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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 415 records · Page 23
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DOE OSTI2016

Lyman continuum escape fraction of star-forming dwarf galaxies at z ∼ 1

To date, no direct detection of Lyman continuum emission has been measured for intermediate-redshift (z∼1) star-forming galaxies. We combine Hubble Space Telescope grism spectroscopy with GALEX UV and ground-based optical imaging to extend the search for escaping Lyman continuum to a large (∼600) sample of z∼1 low-mass (log( M-bar ) ≃ 9.3M{sub ⊙}), moderately star-forming ( Ψ-bar ≲ 10M{sub ⊙} yr{sup −1}) galaxies selected initially on Hα emission. The characteristic escape fraction of LyC from star-forming galaxies (SFGs) that populate this parameter space remains weakly constrained by previous surveys, but these faint (sub-L{sub ⋆}) SFGs are assumed to play a significant role in the reionization of neutral hydrogen in the intergalactic medium (IGM) at high redshift z>6. We do not make an unambiguous detection of escaping LyC radiation from this z∼1 sample, individual non-detections to constrain the absolute Lyman continuum escape fraction, f{sub esc} < 2.1% (3σ). We measure an upper limit of f{sub esc} < 9.6% from a sample of SFGs selected on high Hα equivalent width (EW > 200 A-ring ), which are thought to be close analogs of high redshift sources of reionization. For reference, we also present an emissivity-weighted escape fraction that is useful for measuring the general contribution SFGs to the ionizing UV background. In the discussion, we consider the implications of these intermediate redshift constraints for the reionization of hydrogen in the IGM at high (z>6) redshift. If we assume our z∼1 SFGs, for which we measure this emissivity-weighted f{sub esc}, are analogs to the high redshift sources of reionization, we find it is difficult to reconcile reionization by faint (M{sub UV}≲−13) SFGs with a low escape fraction (f{sub esc} < 3%), with constraints from independent high redshift observations. If f{sub esc} evolves with redshift, reionization by SFGs may be consistent with observations from Planck.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE KENNICUTT–SCHMIDT RELATION IN EXTREMELY METAL-POOR DWARF GALAXIES

The Kennicutt–Schmidt (KS) relation between the gas mass and star formation rate (SFR) describes the star formation regulation in disk galaxies. It is a function of gas metallicity, but the low-metallicity regime of the KS diagram is poorly sampled. We have analyzed data for a representative set of extremely metal-poor galaxies (XMPs), as well as auxiliary data, and compared these to empirical and theoretical predictions. The majority of the XMPs possess high specific SFRs, similar to high-redshift star-forming galaxies. On the KS plot, the XMP H i data occupy the same region as dwarfs and extend the relation for low surface brightness galaxies. Considering the H i gas alone, a considerable fraction of the XMPs already fall off the KS law. Significant quantities of “dark” H{sub 2} mass (i.e., not traced by CO) would imply that XMPs possess low star formation efficiencies (SFE{sub gas}). Low SFE{sub gas} in XMPs may be the result of the metal-poor nature of the H i gas. Alternatively, the H i reservoir may be largely inert, the star formation being dominated by cosmological accretion. Time lags between gas accretion and star formation may also reduce the apparent SFE{sub gas}, as may galaxy winds, which can expel most of the gas into the intergalactic medium. Hence, on global scales, XMPs could be H i-dominated, high-specific-SFR (≳10{sup −10} yr{sup −1}), low-SFE{sub gas} (≲10{sup −9} yr{sup −1}) systems, in which the total H i mass is likely not a good predictor of the total H{sub 2} mass, nor of the SFR.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

CONSTRAINING POLARIZED FOREGROUNDS FOR EoR EXPERIMENTS. I. 2D POWER SPECTRA FROM THE PAPER-32 IMAGING ARRAY

Current generation low-frequency interferometers constructed with the objective of detecting the high-redshift 21 cm background aim to generate power spectra of the brightness temperature contrast of neutral hydrogen in primordial intergalactic medium. Two-dimensional (2D) power spectra (power in Fourier modes parallel and perpendicular to the line of sight) that formed from interferometric visibilities have been shown to delineate a boundary between spectrally smooth foregrounds (known as the wedge ) and spectrally structured 21 cm background emission (the EoR window ). However, polarized foregrounds are known to possess spectral structure due to Faraday rotation, which can leak into the EoR window. In this work we create and analyze 2D power spectra from the PAPER-32 imaging array in Stokes I, Q, U, and V. These allow us to observe and diagnose systematic effects in our calibration at high signal-to-noise within the Fourier space most relevant to EoR experiments. We observe well-defined windows in the Stokes visibilities, with Stokes Q, U, and V power spectra sharing a similar wedge shape to that seen in Stokes I. With modest polarization calibration, we see no evidence that polarization calibration errors move power outside the wedge in any Stokes visibility to the noise levels attained. Deeper integrations will be required to confirm that this behavior persists to the depth required for EoR detection.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The wide-angle outflow of the lensed z = 1.51 AGN HS 0810+2554

We present results from X-ray observations of the gravitationally lensed z = 1.51 active galactic nucleus (AGN) HS 0810+2554 performed with the Chandra X-ray Observatory and XMM-Newton. Blueshifted absorption lines are detected in both observations at rest-frame energies ranging between ∼1 and 12 keV at ≳99% confidence. The inferred velocities of the outflowing components range between ∼0.1c and ∼0.4c. A strong emission line at ∼6.8 keV that is accompanied by a significant absorption line at ∼7.8 keV is also detected in the Chandra observation. The presence of these lines is a characteristic feature of a P-Cygni profile supporting the presence of an expanding, outflowing, highly ionized iron absorber in this quasar. Modeling of the P-Cygni profile constrains the covering factor of the wind to be ≳0.6, assuming disk shielding. A disk-reflection component is detected in the XMM-Newton observation accompanied by blueshifted absorption lines. The XMM-Newton observation constrains the inclination angle to be <45° at 90% confidence, assuming that the hard excess is due to blurred reflection from the accretion disk. The detection of an ultrafast and wide-angle wind in an AGN with intrinsic narrow absorption lines (NALs) would suggest that quasar winds may couple efficiently with the intergalactic medium and provide significant feedback if ubiquitous in all NAL and broad absorption line (BAL) quasars. We estimate the mass-outflow rate of the absorbers to lie in the range of 1.5–3.4 M {sub ⊙} yr{sup −1} for the two observations. We find that the fraction of kinetic to electromagnetic luminosity released by HS 0810+2554 is large (ϵ {sub k} = 9{sub −6}{sup +8}), which suggests that magnetic driving is likely a significant contributor to the acceleration of this outflow.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The kinetic Sunyaev-Zel'dovich effect from reionization: simulated full-sky maps at arcminute resolution

The kinetic Sunyaev–Zel’dovich (kSZ) effect results from Thomson scattering by coherent flows in the reionized intergalactic medium. We present new results based on ray-tracing an 8 Gpc/h realization of reionization with resolution elements 2 Mpc/h (subtending ∼1′ at z = 6) on a side to create a full-sky kSZ map. The realization includes, self-consistently, the effects of reionization on scales corresponding to multipoles 10≲ℓ≲5000. We separate the kSZ map into Doppler (v), Ostriker–Vishniac (δv), patchy (xv), and third-order (xδv) components, and compute explicitly all the auto- and cross-correlations (e.g., 〈vv〉, 〈δvxv〉, etc.) that contribute to the total power. We find a complex and nonmonotonic dependence on the duration of reionization at ℓ∼300 and evidence for a non-negligible (10%–30%) contribution from connected four-point correlations, 〈xvxv〉{sub c}, usually neglected in analytical models. We also investigate the cross-correlation of linear matter and large-scale kSZ temperature fluctuations, focusing on (1) cross-power spectra with biased tracers of the matter density and (2) cold spots from infall onto large, rare H ii regions centered on peaks in the matter distribution at redshifts z>10 that are a generic non-Gaussian feature of patchy reionization. Finally, we show that the reionization history can be reconstructed at 5σ–10σ significance by correlating full-sky 21 cm maps stacked in bins with Δν = 10 MHz with existing cosmic microwave background (CMB) temperature maps at ℓ<500, raising the prospects for probing reionization by correlating CMB and LSS measurements. The resulting kSZ maps have been made publicly available at www.cita.utoronto.ca/∼malvarez/research/ksz-data/.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The He II proximity effect and the lifetime of quasars

The lifetime of quasars is fundamental for understanding the growth of supermassive black holes, and is an important ingredient in models of the reionization of the intergalactic medium (IGM). However, despite various attempts to determine quasar lifetimes, current estimates from a variety of methods are uncertain by orders of magnitude. This work combines cosmological hydrodynamical simulations and 1D radiative transfer to investigate the structure and evolution of the He ii Lyα proximity zones around quasars at z ≃ 3–4. We show that the time evolution in the proximity zone can be described by a simple analytical model for the approach of the He ii fraction x{sub HeII}(t) to ionization equilibrium, and use this picture to illustrate how the transmission profile depends on the quasar lifetime, quasar UV luminosity, and the ionization state of Helium in the ambient IGM (i.e., the average He ii fraction, or equivalently the metagalactic He ii ionizing background). A significant degeneracy exists between the lifetime and the average He ii fraction, however the latter can be determined from measurements of the He ii Lyα optical depth far from quasars, allowing the lifetime to be measured. We advocate stacking existing He ii quasar spectra at z ∼ 3, and show that the shape of this average proximity zone profile is sensitive to lifetimes as long as ∼30 Myr. At higher redshift z ∼ 4 where the He ii fraction is poorly constrained, degeneracies will make it challenging to determine these parameters independently. Our analytical model for He ii proximity zones should also provide a useful description of the properties of H i proximity zones around quasars at z ≃ 6–7.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Hubble imaging of the ionizing radiation from a star-forming galaxy at Z = 3.2 with f{sub esc}>50%

Star-forming galaxies are considered to be the leading candidate sources dominating cosmic reionization at z>7: the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently an active line of research. We have observed a star-forming galaxy at z = 3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730–890 Å rest-frame, and detected LyC emission. This galaxy is very compact and also has a large Oxygen ratio [O III]λ5007/[O II]λ3727 (≳10). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6 Ms X-ray Chandra observations). The measured escape fraction of ionizing radiation spans the range 50%–100%, depending on the intergalactic medium (IGM) attenuation. The LyC emission is measured at m{sub F336W}=27.57±0.11 (with signal-to-noise ratio (S/N) = 10) and is spatially unresolved, with an effective radius of R{sub e}<200 pc. Predictions from photoionization and radiative transfer models are in line with the properties reported here, indicating that stellar winds and supernova explosions in a nucleated star-forming region can blow cavities generating density-bounded conditions compatible to optically thin media. Irrespective of the nature of the ionizing radiation, spectral signatures of these sources over the entire electromagnetic spectrum are of central importance for their identification during the epoch of reionization when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric signature of intense rest-frame optical emissions ([O iii]λλ4959,5007 + Hβ) recently observed at z≃7.5--8.5 is similar to what is observed in this galaxy. Only the James Webb Space Telescope will measure optical line ratios at z>7, allowing a direct comparison with the lower-redshift LyC emitters, such as that reported here.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The evolution of normal galaxy X-ray emission through cosmic history: constraints from the 6 Ms Chandra deep field-south

We present measurements of the evolution of normal-galaxy X-ray emission from z ≈ 0–7 using local galaxies and galaxy samples in the ≈6 Ms Chandra Deep Field-South (CDF-S) survey. The majority of the CDF-S galaxies are observed at rest-frame energies above 2 keV, where the emission is expected to be dominated by X-ray binary (XRB) populations; however, hot gas is expected to provide small contributions to the observed-frame ≲1 keV emission at z ≲ 1. We show that a single scaling relation between X-ray luminosity (L{sub X}) and star-formation rate (SFR) literature, is insufficient for characterizing the average X-ray emission at all redshifts. We establish that scaling relations involving not only SFR, but also stellar mass (M{sub ⋆}) and redshift, provide significantly improved characterizations of the average X-ray emission from normal galaxy populations at z ≈ 0–7. We further provide the first empirical constraints on the redshift evolution of X-ray emission from both low-mass XRB (LMXB) and high-mass XRB (HMXB) populations and their scalings with M{sub ⋆} and SFR, respectively. We find L{sub 2−10keV}(LMXB)/M{sub ⋆}∝(1+z){sup 2−3} and L{sub 2−10keV}(HMXB)/SFR ∝ (1+z), and show that these relations are consistent with XRB population-synthesis model predictions, which attribute the increase in LMXB and HMXB scaling relations with redshift as being due to declining host galaxy stellar ages and metallicities, respectively. We discuss how emission from XRBs could provide an important source of heating to the intergalactic medium in the early universe, exceeding that of active galactic nuclei.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Early and extended helium reionization over more than 600 million years of cosmic time

We measure the effective optical depth of He ii Lyα absorption τ{sub eff,HeII} at 2.3 3 intergalactic medium (IGM) remain transmissive (τ{sub eff,HeII}<4), and the gradual trend with redshift appears consistent with density evolution of a fully reionized IGM. These modest optical depths imply average He ii fractions of x{sub HeII}<0.01 and He ii ionizing photon mean free paths of ≃50 comoving Mpc at z≃3.4, thus requiring that a substantial volume of the helium in the universe was already doubly ionized at early times; this stands in conflict with current models of He ii reionization driven by luminous quasars. Along 10 sightlines we measure the coeval H i Lyα effective optical depths, allowing us to study the density dependence of τ{sub eff,HeII} at z∼3. We establish that the dependence of τ{sub eff,HeII} on increasing τ{sub eff,HI} is significantly shallower than expected from simple models of an IGM reionized in He ii. This requires higher He ii photoionization rates in overdense regions or underdense regions being not in photoionization equilibrium. Moreover, there are very large fluctuations in τ{sub eff,HeII} at all τ{sub eff,HI} which greatly exceed the expectations from these simple models. These data present a distinct challenge to scenarios of He ii reionization—an IGM where He ii appears to be predominantly ionized at z≃3.4, and with a radiation field strength that may be correlated with the density field, but exhibits large fluctuations at all densities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Keck/mosfire spectroscopy of z = 7-8 galaxies: Lyα emission from a galaxy at z = 7.66

We report the results from some of the deepest Keck/Multi-Object Spectrometer For Infra-Red Exploration data yet obtained for candidate z ≳ 7 galaxies. Our data show one significant line detection with 6.5σ significance in our combined 10 hr of integration which is independently detected on more than one night, thus ruling out the possibility that the detection is spurious. The asymmetric line profile and non-detection in the optical bands strongly imply that the detected line is Lyα emission from a galaxy at z(Lyα) = 7.6637 ± 0.0011, making it the fourth spectroscopically confirmed galaxy via Lyα at z > 7.5. This galaxy is bright in the rest-frame ultraviolet (UV; M {sub UV} ∼ −21.2) with a moderately blue UV slope (β=−2.2{sub −0.2}{sup +0.3}), and exhibits a rest-frame Lyα equivalent width of EW(Lyα) ∼ 15.6{sub −3.6}{sup +5.9} Å. The non-detection of the 11 other z ∼ 7–8 galaxies in our long 10 hr integration, reaching a median 5σ sensitivity of 28 Å in the rest-frame EW(Lyα), implies a 1.3σ deviation from the null hypothesis of a non-evolving distribution in the rest-frame EW(Lyα) between 3 < z < 6 and z = 7–8. Our results are consistent with previous studies finding a decline in Lyα emission at z > 6.5, which may signal the evolving neutral fraction in the intergalactic medium at the end of the reionization epoch, although our weak evidence suggests the need for a larger statistical sample to allow for a more robust conclusion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

REDUNDANT ARRAY CONFIGURATIONS FOR 21 cm COSMOLOGY

Realizing the potential of 21 cm tomography to statistically probe the intergalactic medium before and during the Epoch of Reionization requires large telescopes and precise control of systematics. Next-generation telescopes are now being designed and built to meet these challenges, drawing lessons from first-generation experiments that showed the benefits of densely packed, highly redundant arrays—in which the same mode on the sky is sampled by many antenna pairs—for achieving high sensitivity, precise calibration, and robust foreground mitigation. In this work, we focus on the Hydrogen Epoch of Reionization Array (HERA) as an interferometer with a dense, redundant core designed following these lessons to be optimized for 21 cm cosmology. We show how modestly supplementing or modifying a compact design like HERA’s can still deliver high sensitivity while enhancing strategies for calibration and foreground mitigation. In particular, we compare the imaging capability of several array configurations, both instantaneously (to address instrumental and ionospheric effects) and with rotation synthesis (for foreground removal). We also examine the effects that configuration has on calibratability using instantaneous redundancy. We find that improved imaging with sub-aperture sampling via “off-grid” antennas and increased angular resolution via far-flung “outrigger” antennas is possible with a redundantly calibratable array configuration.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Helium reionization simulations. I. Modeling quasars as radiation sources

We introduce a new project to understand helium reionization using fully coupled N-body, hydrodynamics, and radiative transfer simulations. This project aims to capture correctly the thermal history of the intergalactic medium as a result of reionization and make predictions about the Lyα forest and baryon temperature–density relation. The dominant sources of radiation for this transition are quasars, so modeling the source population accurately is very important for making reliable predictions. In this first paper, we present a new method for populating dark matter halos with quasars. Our set of quasar models includes two different light curves, a lightbulb (simple on/off) and symmetric exponential model, and luminosity-dependent quasar lifetimes. Our method self-consistently reproduces an input quasar luminosity function given a halo catalog from an N-body simulation, and propagates quasars through the merger history of halo hosts. After calibrating quasar clustering using measurements from the Baryon Oscillation Spectroscopic Survey, we find that the characteristic mass of quasar hosts is M{sub h}∼2.5×10{sup 12} h{sup −1} M{sub ⊙} for the lightbulb model, and M{sub h}∼2.3×10{sup 12} h{sup −1} M{sub ⊙} for the exponential model. In the latter model, the peak quasar luminosity for a given halo mass is larger than that in the former, typically by a factor of 1.5–2. The effective lifetime for quasars in the lightbulb model is 59 Myr, and in the exponential case, the effective time constant is about 15 Myr. We include semi-analytic calculations of helium reionization, and discuss how to include these quasars as sources of ionizing radiation for full hydrodynamics with radiative transfer simulations in order to study helium reionization.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE BARYON CYCLE AT HIGH REDSHIFTS: EFFECTS OF GALACTIC WINDS ON GALAXY EVOLUTION IN OVERDENSE AND AVERAGE REGIONS

We employ high-resolution cosmological zoom-in simulations focusing on a high-sigma peak and an average cosmological field at z ∼ 6–12 in order to investigate the influence of environment and baryonic feedback on galaxy evolution in the reionization epoch. Strong feedback, e.g., galactic winds, caused by elevated star formation rates (SFRs) is expected to play an important role in this evolution. We compare different outflow prescriptions: (i) constant wind velocity (CW), (ii) variable wind scaling with galaxy properties (VW), and (iii) no outflows (NW). The overdensity leads to accelerated evolution of dark matter and baryonic structures, absent from the “normal” region, and to shallow galaxy stellar mass functions at the low-mass end. Although CW shows little dependence on the environment, the more physically motivated VW model does exhibit this effect. In addition, VW can reproduce the observed specific SFR (sSFR) and the sSFR–stellar mass relation, which CW and NW fail to satisfy simultaneously. Winds also differ substantially in affecting the state of the intergalactic medium (IGM). The difference lies in the volume-filling factor of hot, high-metallicity gas, which is near unity for CW, while such gas remains confined in massive filaments for VW, and locked up in galaxies for NW. Such gas is nearly absent from the normal region. Although all wind models suffer from deficiencies, the VW model seems to be promising in correlating the outflow properties with those of host galaxies. Further constraints on the state of the IGM at high z are needed to separate different wind models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Compressible flow in front of an axisymmetric blunt object: analytic approximation and astrophysical implications

Compressible flows around blunt objects have diverse applications, but current analytic treatments are inaccurate and limited to narrow parameter regimes. We show that the gas-dynamic flow in front of an axisymmetric blunt body is accurately derived analytically using a low order expansion of the perpendicular gradients in terms of the parallel velocity. This reproduces both subsonic and supersonic flows measured and simulated for a sphere, including the transonic regime and the bow shock properties. Some astrophysical implications are outlined, in particular for planets in the solar wind and for clumps and bubbles in the intergalactic medium. The bow shock standoff distance normalized by the obstacle curvature is ∼2/(3g) in the strong shock limit, where g is the compression ratio. For a subsonic Mach number M approaching unity, the thickness δ of an initially weak, draped magnetic layer is a few times larger than in the incompressible limit, with amplification ∼(1+1.3M{sup 2.6})/(3δ).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The molecular baryon cycle of M82

Baryons cycle into galaxies from the intergalactic medium and are converted into stars; a fraction of the baryons are ejected out of galaxies by stellar feedback. Here we present new high-resolution (3.″9; 68 pc) {sup 12}CO(2–1) and {sup 12}CO(3–2) images that probe these three stages of the baryon cycle in the nearby starburst M82. We combine these new observations with previous {sup 12}CO(1–0) and [Fe ii] images to study the physical conditions within the molecular gas. Using a Bayesian analysis and the radiative transfer code RADEX, we model temperatures and densities of molecular hydrogen, as well as column densities of CO. Besides the disk, we concentrate on two regions within the galaxy: an expanding super-bubble and the base of a molecular streamer. Shock diagnostics, kinematics, and optical extinction suggest that the streamer is an inflowing filament, with a mass inflow rate of molecular gas of 3.5 M{sub ⊙} yr{sup −1}. We measure the mass outflow rate of molecular gas of the expanding super-bubble to be 17 M{sub ⊙} yr{sup −1}, five times higher than the inferred inflow rate and 1.3 times the star formation rate of the galaxy. The high mass outflow rate and large star formation rate will deplete the galaxy of molecular gas within eight million years, unless there are additional sources of molecular gas.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Contribution of neutron star mergers to the r-process chemical evolution in the hierarchical galaxy formation

The main astronomical source of r-process elements has not yet been identified. One plausible site is neutron star mergers (NSMs), but from the perspective of the Galactic chemical evolution, it has been pointed out that NSMs cannot reproduce the observed r-process abundance distribution of metal-poor stars at [Fe/H]<−3. Recently, Tsujimoto and Shigeyama pointed out that NSM ejecta can spread into a much larger volume than ejecta from a supernova. We re-examine the enrichment of r-process elements by NSMs considering this difference in propagation using the chemical evolution model under the hierarchical galaxy formation. The observed r-process enhanced stars around [Fe/H]∼−3 are reproduced if the star formation efficiency is lower for low-mass galaxies under a realistic delay-time distribution for NSMs. We show that a significant fraction of NSM ejecta escape from its host proto-galaxy to pollute intergalactic matter and other proto-galaxies. The propagation of r-process elements over proto-galaxies changes the abundance distribution at [Fe/H]<−3 and obtains distribution compatible with observations of the Milky Way halo stars. In particular, the pre-enrichment of intergalactic medium explains the observed scarcity of extremely metal-poor stars without Ba and abundance distribution of r-process elements at [Fe/H]≲−3.5.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Revisiting the completeness and luminosity function in high-redshift low-luminosity quasar surveys

Recent studies have derived quasar luminosity functions (QLFs) at various redshifts. However, the faint side of the QLF at high redshifts is still too uncertain. An accurate estimate of the survey completeness is essential to derive an accurate QLF for use in studying the luminosity-dependent density evolution (LDDE) of the quasar population. Here we investigate how the luminosity dependence of quasar spectra (the Baldwin effect) and the attenuation model for the intergalactic medium (IGM) affect the completeness estimates. For this purpose, we specifically revisit the completeness of quasar surveys at z∼4--5, using the COSMOS images observed with Subaru/Suprime-Cam. As a result, we find that the completeness estimates are sensitive to the luminosity dependence of the quasar spectrum and to different the IGM attenuation models. At z∼4, the number density of quasars when we adopt the latest IGM model and take the luminosity dependence of spectra into account is (3.49±1.62)×10{sup −7} Mpc{sup −3} mag{sup −1} for −24.09<M{sub 1450}<−23.09 and (5.24±2.13)×10{sup −7} Mpc{sup −3} mag{sup −1} for −23.09<M{sub 1450}<−22.09, which are ∼24% lower than that estimated by the conventional method. On the other hand, at z∼5, the 1σ confidence upper limit of the number density at −24.5<M{sub 1450}<−22.5 in our new estimates is ∼43% higher than that estimated previously. The results suggest that the luminosity dependence of the quasar spectrum and the IGM model are important for deriving the accurate number density of high-z quasars. Even taking these effects into account, the inferred LDDE of quasars is consistent with the AGN downsizing evolutionary picture.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Smooth H i Low Column Density Outskirts in Nearby Galaxies

The low column density gas at the outskirts of galaxies as traced by the 21 cm hydrogen line emission (H i) represents the interface between galaxies and the intergalactic medium, i.e., where galaxies are believed to get their supply of gas to fuel future episodes of star formation. Photoionization models predict a break in the radial profiles of H i at a column density of ∼5 × 10{sup 19} cm{sup −2} due to the lack of self-shielding against extragalactic ionizing photons. To investigate the prevalence of such breaks in galactic disks and to characterize what determines the potential edge of the H i disks, we study the azimuthally averaged H i column density profiles of 17 nearby galaxies from the H i Nearby Galaxy Survey and supplemented in two cases with published Hydrogen Accretion in LOcal GAlaxieS data. To detect potential faint H i emission that would otherwise be undetected using conventional moment map analysis, we line up individual profiles to the same reference velocity and average them azimuthally to derive stacked radial profiles. To do so, we use model velocity fields created from a simple extrapolation of the rotation curves to align the profiles in velocity at radii beyond the extent probed with the sensitivity of traditional integrated H i maps. With this method, we improve our sensitivity to outer-disk H i emission by up to an order of magnitude. Except for a few disturbed galaxies, none show evidence of a sudden change in the slope of the H i radial profiles: the alleged signature of ionization by the extragalactic background.

79 ASTRONOMY AND ASTROPHYSICS↗