276 research outputs found
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
Self-Consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations
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 z ~ 15 than they should. This problem
arises because at z > 6, 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 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 Lyman-alpha 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.Comment: 28 pages, 15 figures, 5 tables, matches version accepted in Ap
Joint Bayesian Estimation of Quasar Continua and the Lyman-Alpha Forest Flux Probability Distribution Function
We present a new Bayesian algorithm making use of Markov Chain Monte Carlo
sampling that allows us to simultaneously estimate the unknown continuum level
of each quasar in an ensemble of high-resolution spectra, as well as their
common probability distribution function (PDF) for the transmitted Ly
forest flux. This fully automated PDF regulated continuum fitting method models
the unknown quasar continuum with a linear Principal Component Analysis (PCA)
basis, with the PCA coefficients treated as nuisance parameters. The method
allows one to estimate parameters governing the thermal state of the
intergalactic medium (IGM), such as the slope of the temperature-density
relation , while marginalizing out continuum uncertainties in a fully
Bayesian way. Using realistic mock quasar spectra created from a simplified
semi-numerical model of the IGM, we show that this method recovers the
underlying quasar continua to a precision of and at
and , respectively. Given the number of principal component spectra,
this is comparable to the underlying accuracy of the PCA model itself. Most
importantly, we show that we can achieve a nearly unbiased estimate of the
slope of the IGM temperature-density relation with a precision of
at , at , for an ensemble of ten mock
high-resolution quasar spectra. Applying this method to real quasar spectra and
comparing to a more realistic IGM model from hydrodynamical simulations would
enable precise measurements of the thermal and cosmological parameters
governing the IGM, albeit with somewhat larger uncertainties given the
increased flexibility of the model.Comment: 21 pages (+ Appendix), accepted at Ap
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