QSO obscuration at high redshift (z ≳ 7): predictions from the BLUETIDES simulation

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 (> 90 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<SUB>H</SUB>. The brightest quasars can have large angular variations in galactic obscuration, over 2 orders of magnitude (ranging from column density N<SUB>H</SUB> ∼ 10<SUP>21.5-24</SUP> cm<SUP>-2</SUP>), 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<SUB>H</SUB> > 10<SUP>23</SUP> cm<SUP>-2</SUP>) typically ranges from 0.6 to 1.0 for increasing L<SUB>X</SUB> (with L<SUB>X</SUB> > 10<SUP>43</SUP> erg cm<SUP>-1</SUP>), with no clear trend of redshift evolution. Due to the angular variation in N<SUB>H</SUB>, all relations between N<SUB>H</SUB> and L<SUB>X</SUB>, M<SUB>BH</SUB>, and galaxy host properties (global M<SUB>*</SUB>, M<SUB>H<SUB>2</SUB></SUB>, and star formation rate) show appreciable scatter. The dust optical depth in the UV band τ<SUB>UV</SUB> has tight positive correlation with N<SUB>H</SUB>. 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.