Topic sentences of each paragraph

• Galaxy-scale obscuration in Active Galactic Nuclei Galaxy gas as obscurer: II.
• The ``torus'' obscurer of Active Galactic Nuclei, is poorly understood in terms of its density, substructure and physical mechanisms.
• The vast majority of Active Galactic Nuclei (AGN) are obscured by thick columns of gas and dust.
• Local galaxies exemplify that several scales can contribute to the obscured columns.
• However, the Milky Way and local galaxies are limited in their use as templates for the high-redshift universe, specifically at peak SMBH growth (; e.g., Aird2010).
• This paper is organised as follows: In Section we present our computation of the galaxy-scale obscuration using observational results from Paper I of the obscuring column distribution of galaxies, applied to the AGN population.
• Independently, Section looks into simulated galaxies in hydro-dynamic cosmological simulations.
• Our goal is to predict the fraction of obscured AGN from the obscuration of host galaxy-scale gas alone, i.e.
• We apply this relation to the AGN host galaxy population to estimate host galaxy obscuration.
• We start with the stellar mass function (SMF) of the galaxy population.
• The obscured fraction can then be simply computed by Monte Carlo simulations.
• After inserting the factorised powerlaw relationship of the SARD, the result has the form
• While the absolute probability of finding an AGN is a function of luminosity, the mass distribution is independent of luminosity.
• We adopt an AGN definition of X.
• Finally the obscured AGN fraction is the cumulative distribution, i.e.
• Into the calculation of we propagate the uncertainties from the obscuration relation for Paper I.
• The obscured fraction of the AGN population from putting together the observed relationships are shown in Figure X.
• Firstly, galaxy-scale gas does not provide Compton-thick column densities ().
• We therefore focus on the Compton-thin obscurer and compare our obscuration results to the fraction of Compton-thin AGN with , a common definition of ``obscured'' AGN.
• We now compare to measurements of the obscured fraction of Compton-thin AGN from surveys.
• Note that our results are meaningful for the AGN population -- the obscuration of individual host galaxies is stellar-mass dependent with substantial variations between individual galaxies (see Equation ).
• Our main result is that the host galaxy gas provides a luminosity-independent obscurer, for which we compute covering fractions.
• Our results give, for the first time, constraints on the galaxy-scale obscurer alone.
• We can now discuss the luminosity-dependence of the obscurer.
• Since neither semi-analytic nor hydro-dynamic cosmological simulations can resolve the nuclear obscurer of AGN, we present a sub-grid model for post-processing.
• We assume that a nuclear Compton-thick obscurer covers a fraction of the SMBH, X.
• * We propose that the remaining Compton-thin sky is obscured by galaxy-scale gas as well as a nuclear Compton-thin obscurer according to the formula:
• The luminosity-independent obscuration, is on average .
• As motivated in the above Section , the peak luminosity is in turn a function of mass:
• Here, at the distribution peaks at , which is using the conversion of Marconi2004.
• The ratio of dust re-radiated infrared luminosity to bolometric, illuminating luminosity has been used to measure the irradiated area (obscurer covering factor) of individual AGN.
• Figure illustrates the behaviour of the PuffedTorus model.
• Physical processes giving rise to the luminosity and mass-dependent behaviour can not be discussed rigorously within the scope of this paper.
• We take note of the analytic wind model formalism described in Elitzur2016 and of the radiation-driven fountain model by Wada2015.
• A further, commonly overlooked aspect is the evolution of AGN.
• no accretion, and therefore no AGN detection 89 , nearby gas leading to obscuration and triggering of a faint AGN 10 , either in the onset of a merger or due to secular events, major merger triggering a bright AGN, which immediately clears the vertically extended obscurer but shines for a period of time 1 before fading.
• Such a duty cycle would give rise to the observed obscured fractions (Figure ) while also respecting the luminosity function of AGN.
• We now assess the gas content in simulated galaxies.
• The Evolution and Assembly of Galaxies and their Environment (EAGLE) simulation Schaye2015,Crain2015 reproduces many observed quantities; it reproduces very well the stellar mass function Furlong2015a and size distribution Furlong2015 of galaxies as a function of cosmic time, being tuned to reproduce these at X.
• We also consider Illustris Vogelsberger2014,Vogelsberger2014a, another hydro-dynamic cosmological simulation.
• We first investigate the gas distribution in the reference simulations.
• We apply ray tracing, starting from the most massive black hole particle of each simulated galaxy (subhalo).
• We present the fraction of AGN showing column densities larger than a given value in Figure X.
• We discuss three aspects which affect the results: (1) different sub-grid physics, most notably stronger feedback mechanisms, (2) differences between active and passive galaxies, (3) unresolved substructure of the gas.
• The strength of EAGLE is that we can explore how variations of the physics affect the results.
• Star formation-related feedback (supernovae, stellar winds, radiation pressure, cosmic rays) was altered in the WeakFB and StrongFB models.
• Next we discuss the effects of feedback from accreting SMBHs.
• AGN outflows or radiation pressure may decrease the covering fractions momentarily.
• Clumpy ISM may decrease the covering fractions.
• To summarise, our obscured fraction diagnostic is a highly sensitive test of feedback recipes.
• Using only observational relations, we predict the covering fractions of galaxy-scale gas as relevant for the AGN population.
• Galaxy-scale gas does not provide Compton-thick lines of sight.
• We therefore conclude that heavily obscured AGN are associated with nuclear obscuration, and propose the value as a demarcation line singling out the nuclear obscurer
• We subtracted the galaxy-scale obscuration and concluded regarding the remaining nuclear obscurer, that
• a nuclear Compton-thick obscurer with covering is necessary.
• The result is formalised into a semi-analytic model for cosmological simulations, called PuffedTorus (Section ).
• We also investigated the inside of simulated galaxies from state-of-the-art hydro-dynamic, cosmological simulations and apply ray-tracing from their black holes.
• JB thanks Antonis Georgakakis and Dave Alexander for insightful conversations.
• Important constraints on how much gas resides in galaxies can be drawn from cosmological simulations.
• We present a simple calculation to show that Compton-thick column densities, i.e
• * For example, a 1kpc ray in a region of metal gas density results in a measured column density of X
• The gas inside a galaxy may be arranged in a multitude of ways to achieve a covering with column density X.
• Converting to metals using the factor and expressing in conventional units, this limit is
• Therefore, a metal mass larger than is required to create a Compton-thick obscurer outside the central X.
• Combining this simple limit with the masses of Figure , we can now conclude that galaxies simply do not have the required gas to provide Compton-thick obscurers with substantial covering factors outside the central X.
• This paper focused on metal column densities, not hydrogen column densities.
• For completeness we also present the expected metallicities for GRB sightlines in Figure X.