QSO obscuration at high redshift (z ≳ 7): predictions from the bluetides simulation. Issue 2 (11th May 2020)

• Record Type: Journal Article
• Authors: Ni, Yueying; Di Matteo, Tiziana; Gilli, Roberto; Croft, Rupert A C; Feng, Yu; Norman, Colin
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 495:Issue 2(2020)
• Page Start: 2135
• Page End: 2151
• Publication Date: 2020-05-11
• Subjects: galaxies: formation -- galaxies: high-redshift -- quasars: supermassive black holes; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa1313
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: High-$z$ AGNs hosted in gas-rich galaxies are expected to grow through significantly obscured accretion phases. This may limit or bias their observability. In this work, we use bluetides, a large volume cosmological simulation of galaxy formation to examine quasar obscuration for the highest redshift ($z$ ≥ 7) supermassive black holes residing in the centre of galaxies. We find that for the bright quasars, most of the high-column density gas ($\rm {\gt} 90 {\rm {per\ cent}}$ ) resides in the innermost regions of the host galaxy (typically within <10 ckpc), while the gas in the outskirts is a minor contributor to the N H . The brightest quasars can have large angular variations in galactic obscuration, over 2 orders of magnitude (ranging from column density $N_\mathrm{H} \sim 10^{21.5 \!-\! 24}\, \rm {cm}^{-2}$ ), where the lines of sight with the lowest obscuration are those formed via strong gas outflows driven by AGN feedback. The obscured fraction P ( N H > 10 23 cm −2 ) typically ranges from 0.6 to 1.0 for increasing L X (with $L_\mathrm{ X} \gt 10^{43} \, \rm {erg\, s}^{-1}$ ), with no clear trend of redshift evolution. Due to the angular variation in N H, all relations between N H and L X, M BH, and galaxy host properties (global M *, $M_{\rm H_2}$, and star formation rate) show appreciable scatter. The dust optical depth in the UV band τUV has tight positive correlation with N H . Our dust-extincted UV luminosity function (UVLF) is about 1.5 dex lower than the intrinsic UVLF, implying that more than 99 per cent of the $z$ ∼ 7 AGNs are heavily dust extincted and therefore would be missed by the UV-band observation.