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Register a new userProbing the structure and evolution of active galactic nuclei with the ultraviolet polarimeter POLLUX aboard LUVOIR
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Added by
Fr\\'ed\\'eric Marin
Publication date
2018
fields
Physics
and research's language is
English
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The ultraviolet (UV) polarization spectrum of nearby active galactic nuclei (AGN) is poorly known. The Wisconsin Ultraviolet Photo-Polarimeter Experiment and a handful of instruments on board the Hubble Space Telescope were able to probe the near- and mid-UV polarization of nearby AGN, but the far-UV band (from 1200 angs down to the Lyman limit at 912 angs) remains completely uncharted. In addition, the linewidth resolution of previous observations was at best 1.89 angs. Such a resolution is not sufficient to probe in detail quantum mechanical effects, synchrotron and cyclotron processes, scattering by electrons and dust grains, and dichroic extinction by asymmetric dust grains. Exploring those physical processes would require a new, high-resolution, broadband polarimeter with full ultraviolet-band coverage. In this context, we discuss the AGN science case for POLLUX, a high-resolution UV spectropolarimeter, proposed for the 15-meter primary mirror option of LUVOIR (a multi-wavelength space observatory concept being developed by the Goddard Space Flight Center and proposed for the 2020 Decadal Survey Concept Study).
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Many open questions remain about massive stars, for example about their evolution, their wind, and their maximum mass at formation. These issues could be ideally adressed by the Pollux UV spectropolarimeter onboard LUVOIR. Here we present examples of the science themes that one could study with Pollux regarding massive stars.
The present paper describes the current baseline optical design of POLLUX, a high-resolution spectropolarimeter for the future LUVOIR mission. The instrument will operate in the ultraviolet (UV) domain from 90 to 390 nm in both spectropolarimetric and pure spectroscopic modes. The working range is split between 3 channels -- far (90-124.5 nm), medium (118.5-195 nm) and near (195-390 nm) UV. Each of the channels is composed of a polarimeter followed by an echelle spectrograph consisting of a classical off-axis paraboloid collimator, echelle grating with a high grooves frequency and a cross-disperser grating operating also as a camera. The latter component integrates some advanced technologies: it is a blazed grating with a complex grooves pattern formed by holographic recording, which is manufactured on a freeform surface. One of the key features underlying the current design is the large spectral length of each order ~6 nm, which allows to record wide spectral lines without any discontinuities. The modelling results show that the optical design will provide the required spectral resolving power higher than R ~ 120,000 over the entire working range for a point source object with angular size of 30 mas. It is also shown that with the 15-m primary mirror of the LUVOIR telescope the instrument will provide an effective collecting area up to 38 569 cm 2. Such a performance will allow to perform a number of groundbreaking scientific observations. Finally, the future work and the technological risks of the design are discussed in details.
We aim to constrain the evolution of AGN as a function of obscuration using an X-ray selected sample of $sim2000$ AGN from a multi-tiered survey including the CDFS, AEGIS-XD, COSMOS and XMM-XXL fields. The spectra of individual X-ray sources are analysed using a Bayesian methodology with a physically realistic model to infer the posterior distribution of the hydrogen column density and intrinsic X-ray luminosity. We develop a novel non-parametric method which allows us to robustly infer the distribution of the AGN population in X-ray luminosity, redshift and obscuring column density, relying only on minimal smoothness assumptions. Our analysis properly incorporates uncertainties from low count spectra, photometric redshift measurements, association incompleteness and the limited sample size. We find that obscured AGN with $N_{H}>{rm 10^{22}, cm^{-2}}$ account for ${77}^{+4}_{-5}%$ of the number density and luminosity density of the accretion SMBH population with $L_{{rm X}}>10^{43}text{ erg/s}$, averaged over cosmic time. Compton-thick AGN account for approximately half the number and luminosity density of the obscured population, and ${38}^{+8}_{-7}%$ of the total. We also find evidence that the evolution is obscuration-dependent, with the strongest evolution around $N_{H}thickapprox10^{23}text{ cm}^{-2}$. We highlight this by measuring the obscured fraction in Compton-thin AGN, which increases towards $zsim3$, where it is $25%$ higher than the local value. In contrast the fraction of Compton-thick AGN is consistent with being constant at $approx35%$, independent of redshift and accretion luminosity. We discuss our findings in the context of existing models and conclude that the observed evolution is to first order a side-effect of anti-hierarchical growth.
This Astro2020 white paper summarizes the unknowns of active galactic nuclei (AGN) physics that could be unveiled thanks to a new, space-born, ultraviolet spectropolarimeter. The unique capabilities of high energy polarimetry would help us to understand the precise mechanisms of matter and energy transfer and supermassive black holes growth, together with the impact of AGN feedback on galaxy evolution.
There are several key open questions as to the nature and origin of AGN including: 1) what initiates the active phase, 2) the duration of the active phase, and 3) the effect of the AGN on the host galaxy. Critical new insights to these can be achieved by probing the central regions of AGN with sub-mas angular resolution at UV/optical wavelengths. In particular, such observations would enable us to constrain the energetics of the AGN feedback mechanism, which is critical for understanding the role of AGN in galaxy formation and evolution. These observations can only be obtained by long-baseline interferometers or sparse aperture telescopes in space, since the aperture diameters required are in excess of 500 m - a regime in which monolithic or segmented designs are not and will not be feasible and because these observations require the detection of faint emission near the bright unresolved continuum source, which is impossible from the ground, even with adaptive optics. Two mission concepts which could provide these invaluable observations are NASAs Stellar Imager (SI; Carpenter et al. 2008 & http://hires.gsfc.nasa.gov/si/) interferometer and ESAs Luciola (Labeyrie 2008) sparse aperture hypertelescope.
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