Radio-loud fraction of z>6 quasars
arxiv(2024)
摘要
Quasars at redshifts z>6 are an excellent probe of the formation and
evolution of supermassive black holes in the early Universe. The population of
radio-luminous quasars is of particular interest, as such quasars could
potentially be used to study the neutral intergalactic medium during cosmic
reionisation via HI 21cm absorption studies. However, the lack of deep
radio observations of z>6 quasars leaves the population poorly constrained,
and suitable candidates for an HI 21cm absorption study have yet to be
found. In this work, we present Jansky Very Large Array (VLA) 1-2 GHz radio
continuum observations of 138 quasars at redshifts 6.0 ≤ z<7.6. We detect
the radio continuum emission of the z=6.1 quasar J1034-1425, with a 1.6 GHz
flux density of 170± 36 μJy. This quasar is radio-quiet with
radio-loudness, R ≡ f_5 GHz/f_ν,4400 A = 2.4±0.5.
In addition, we detect 7 other quasars at z>6, which have previously been
characterised in the literature at these frequencies. Using the full sample, we
estimate the radio-loud fraction to be 3.8^+6.2_-2.4%, where the
uncertainties are 95
of the radio-loud fraction in the literature, but is still marginally
consistent with no redshift evolution of the radio-loud fraction. We explore
the undetected quasar population by stacking their continuum images at their
optical positions and obtain a median stacked flux density of 13.8±
3.9 μJy and luminosity of
logL_5 GHz/(W Hz^-1)=24.2±0.1.
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