26 research outputs found
The Impact of Dust in Host Galaxies on Quasar Luminosity Functions
We have investigated effects of dust attenuation on quasar luminosity
functions using a semi-analytic galaxy formation model combined with a large
cosmological N-body simulation. We estimate the dust attenuation of quasars
self-consistently with that of galaxies by considering the dust in their host
bulges. We find that the luminosity of the bright quasars is strongly dimmed by
the dust attenuation, about 2 mag in the B-band. Assuming the empirical
bolometric corrections for active galactic nuclei (AGNs) by Marconi et al., we
find that this dust attenuation is too strong to explain the B-band and X-ray
quasar luminosity functions simultaneously. We consider two possible mechanisms
that weaken the dust attenuation. As such a mechanism, we introduce a time
delay for AGN activity, that is, gas fueling to a central black hole starts
some time after the beginning of the starburst induced by a major merger. The
other is the anisotropy in the dust distribution. We find that in order to make
the dust attenuation of the quasars negligible, either the gas accretion into
the black holes has to be delayed at least three times the dynamical timescale
of their host bulges or the dust covering factor is as small as 0.1.Comment: 5 pages, 4 figures, 1 tables. Accepted for publication by MNRAS
Letter
Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) VIII. A less biased view of the early co-evolution of black holes and host galaxies
We present ALMA [CII] line and far-infrared (FIR) continuum observations of
three low-luminosity quasars () discovered by our
Subaru Hyper Suprime-Cam (HSC) survey. The [CII] line was detected in all three
targets with luminosities of , about one order
of magnitude smaller than optically luminous ()
quasars. The FIR continuum luminosities range from
(3 limit) to , indicating a wide range
in star formation rates in these galaxies. Most of the HSC quasars studied thus
far show [CII]/FIR luminosity ratios similar to local star-forming galaxies.
Using the [CII]-based dynamical mass () as a surrogate for bulge
stellar mass (), we find that a significant fraction of
low-luminosity quasars are located on or even below the local relation, particularly at the massive end of the galaxy mass
distribution. In contrast, previous studies of optically luminous quasars have
found that black holes are overmassive relative to the local relation. Given
the low luminosities of our targets, we are exploring the nature of the early
co-evolution of supermassive black holes and their hosts in a less biased way.
Almost all of the quasars presented in this work are growing their black hole
mass at much higher pace at than the parallel growth model, in which
supermassive black holes and their hosts grow simultaneously to match the local
relation at all redshifts. As the low-luminosity
quasars appear to realize the local co-evolutionary relation even at , they should have experienced vigorous starbursts prior to the currently
observed quasar phase to catch up with the relation.Comment: 19 pages, 11 figures, 4 tables. Accepted for publication in
Publications of the Astronomical Society of Japan (PASJ
Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) III. Star formation properties of the host galaxies at studied with ALMA
We present our ALMA Cycle 4 measurements of the [CII] emission line and the
underlying far-infrared (FIR) continuum emission from four optically
low-luminosity () quasars at discovered by
the Subaru Hyper Suprime Cam (HSC) survey. The [CII] line and FIR continuum
luminosities lie in the ranges
and , which are at least one
order of magnitude smaller than those of optically-luminous quasars at . We estimate the star formation rates (SFR) of our targets as
. Their line and continuum-emitting
regions are marginally resolved, and found to be comparable in size to those of
optically luminous quasars, indicating that their SFR or likely gas mass
surface densities (key controlling parameter of mass accretion) are accordingly
different. The ratios of the hosts, , are fully consistent with local star-forming
galaxies. Using the [CII] dynamics, we derived their dynamical masses within a
radius of 1.5-2.5 kpc as . By
interpreting these masses as stellar ones, we suggest that these faint quasar
hosts are on or even below the star-forming main sequence at , i.e.,
they appear to be transforming into quiescent galaxies. This is in contrast to
the optically luminous quasars at those redshifts, which show starburst-like
properties. Finally, we find that the ratios of black hole mass to host galaxy
dynamical mass of the most of low-luminosity quasars including the HSC ones are
consistent with the local value. The mass ratios of the HSC quasars can be
reproduced by a semi-analytical model that assumes merger-induced black
hole-host galaxy evolution.Comment: 20 pages, 9 figures, 4 tables. Accepted for publication in PAS