ABSTRACT
Determinations of the ultraviolet (UV) luminosity function of active galactic nuclei (AGN) at high redshifts are important for constraining the AGN contribution to reionization and ...understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from inconsistencies arising from inhomogeneous selection and analysis of data. We address this problem by constructing a sample of more than 80 000 colour-selected AGN from redshift $z$ = 0 to 7.5 using multiple data sets homogenized to identical cosmologies, intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the AGN UV luminosity function from redshift $z$ = 0 to 7.5. The luminosity function has a double power-law form at all redshifts. The break magnitude M* shows a steep brightening from M* ∼ −24 at $z$ = 0.7 to M* ∼ −29 at $z$ = 6. The faint-end slope β significantly steepens from −1.9 at $z$ < 2.2 to −2.4 at $z$ ≃ 6. In spite of this steepening, the contribution of AGN to the hydrogen photoionization rate at $z$ ∼ 6 is subdominant (<3 per cent), although it can be non-negligible (∼10 per cent) if these luminosity functions hold down to M1450 = −18. Under reasonable assumptions, AGN can reionize He ii by redshift $z$ = 2.9. At low redshifts ($z$ < 0.5), AGN can produce about half of the hydrogen photoionization rate inferred from the statistics of H i absorption lines in the intergalactic medium. Our analysis also reveals important systematic errors in the data, which need to be addressed and incorporated in the AGN selection function in future in order to improve our results. We make various fitting functions, codes, and data publicly available.
We publicly release a new sample of 34 medium resolution quasar spectra at 5.77 ≤ zem ≤ 6.54 observed with the Echellette Spectrograph and Imager on the Keck telescope. This quasar sample represents ...an ideal laboratory to study the intergalactic medium (IGM) during the end stages of the epoch of reionization, and constrain the timing and morphology of the phase transition. For a subset of 23 of our highest signal-to-noise ratio spectra (S/N > 7, per 10 km s−1 pixel), we present a new measurement of the Ly forest opacity spanning the redshift range 4.8 z 6.3. We carefully eliminate spectral regions that could be causing biases in our measurements due to additional transmitted flux in the proximity zone of the quasars, or extra absorption caused by strong intervening absorption systems along the line of sight. We compare the observed evolution of the IGM opacity with redshift to predictions from a hydrodynamical simulation with uniform ultraviolet background (UVB) radiation, as well as two semi-numerical patchy reionization models, one with a fluctuating UVB and another with a fluctuating temperature field. Our measurements show a steep rise in opacity at z 5.0 and an increased scatter and thus support the picture of a spatially inhomogeneous reionization process, consistent with previous work. However, we measure significantly higher optical depths at 5.3 z 5.7 than previous studies, which reduces the contrast between the highest opacity Gunn-Peterson troughs and the average opacity trend of the IGM, which may relieve some of the previously noted tension between these measurements and reionization models.
Abstract
We determine the thermal evolution of the intergalactic medium (IGM) over 3 Gyr of cosmic time
by comparing measurements of the Ly
α
forest power spectrum to a suite of ∼70 hydrodynamical ...simulations. We conduct Bayesian inference of IGM thermal parameters using an end-to-end forward modeling framework whereby mock spectra generated from our simulation grid are used to build a custom emulator that interpolates the power spectrum between thermal grid points. The temperature at mean density
T
0
rises steadily from
at
z
= 5.4, peaks at 14,000 K for
z
∼ 3.4, and decreases at lower redshift, reaching
T
0
∼ 7000 K by
z
∼ 1.8. This evolution provides conclusive evidence for photoionization heating resulting from the reionization of
, as well as the subsequent cooling of the IGM due to the expansion of the universe after all reionization events are complete. Our results are broadly consistent with previous measurements of thermal evolution based on a variety of approaches, but the sensitivity of the power spectrum, the combination of high-precision measurements of large-scale modes (
) from the Baryon Oscillation Spectroscopic Survey with our recent determination of the small-scale power, our large grid of models, and our careful statistical analysis allow us to break the well-known degeneracy between the temperature at mean density
T
0
and the slope of the temperature–density relation
γ
that has plagued previous analyses. At the highest redshifts,
z
≥ 5, we infer lower temperatures than expected from the standard picture of IGM thermal evolution leaving little room for additional smoothing of the Ly
α
forest by free streaming of warm dark matter.
The ultraviolet background (UVB) emitted by quasars and galaxies governs the ionization and thermal state of the intergalactic medium (IGM), regulates the formation of high-redshift galaxies, and is ...thus a key quantity for modeling cosmic reionization. The vast majority of cosmological hydrodynamical simulations implement the UVB via a set of spatially uniform photoionization and photoheating rates derived from UVB synthesis models. We show that simulations using canonical UVB rates reionize and, perhaps more importantly, spuriously heat the IGM, much earlier ( ) than they should. This problem arises because at , where observational constraints are nonexistent, the UVB amplitude is far too high. We introduce a new methodology to remedy this issue, and we generate self-consistent photoionization and photoheating rates to model any chosen reionization history. Following this approach, we run a suite of hydrodynamical simulations of different reionization scenarios and explore the impact of the timing of reionization and its concomitant heat injection on the thermal state of the IGM. We present a comprehensive study of the pressure smoothing scale of IGM gas, illustrating its dependence on the details of both hydrogen and helium reionization, and argue that it plays a fundamental role in interpreting Ly forest statistics and the thermal evolution of the IGM. The premature IGM heating we have uncovered implies that previous work has likely dramatically overestimated the impact of photoionization feedback on galaxy formation, which sets the minimum halo mass able to form stars at high redshifts. We make our new UVB photoionization and photoheating rates publicly available for use in future simulations.
The supermassive black holes (SMBHs) observed at the centers of all massive galaxies are believed to have grown via luminous accretion during quasar phases in the distant past. The fraction of ...inflowing rest mass energy emitted as light, the radiative efficiency, has been inferred to be 10%, in agreement with expectations from thin disk accretion models. But the existence of billion solar-mass SMBHs powering quasars at z > 7 challenges this picture: provided they respect the Eddington limit, there is not enough time to grow z > 7 SMBHs from stellar remnant seeds unless the radiative efficiency is below 10%. Here we show that one can constrain the radiative efficiencies of the most distant quasars known using foreground neutral intergalactic gas as a cosmological-scale ionizing photon counter. From the Ly absorption profiles of ULAS J1120+0641 (z = 7.09) and ULAS J1342+0928 (z = 7.54), we determine posterior median radiative efficiencies of 0.08% and 0.1%, respectively, and the combination of the two measurements rules out the canonical 10% efficiency at 99.8% credibility after marginalizing over the unknown obscured fraction. This low radiative efficiency implies rapid mass accretion for the earliest SMBHs, greatly easing the tension between the age of the universe and the SMBH masses. However, our measured efficiency may instead reflect nearly complete obscuration by dusty gas in the quasar host galaxies over the vast majority of their SMBH growth. Assuming 10% efficiency during unobscured phases, we find that the obscured fraction would be >82% at 95% credibility, and imply a times larger obscured than unobscured luminous quasar population at z > 7.
We measure the effective optical depth of He II Lyalpha absorption tau sub(eff,HeII) at 2.3 < z< 3.5 in 17 UV-transmitting quasars observed with UV spectrographs on the Hubble Space Telescope. The ...median tau sub(eff,HeII) values increase gradually from 1.95 at z= 2.7 to 5.17 at z= 3.4, but with a strong sightline-to-sightline variance. Many Asymptotically = to35 comoving Mpc regions of the z> 3 intergalactic medium (IGM) remain transmissive (tau 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(eff,HeII)< 0.01 and He II ionizing photon mean free paths of Asymptotically = to50 comoving Mpc at zAsymptotically = to 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 Lyalpha effective optical depths, allowing us to study the density dependence of tau sub(eff,HeII) at z~ 3. We establish that the dependence of tau sub(eff,HeII) on increasing tau sub(eff,HeI) 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 tau sub(eff,HeII) at all tau sub(eff,HeI) 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 zAsymptotically = to 3.4, and with a radiation field strength that may be correlated with the density field, but exhibits large fluctuations at all densities.
First Spectroscopic Study of a Young Quasar Eilers, Anna-Christina; Hennawi, Joseph F.; Davies, Frederick B.
The Astrophysical journal,
11/2018, Letnik:
867, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The quasar lifetime tQ is one of the most fundamental quantities for understanding quasar evolution and the growth of supermassive black holes (SMBHs), but remains uncertain by several orders of ...magnitude. In a recent study we uncovered a population of very young quasars (tQ 104-105 yr), based on the sizes of their proximity zones, which are regions of enhanced Ly forest transmission near the quasar resulting from its own ionizing radiation. The presence of such young objects poses significant challenges to models of SMBH formation, which already struggle to explain the existence of SMBHs (∼109 M ) at such early cosmic epochs. We conduct the first comprehensive spectroscopic study of the youngest quasar known, SDSS J1335+3533 at z = 5.9012, whose lifetime is tQ = 103.0 0.8 yr (95% confidence). A careful search of our deep optical and near-infrared spectra for H i and metal absorption lines allows us to convincingly exclude the possibility that its small proximity zone results from an associated absorption system rather than a short lifetime. From the Mg ii emission line we measure a black hole mass , implying an Eddington ratio of -comparable to other co-eval quasars of similar luminosity. The only possible anomaly associated with SDSS J1335+3533's youth are its weak emission lines, but larger samples are needed to shed light on the potential causality. We discuss the implications of short lifetimes for various SMBH growth and formation scenarios, and argue that future observations of young quasars with the James Webb Space Telescope could distinguish between them.
All galaxies once passed through a hyperluminous quasar phase powered by accretion onto a supermassive black hole. But because these episodes are brief, quasars are rare objects typically separated ...by cosmological distances. In a survey for Lyman-α emission at redshift z ≈ 2, we discovered a physical association of four quasars embedded in a giant nebula. Located within a substantial overdensity of galaxies, this system is probably the progenitor of a massive galaxy cluster. The chance probability of finding a quadruple quasar is estimated to be ∼10–7, implying a physical connection between Lyman-α nebulae and the locations of rare protoclusters. Our findings imply that the most massive structures in the distant universe have a tremendous supply (≃1011 solar masses) of cool dense (volume density ≃ 1 cm–3) gas, which is in conflict with current cosmological simulations.
ABSTRACT We develop a new method to constrain the physical conditions in the cool (∼104 K) circumgalactic medium (CGM) from measurements of ionic column densities by assuming that the cool CGM spans ...a large range of gas densities and that small high-density clouds are hierarchically embedded in large low-density clouds. The new method combines the information available from different sightlines during the photoionization modeling, thus yielding tighter constraints on CGM properties compared to traditional methods that model each sightline individually. Applying this new technique to the COS Halos survey of low-redshift ∼L* galaxies, we find that we can reproduce all observed ion columns in all 44 galaxies in the sample, from the low ions to , with a single universal density structure for the cool CGM. The gas densities span the range ( is the cosmic mean), while the physical size of individual clouds scales as ∼ −1, from 35 kpc for the low-density clouds to 6 pc for the highest-density low-ion clouds. The deduced cloud sizes are too small for this density structure to be driven by self-gravity; thus, its physical origin is unclear. The implied cool CGM mass within the virial radius is (1.3 0.4) × 1010 (∼1% of the halo mass), distributed rather uniformly over the 4 decades in density. The mean cool gas density profile scales as , where R is the distance from the galaxy center. We construct a 3D model of the cool CGM based on our results, which we argue provides a benchmark for the CGM structure in hydrodynamic simulations. Our results can be tested by measuring the coherence scales of different ions.
ABSTRACT We characterize the physical properties of the cool T ∼ 104 K circumgalactic medium (CGM) surrounding z ∼ 2-3 quasar host galaxies, which are predicted to evolve into present-day massive ...ellipticals. Using a statistical sample of 14 quasar pairs with projected separation <300 kpc and spectra of high dispersion and high signal-to-noise ratio, we find extreme kinematics with low metal ion lines typically spanning 500 km s−1, exceeding any previously studied galactic population. The CGM is significantly enriched, even beyond the virial radius, with a median metallicity M/H −0.6. The /Fe abundance ratio is enhanced, suggesting that halo gas is primarily enriched by core-collapse supernovae. The projected cool gas mass within the virial radius is estimated to be 1.9 × 1011 M (R /160 kpc)2, accounting for 1/3 of the baryonic budget of the galaxy halo. The ionization state of CGM gas increases with projected distance from the foreground quasars, contrary to expectation if the quasar dominates the ionizing radiation flux. However, we also found peculiarities not exhibited in the CGM of other galaxy populations. In one absorption system, we may be detecting unresolved fluorescent Ly emission, and another system shows strong N v lines. Taken together, these anomalies suggest that transverse sightlines are-at least in some cases-possibly illuminated. We also discovered a peculiar case where detection of the C ii fine-structure line implies an electron density >100 cm−3 and sub-parsec-scale gas clumps.
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