56 research outputs found
Minor Contribution of Quasars to Ionizing Photon Budget at z~6: Update on Quasar Luminosity Function at the Faint-end with Subaru/Suprime-Cam
We constrain the quasar contribution to cosmic reionization based on our deep
optical survey of z~6 quasars down to z_R=24.15 using Subaru/Suprime-Cam in
three UKIDSS-DXS fields covering 6.5 deg^2. In Kashikawa et al. (2015), we
select 17 quasar candidates and report our initial discovery of two
low-luminosity quasars (M_1450~ -23) from seven targets, one of which might be
a Lyman alpha emitting galaxy. From an additional optical spectroscopy, none of
the four candidates out of the remaining ten turn out to be genuine quasars.
Moreover, the deeper optical photometry provided by the Hyper Suprime-Cam
Subaru Strategic Program (HSC-SSP) shows that, unlike the two already-known
quasars, the i-z and z-y colors of the last six candidates are consistent with
M- or L-type brown dwarfs. Therefore, the quasar luminosity function (QLF) in
the previous paper is confirmed. Compiling QLF measurements from the literature
over a wide magnitude range, including an extremely faint AGN candidate from
Parsa et al. (2017}, to fit them with a double power-law, we find that the
best-fit faint-end slope is alpha=-2.04^+0.33_-0.18 (-1.98^+0.48_-0.21) and
characteristic magnitude is M_1450^*=-25.8^+1.1_-1.9 (-25.7^+1.0_-1.8) in the
case of two (one) quasar detection. Our result suggests that, if the QLF is
integrated down to M_1450=-18, quasars produce ~1-12% of the ionizing photons
required to ionize the whole universe at z~6 with 2sigma confidence level,
assuming that the escape fraction is f_esc=1 and the IGM clumpy factor is C=3.
Even when the systematic uncertainties are taken into account, our result
supports the scenario that quasars are the minor contributors of reionization.Comment: 8 pages, 3 figures, ApJL accepte
Subaru High- Exploration of Low-luminosity Quasars (SHELLQs). XVII. Black Hole Mass Distribution at Estimated via Spectral Comparison with Low- Quasars
We report the distribution of black hole (BH) masses and Eddingont ratios
estimated for a sample of 131 low luminosity quasars in the early cosmic epoch
(). Our work is based on Subaru High- Exploration of
Low-Luminosity Quasars (SHELLQs) project, which has constructed a low
luminosity quasar sample down to mag, exploiting the
survey data of Hyper Suprime-Cam installed on Subaru Telescope. The discovery
spectra of these quasars are limited to the rest-frame wavelengths of
1200 -- 1400 \AA, which contains no emission lines that can be used as BH mass
estimators. In order to overcome this problem, we made use of low-
counterpart spectra from the Sloan Digital Sky Survey, which are spectrally
matched to the high- spectra in overlapping wavelengths. We then combined
the C~{\sc iv} emission line widths of the counterparts with the continuum
luminosity from the SHELLQs data to estimate BH masses. The resulting BH mass
distribution has a range of , with most of the
quasars having BH masses with sub-Eddington accretion.
The present study provides not only a new insight into normal quasars in the
reionization epoch, but also a new promising way to estimate BH masses of
high- quasars without near-infrared spectroscopy.Comment: 18 pages, 17 figures, accepted for publication in Ap
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). XIV. A Candidate Type-II Quasar at z=6.1292
We present deep Keck/MOSFIRE near-infrared spectroscopy of a strong Lyman
alpha emitting source at z=6.1292, HSC J142331.71-001809.1, which was
discovered by the SHELLQS program from imaging data of the Subaru Hyper
Suprime-Cam (HSC) survey. This source is one of five objects that show
unresolved (10^44 erg s-1) Lyman alpha emission
lines at absolute 1450 angstrom continuum magnitudes of M1450~-22 mag. Its
rest-frame Lyman alpha equivalent width (EW) is 370+/-30 angstrom. In the 2
hour Keck/MOSFIRE spectrum in Y band, the high-ionization CIV 1548,1550 doublet
emission line was clearly detected with FWHM =120+/-20 km s-1 and a total
rest-frame EW of 37-5+6 angstrom. We also report the detection of weak
continuum emission, and the tentative detection of OIII] 1661,1666 in the 4
hour J band spectrum. Judging from the UV magnitude, line widths, luminosities,
and EWs of Lyman alpha and CIV, we suggest that this source is a
reionization-era analog of classical type-II AGNs, although there is a
possibility that it represents a new population of AGN/galaxy composite objects
in the early universe. We compare the properties of J1423-0018 to
intermediate-redshift type-II AGNs and CIV emitters seen in z=6-7 galaxy
samples. Further observations of other metal emission lines in the rest-frame
UV or optical, and X-ray follow-up observations of the z=6-7 narrow-line
quasars are needed for more robust diagnostics and to determine their nature.Comment: 15 pages, 8 figures, accepted for publication in the Astrophysical
Journa
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
Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). IX. Identification of Two Red Quasars at z > 5.6
We present the first discovery of dust-reddened quasars (red quasars) in the
high-z universe (z >5.6). This is a result from the Subaru High-z Exploration
of Low-Luminosity Quasars (SHELLQs) project, which is based on the sensitive
multi-band optical imaging data produced by the Hyper Suprime-Cam (HSC) Subaru
Strategic Program survey. We identified four red quasar candidates from the
spectroscopically confirmed 93 high-z quasars in the SHELLQs sample, based on
detections in the Wide-field Infrared Survey Explorer (WISE) data at 3.4 and
4.6 um (rest-frame ~5000-6500 A). The amount of dust reddening was estimated
with spectral energy distribution (SED) fits over optical and mid-infrared
wavelengths. Two of the four candidates were found to be red quasars with dust
reddening of E(B-V) > 0.1. The remaining SHELLQs quasars without individual
WISE detections are significantly fainter in the WISE bands and bluer than the
red quasars, although we did detect them in the W1 band in a stacked image. We
also conducted the same SED fits for high-z optically-luminous quasars, but no
red quasar was found. This demonstrates the power of Subaru HSC to discover
high-z red quasars, which are fainter than the limiting magnitudes of past
surveys in the rest-frame ultraviolet, due to dust extinction.Comment: Accepted for publication in PAS
Hyper-luminous Dust Obscured Galaxies discovered by the Hyper Suprime-Cam on Subaru and WISE
We present the photometric properties of a sample of infrared (IR) bright
dust obscured galaxies (DOGs). Combining wide and deep optical images obtained
with the Hyper Suprime-Cam (HSC) on the Subaru Telescope and all-sky mid-IR
(MIR) images taken with Wide-Field Infrared Survey Explorer (WISE), we
discovered 48 DOGs with and , where
, , and [22] represent AB magnitude in the -band,
-band, and 22 m, respectively, in the GAMA 14hr field
( 9 deg). Among these objects, 31 ( 65 %) show power-law
spectral energy distributions (SEDs) in the near-IR (NIR) and MIR regime, while
the remainder show a NIR bump in their SEDs. Assuming that the redshift
distribution for our DOGs sample is Gaussian, with mean and sigma = 1.99
0.45, we calculated their total IR luminosity using an empirical relation
between 22 m luminosity and total IR luminosity. The average value of the
total IR luminosity is (3.5 1.1) L, which
classifies them as hyper-luminous infrared galaxies (HyLIRGs). We also derived
the total IR luminosity function (LF) and IR luminosity density (LD) for a
flux-limited subsample of 18 DOGs with 22 m flux greater than 3.0 mJy and
with -band magnitude brighter than 24 AB magnitude. The derived space
density for this subsample is log = -6.59 0.11 [Mpc]. The
IR LF for DOGs including data obtained from the literature is well fitted by a
double-power law. The derived lower limit for the IR LD for our sample is
3.8 10 [L Mpc] and
its contributions to the total IR LD, IR LD of all ultra-luminous infrared
galaxies (ULIRGs), and that of all DOGs are 3 %, 9 %, and 15 %,
respectively.Comment: 15 pages, 15 figures, and 3 tables, accepted for publication in PASJ
(Subaru special issue
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