31 research outputs found
The transverse proximity effect in quasar spectra
The intergalactic medium is kept highly photoionised by the intergalactic UV background radiation field generated by the overall population of quasars and galaxies. In the vicinity of sources of UV photons, such as luminous high-redshift quasars, the UV radiation field is enhanced due to the local source contribution. The higher degree of ionisation is visible as a reduced line density or generally as a decreased level of absorption in the Lyman alpha forest of neutral hydrogen. This so-called proximity effect has been detected with high statistical significance towards luminous quasars. If quasars radiate rather isotropically, background quasar sightlines located near foreground quasars should show a region of decreased Lyman alpha absorption close to the foreground quasar. Despite considerable effort, such a transverse proximity effect has only been detected in a few cases...thesi
Evolution of the AGN UV luminosity function from redshift 7.5
Determinations of the UV luminosity function of 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 AGN data. We address this problem by constructing a
sample of more than 80,000 colour-selected AGN from redshift z=0 to 7.5. While
this sample is composed of multiple data sets with spectroscopic redshifts and
completeness estimates, we homogenise these data sets 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 of the
AGN luminosity function shows a steep brightening from at z=0.7
to at z=6. The faint-end slope significantly steepens
from at to at . In spite of this steepening,
the contribution of AGN to the hydrogen photoionization rate at is
subdominant (< 3%), although it can be non-negligible (~10%) if these
luminosity functions hold down to . Under reasonable assumptions,
AGN can reionize HeII by redshift z=2.9. At low redshifts (z<0.5), AGN can
produce about half of the hydrogen photoionization rate inferred from the
statistics of HI absorption lines in the IGM. Our global analysis of the
luminosity function also reveals important systematic errors in the data,
particularly at z=2.2--3.5, 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, luminosity function analysis codes, and homogenised
AGN data publicly available.Comment: 30 pages, 15 figures; accepted in MNRAS; code, data, and various fits
at https://github.com/gkulkarni/QL
Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration
The HeII transverse proximity effect - enhanced HeII Ly{\alpha} transmission
in a background sightline caused by the ionizing radiation of a foreground
quasar - offers a unique opportunity to probe the emission properties of
quasars, in particular the emission geometry (obscuration, beaming) and the
quasar lifetime. Building on the foreground quasar survey published in
Schmidt+2017, we present a detailed model of the HeII transverse proximity
effect, specifically designed to include light travel time effects, finite
quasar ages, and quasar obscuration. We post-process outputs from a
cosmological hydrodynamical simulation with a fluctuating HeII UV background
model, plus the added effect of the radiation from a single bright foreground
quasar. We vary the age and obscured sky fractions
of the foreground quasar, and explore the resulting
effect on the HeII transverse proximity effect signal. Fluctuations in IGM
density and the UV background, as well as the unknown orientation of the
foreground quasar, result in a large variance of the HeII Ly{\alpha}
transmission along the background sightline. We develop a fully Bayesian
statistical formalism to compare far UV HeII Ly{\alpha} transmission spectra of
the background quasars to our models, and extract joint constraints on
and for the six Schmidt+2017 foreground
quasars with the highest implied HeII photoionization rates. Our analysis
suggests a bimodal distribution of quasar emission properties, whereby one
foreground quasar, associated with a strong HeII transmission spike, is
relatively old and unobscured ,
whereas three others are either younger than or highly
obscured .Comment: 19 pages, 6 figures, submitted to Ap
A Refined Measurement of the Mean Transmitted Flux in the Ly-alpha Forest over 2 < z < 5 Using Composite Quasar Spectra
We present new measurements of the mean transmitted flux in the Ly-alpha
forest over 2 < z < 5 made using 6065 quasar spectra from the Sloan Digital Sky
Survey DR7. We exploit the general lack of evolution in the mean quasar
continuum to avoid the bias introduced by continuum fitting over the Ly-alpha
forest at high redshifts, which has been the primary systematic uncertainty in
previous measurements of the mean Ly-alpha transmission. The individual spectra
are first combined into twenty-six composites with mean redshifts spanning 2.25
< z_comp < 5.08. The flux ratios of separate composites at the same rest
wavelength are then used, without continuum fitting, to infer the mean
transmitted flux, F(z), as a fraction of its value at z~2. Absolute values for
F(z) are found by scaling our relative values to measurements made from
high-resolution data by Faucher-Giguere et al. (2008) at z < 2.5, where
continuum uncertainties are minimal. We find that F(z) evolves smoothly with
redshift, with no evidence of a previously reported feature at z~3.2. This
trend is consistent with a gradual evolution of the ionization and thermal
state of the intergalactic medium over 2 < z < 5. Our results generally agree
with the most careful measurements to date made from high-resolution data, but
offer much greater precision and extend to higher redshifts. This work also
improves upon previous efforts using SDSS spectra by significantly reducing the
level of systematic error.Comment: 14 pages, 11 figures, MNRAS, in press. Supplementary materials may be
downloaded from http://www.ast.cam.ac.uk/~gdb/mean_flu
The Evolution of O i over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium
We present a survey for metal absorption systems traced by neutral oxygen over 3.2 0.05 Å, of which there are 49 nonproximate systems in our sample. We find that the number density does not monotonically increase with decreasing redshift, as would naively be expected from the buildup of metal-enriched circumgalactic gas with time. The number density over 4.9 < z < 5.7 is a factor of 1.7–4.1 lower (68% confidence) than that over 5.7 < z < 6.5, with a lower value at z < 5.7 favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at z ~ 6 than at lower redshifts. Absorption from highly ionized metals traced by C iv is also weaker in higher-redshift O i systems, supporting this picture. The evolution of O i absorbers implies that metal-enriched circumgalactic gas at z ~ 6 is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch
The Lyman α forest power spectrum from the XQ-100 legacy survey
We present the Lyman α flux power spectrum measurements of the XQ-100 sample of quasar spectra obtained in the context of the European Southern Observatory Large Programme ‘Quasars and their absorption lines: a legacy survey of the high redshift universe with VLT/XSHOOTER’. Using 100 quasar spectra with medium resolution and signal-to-noise ratio, we measure the power spectrum over a range of redshifts z = 3–4.2 and over a range of scales k = 0.003–0.06 km−1 s. The results agree well with the measurements of the one-dimensional power spectrum found in the literature. The data analysis used in this paper is based on the Fourier transform and has been tested on synthetic data. Systematic and statistical uncertainties of our measurements are estimated, with a total error (statistical and systematic) comparable to the one of the BOSS data in the overlapping range of scales, and smaller by more than 50 per cent for higher redshift bins (z > 3.6) and small scales (k > 0.01 km−1 s). The XQ-100 data set has the unique feature of having signal-to-noise ratios and resolution intermediate between the two data sets that are typically used to perform cosmological studies, i.e. BOSS and high-resolution spectra (e.g. UVES/VLT or HIRES). More importantly, the measured flux power spectra span the high-redshift regime that is usually more constraining for structure formation models
Reionization with star-forming galaxies: insights from the Low-z Lyman Continuum Survey
The fraction of ionizing photons escaping from galaxies, , is at the
same time a crucial parameter in modelling reionization and a very poorly known
quantity, especially at high redshift. Recent observations are starting to
constrain the values of in low-z star-forming galaxies, but the
validity of this comparison remains to be verified. Applying at high-z the
empirical relation between and the UV slope trends derived from the
Low-z Lyman Continuum Survey, we use the DELPHI semi-analytical galaxy
formation model to estimate the global ionizing emissivity of high-z galaxies,
which we use to compute the resulting reionization history. We find that both
the global ionizing emissivity and reionization history match the observational
constraints. Assuming that the low-z correlations hold during the epoch of
reionization, we find that galaxies with
are the main drivers of reionization. We derive a population-averaged at z=4.5, 6, 8.Comment: 5+1 page, 3 figures, submitted to A&