum of 21 cm and near-infrared radiation backgrounds

The cross-correlation between the 21 cm emission from the high-redshift intergalactic medium and the near-infrared (NIR) background light from high-redshift galaxies promises to be a powerful probe of cosmic reionization. In this paper, we investigate the cross-power spectrum during the epoch of reionization. We employ an improved halo approach to derive the distribution of the density field and consider two stellar populations in the star formation model: metal-free stars and metal-poor stars. The reionization history is further generated to be consistent with the electron-scattering optical depth from cosmic microwave background measurements. Then, the intensity of the NIR background is estimated by collecting emission from stars in first-light galaxies. On large scales, we find that the 21 cm and NIR radiation backgrounds are positively correlated during the very early stages of reionization. However, these two radiation backgrounds quickly become anti-correlated as reionization proceeds. The maximum absolute value of the cross-power spectrum is |Δ{sub 21,NIR}{sup 2}|∼10{sup −4} mK nW m{sup –2} sr{sup –1}, reached at ℓ ∼ 1000 when the mean fraction of ionized hydrogen is x-bar{sub i}∼0.9. We find that Square Kilometer Array can measure the 21 cm-NIR cross-power spectrum in conjunction with mild extensions to the existing CIBER survey, provided that the integration time independently adds up to 1000 and 1 hr for 21 cm and NIR observations, and that the sky coverage fraction of the CIBER survey is extended from 4 × 10{sup –4} to 0.1. Measuring the cross-correlation signal as a function of redshift provides valuable information on reionization and helps confirm the origin of the 'missing' NIR background.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The baryon cycle of dwarf galaxies: dark, bursty, gas-rich polluters

We present results from a fully cosmological, very high-resolution, ΛCDM simulation of a group of seven field dwarf galaxies with present-day virial masses in the range M {sub vir} = 4.4 × 10{sup 8}-3.6 × 10{sup 10} M {sub ☉}. The simulation includes a blastwave scheme for supernova feedback, a star-formation recipe based on a high gas density threshold, metal-dependent radiative cooling, a scheme for the turbulent diffusion of metals and thermal energy, and a uniform UV background. The properties of the simulated dwarfs are strongly modulated by the depth of the gravitational potential well. All three halos with M {sub vir} < 10{sup 9} M {sub ☉} are devoid of stars, as they never reach the density threshold for star formation of 100 atoms cm{sup –3}. The other four, M {sub vir} > 10{sup 9} M {sub ☉} dwarfs have blue colors, low star-formation efficiencies, high cold gas-to-stellar mass ratios, and low stellar metallicities. Their bursty star-formation histories are characterized by peak specific star-formation rates in excess of 50-100 Gyr{sup –1}, far outside the realm of normal, more massive galaxies. The median stellar age of the simulated galaxies decreases with decreasing halo mass, with the two M {sub vir} ≅ 2-3 × 10{sup 9} M {sub ☉} dwarfs being predominantly young, and the two more massive systems hosting intermediate and older populations. The cosmologically young dwarfs are lit up by tidal interactions, have compact morphologies, and have metallicities and cold gas fractions similar to the relatively quiescent, extremely metal-deficient dwarf population. Metal-enriched galactic outflows produce sub-solar effective yields and pollute with heavy elements a megaparsec-size region of the intergalactic medium, but are not sufficient to completely quench star-formation activity and are absent in the faintest dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Constraining UV continuum slopes of active galactic nuclei with cloudy models of broad-line region extreme-ultraviolet emission lines

Understanding the composition and structure of the broad-line region (BLR) of active galactic nuclei (AGNs) is important for answering many outstanding questions in supermassive black hole evolution, galaxy evolution, and ionization of the intergalactic medium. We used single-epoch UV spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to measure EUV emission-line fluxes from four individual AGNs with 0.49 ≤ z ≤ 0.64, two AGNs with 0.32 ≤ z ≤ 0.40, and a composite of 159 AGNs. With the CLOUDY photoionization code, we calculated emission-line fluxes from BLR clouds with a range of density, hydrogen ionizing flux, and incident continuum spectral indices. The photoionization grids were fit to the observations using single-component and locally optimally emitting cloud (LOC) models. The LOC models provide good fits to the measured fluxes, while the single-component models do not. The UV spectral indices preferred by our LOC models are consistent with those measured from COS spectra. EUV emission lines such as N IV λ765, O II λ833, and O III λ834 originate primarily from gas with electron temperatures between 37,000 K and 55,000 K. This gas is found in BLR clouds with high hydrogen densities (n {sub H} ≥ 10{sup 12} cm{sup –3}) and hydrogen ionizing photon fluxes (Φ{sub H} ≥ 10{sup 22} cm{sup –2} s{sup –1}).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Rapid decline of Lyα emission toward the reionization era

The observed deficit of strongly Lyα emitting galaxies at z > 6.5 is attributed to increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this we have performed very deep near-IR spectroscopy of z ≳ 7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lyα fraction at z ∼ 8 using two methods. First, we derived N {sub Lyα}/N {sub tot} directly, using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al.) that compares the z > 7 galaxy spectra to models of the Lyα equivalent width (W {sub Lyα}) distribution at z ∼ 6. We explored two simple evolutionary scenarios: pure number evolution where Lyα is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines) and uniform dimming evolution where Lyα is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature, we find that at z ∼ 8 the Lyα fraction has dropped by a factor of >3 (84% confidence interval) using both the dimming and number evolution scenarios, compared to the z ∼ 6 values. Furthermore, we find a tentative positive Bayesian evidence favoring the number evolution scenario over dimming evolution, extending trends observed at z ≲ 7 to higher redshift. A comparison of our results with theoretical models implies the IGM volume averaged neutral hydrogen fraction ≳ 0.3, suggesting that we are likely witnessing reionization in progress at z ∼ 8.

79 ASTRONOMY AND ASTROPHYSICS↗