We present deep near-infrared spectroscopy of six quasars at 6.1 ≤ z ≤ 6.7 with Very Large Telescope/X-Shooter and Gemini-N/GNIRS. Our objects, originally discovered through a wide-field optical ...survey with the Hyper Suprime-Cam (HSC) Subaru Strategic Program (HSC-SSP), have the lowest luminosities (−25.5 mag ≤ M1450 ≤ −23.1 mag) of the z > 5.8 quasars with measured black hole (BH) masses. From single-epoch mass measurements based on Mg ii λ2798, we find a wide range in BH masses, from MBH = 107.6 to 109.3 M . The Eddington ratios Lbol/LEdd range from 0.16 to 1.1, but the majority of the HSC quasars are powered by MBH ∼ 109 M supermassive black holes (SMBHs) accreting at sub-Eddington rates. The Eddington ratio distribution of the HSC quasars is inclined to lower accretion rates than those of Willott et al., who measured the BH masses for similarly faint z ∼ 6 quasars. This suggests that the global Eddington ratio distribution is wider than has previously been thought. The presence of MBH ∼ 109 M SMBHs at z ∼ 6 cannot be explained with constant sub-Eddington accretion from stellar remnant seed BHs. Therefore, we may be witnessing the first buildup of the most massive BHs in the first billion years of the universe, the accretion activity of which is transforming from active growth to a quiescent phase. Measurements of a larger complete sample of z 6 low-luminosity quasars, as well as deeper observations with future facilities, will enable us to better understand the early SMBH growth in the reionization epoch.
We report the discovery of 15 quasars and bright galaxies at 5.7 < z< 6.9. This is the initial result from the Subaru High-z Exploration of Low-Luminosity Quasars project, which exploits the ...exquisite multiband imaging data produced by the Subaru Hyper Suprime-Cam (HSC) Strategic Program survey. The candidate selection is performed by combining several photometric approaches including a Bayesian probabilistic algorithm to reject stars and dwarfs. The spectroscopic identification was carried out with the Gran Telescopio Canarias and the Subaru Telescope for the first 80 deg super(2) of the survey footprint. The success rate of our photometric selection is quite high, approaching 100% at the brighter magnitudes (z sub(AB)< 23.5 mag). Our selection also recovered all the known high-z quasars on the HSC images. Among the 15 discovered objects, six are likely quasars, while the other six with interstellar absorption lines and in some cases narrow emission lines are likely bright Lyman-break galaxies. The remaining three objects have weak continua and very strong and narrow Lyalpha lines, which may be excited by ultraviolet light from both young stars and quasars. These results indicate that we are starting to see the steep rise of the luminosity function of z> or = 6 galaxies, compared with that of quasars, at magnitudes fainter than M sub(1450)~ -22 mag or z sub(AB)~ 24 mag. Follow-up studies of the discovered objects as well as further survey observations are ongoing.
The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is compelling. Detecting dual active galactic nuclei (AGN) – active SMBHs at projected ...separations larger than several parsecs – and binary AGN – probing the scale where SMBHs are bound in a Keplerian binary – is an observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theoretical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydrodynamical conditions of gas in the imminent surroundings of the SMBHs and, at the same time, their duality calls for detailed knowledge on how galaxies assemble through major and minor mergers and grow fed by matter along the filaments of the cosmic web. This review describes the techniques used across the electromagnetic spectrum to detect dual and binary AGN candidates and proposes new avenues for their search. The current observational status is compared with the state-of-the-art numerical simulations and models for formation of dual and binary AGN. Binary SMBHs are among the loudest sources of gravitational waves (GWs) in the Universe. The search for a background of GWs at nHz frequencies from inspiralling SMBHs at low redshifts, and the direct detection of signals from their coalescence by the Laser Interferometer Space Antenna in the next decade, make this a theme of major interest for multi-messenger astrophysics. This review discusses the future facilities and observational strategies that are likely to significantly advance this fascinating field.
We report on the spectroscopic confirmation of a large-scale structure around the luminous
z
= 6.31 quasi-stellar object (QSO) SDSS J1030+0524, powered by a one billion solar mass black hole. The ...structure is populated by at least six members, namely, four Lyman-break galaxies (LBGs), and two Lyman alpha emitters (LAEs). The four LBGs were identified among a sample of 21
i
-band dropouts with
z
AB
< 25.5 selected up to projected separations of 5 physical Mpc (15 arcmin) from the QSO. Their redshifts were determined through multi-object spectroscopic observations at 8−10 m class telescopes lasting up to eight hours. The two LAEs were identified in a 6 h VLT/MUSE observation centered on the QSO. The redshifts of the six galaxies cover the range between 6.129−6.355. Assuming that the peculiar velocities are negligible, this range corresponds to radial separations of ±5 physical Mpc from the QSO, that is comparable to the projected scale of the observed LBG distribution on the sky. We conservatively estimate that this structure is significant at a level > 3.5
σ
and that the level of the galaxy overdensity is at least 1.5−2 within the large volume sampled (∼780 physical Mpc
3
). The spectral properties of the six member galaxies (Ly
α
strength and UV luminosity) are similar to those of field galaxies at similar redshifts. This is the first spectroscopic identification of a galaxy overdensity around a supermassive black hole in the first billion years of the Universe. Our finding lends support to the idea that the most distant and massive black holes form and grow within massive (>10
12
M
⊙
) dark matter halos in large-scale structures and that the absence of earlier detections of such systems is likely due to observational limitations.
Abstract
We present Atacama Large Millimeter/submillimeter Array C
ii
158
μ
m line and underlying far-infrared (FIR) continuum emission observations (0.″70 × 0.″56 resolution) toward HSC ...J124353.93+010038.5 (J1243+0100) at
z
= 7.07, the only low-luminosity (
M
1450
> −25 mag) quasar currently known at
z
> 7. The FIR continuum is bright (1.52 mJy) and resolved with a total luminosity of
L
FIR
= 3.5 × 10
12
L
⊙
. The spatially extended component is responsible for ∼40% of the emission. The area-integrated C
ii
spectrum shows a broad wing (FWHM = 997 km s
−1
,
L
C
ii
= 1.2 × 10
9
L
⊙
), as well as a bright core (FWHM = 235 km s
−1
,
L
C
ii
= 1.9 × 10
9
L
⊙
). This wing is the first detection of a galactic-scale quasar-driven outflow (atomic outflow rate >447
M
⊙
yr
−1
) at
z
> 7. The estimated large mass-loading factor of the total outflow (e.g., ≳9 relative to the C
ii
-based star formation rate) suggests that this outflow will soon quench the star formation of the host. The core gas dynamics are governed by rotation, with a rotation curve suggestive of a compact bulge (∼3.3 × 10
10
M
⊙
), although it is not yet spatially resolved. Finally, we found that J1243+0100 has a black hole mass–to–dynamical mass (and –to–bulge mass) ratio of ∼0.4% (∼1%), consistent with the local value within the uncertainties. Our results therefore suggest that the black hole–host coevolution relation is already in place at
z
∼ 7 for this object.
Abstract
We describe a
Herschel Space Observatory
194–671
μ
m spectroscopic survey of a sample of 121 local luminous infrared galaxies and report the fluxes of the CO
J
to
J
–1 rotational transitions ...for
, the N
ii
205
μ
m line, the C
i
lines at 609 and 370
μ
m, as well as additional and usually fainter lines. The CO spectral line energy distributions (SLEDs) presented here are consistent with our earlier work, which was based on a smaller sample, that calls for two distinct molecular gas components in general: (i) a cold component, which emits CO lines primarily at
J
≲ 4 and likely represents the same gas phase traced by CO (1−0), and (ii) a warm component, which dominates over the mid-
J
regime (4 <
J
≲ 10) and is intimately related to current star formation. We present evidence that the CO line emission associated with an active galactic nucleus is significant only at
J
> 10. The flux ratios of the two C
i
lines imply modest excitation temperatures of 15–30 K; the C
i
370
μ
m line scales more linearly in flux with CO (4−3) than with CO (7−6). These findings suggest that the C
i
emission is predominantly associated with the gas component defined in (i) above. Our analysis of the stacked spectra in different far-infrared (FIR) color bins reveals an evolution of the SLED of the rotational transitions of
vapor as a function of the FIR color in a direction consistent with infrared photon pumping.
Abstract
We present, for the first time, dark matter halo (DMH) mass measurement of quasars at
z
∼ 6 based on a clustering analysis of 107 quasars. Spectroscopically identified quasars are ...homogeneously extracted from the Hyper Suprime-Cam Strategic Survey Program wide layer over 891 deg
2
. We evaluate the clustering strength by three different autocorrelation functions: projected correlation function, angular correlation function, and redshift–space correlation function. The DMH mass of quasars at
z
∼ 6 is evaluated as
5.0
−
4.0
+
7.4
×
10
12
h
−
1
M
⊙
with the bias parameter
b
= 20.8 ± 8.7 by the projected correlation function. The other two estimators agree with these values; though, each uncertainty is large. The DMH mass of quasars is found to be nearly constant ∼10
12.5
h
−1
M
⊙
throughout cosmic time, suggesting that there is a characteristic DMH mass where quasars are always activated. As a result, quasars appear in the most massive halos at
z
∼ 6, but in less extreme halos thereafter. The DMH mass does not appear to exceed the upper limit of 10
13
h
−1
M
⊙
, which suggests that most quasars reside in DMHs with
M
halo
<
10
13
h
-
1
M
⊙
across most of the cosmic time. Our results supporting a significant increasing bias with redshift are consistent with the bias evolution model with inefficient active galactic nucleus feedback at
z
∼ 6. The duty cycle (
f
duty
) is estimated as 0.019 ± 0.008 by assuming that DMHs in some mass interval can host a quasar. The average stellar mass is evaluated from stellar-to-halo mass ratio as
M
*
=
6.5
−
5.2
+
9.6
×
10
10
h
−
1
M
⊙
, which is found to be consistent with C
ii
observational results.
The unique combination of superb spatial resolution, wide-area coverage, and deep depth of the optical imaging from the Hyper Suprime-Cam (HSC) Subaru Strategic Program is utilized to search for dual ...quasar candidates. Using an automated image analysis routine on 34,476 known Sloan Digital Sky Survey quasars, we identify those with two (or more) distinct optical point sources in HSC images covering 796 deg2. We find 421 candidates out to a redshift of 4.5 of which one hundred or so are more likely after filtering out contaminating stars. Angular separations of 0 6-4 0 correspond to projected separations of 3-30 kpc, a range relatively unexplored for population studies of luminous dual quasars. Using Keck I/Low Resolution Imaging Spectrometer and Gemini-N/Near-Infrared Integral Field Spectrometer, we spectroscopically confirm three dual quasar systems at z < 1, two of which are previously unknown out of eight observed, based on the presence of characteristic broad emission lines in each component, while highlighting that the continuum of one object in one of the pairs is reddened. In all cases, the O iiiλ5007 emission lines have mild velocity offsets, thus the joint O iii line profile is not double-peaked. We find a dual quasar fraction of 0.26 0.18% and no evidence for evolution. A comparison with the Horizon-AGN simulation seems to support the case of no evolution in the dual quasar fraction when broadly matching the quasar selection. These results may indicate a scenario in which the frequency of the simultaneous triggering of luminous quasars is not as sensitive as expected to the cosmic evolution of the merger rate or gas content of galaxies.
Abstract
We present ALMA C ii line and far-infrared (FIR) continuum observations of three $z \gt 6$ low-luminosity quasars ($M_{\rm 1450} \gt -25$ mag) discovered by our Subaru Hyper Suprime-Cam ...(HSC) survey. The C ii line was detected in all three targets with luminosities of $(2.4\mbox{--}9.5) \times 10^8\, L_{\odot }$, about one order of magnitude smaller than optically luminous ($M_{\rm 1450} \lesssim -25$ mag) quasars. The FIR continuum luminosities range from $\lt 9 \times 10^{10}\, L_{\odot }$ (3 $\sigma$ limit) to ${\sim } 2 \times 10^{12}\, L_{\odot }$, indicating a wide range in star formation rates in these galaxies. Most of the HSC quasars studied thus far show C ii/ FIR luminosity ratios similar to local star-forming galaxies. Using the C ii-based dynamical mass ($M_{\rm dyn}$) as a surrogate for bulge stellar mass ($M_{\rm\, bulge}$), we find that a significant fraction of low-luminosity quasars are located on or even below the local $M_{\rm\, BH}$–$M_{\rm\, bulge}$ 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 a much higher pace at $z \sim 6$ than the parallel growth model, in which supermassive black holes and their hosts grow simultaneously to match the local $M_{\rm\, BH}$–$M_{\rm\, bulge}$ relation at all redshifts. As the low-luminosity quasars appear to realize the local co-evolutionary relation even at $z \sim 6$, they should have experienced vigorous starbursts prior to the currently observed quasar phase to catch up with the relation.
Abstract
We present deep Keck/MOSFIRE near-infrared spectroscopy of a strong Ly
α
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 narrow (FWHM < 500 km s
−1
) and prominent (
L
Ly
α
> 10
44
erg s
−1
) Ly
α
emission lines at absolute 1450 Å continuum magnitudes of
M
1450
∼ −22 mag. Its rest-frame Ly
α
equivalent width (EW) is 370 ± 30 Å. In the 2 hr Keck/MOSFIRE spectrum in
Y
band, the high-ionization C
iv
λλ
1548,1550 doublet emission line was clearly detected with
FWHM
=
120
−
20
+
20
km s
−1
and a total rest-frame EW of
37
−
5
+
6
Å. We also report the detection of weak continuum emission, and the tentative detection of O
iii
λλ
1661,1666 in the 4 hr
J
-band spectrum. Judging from the UV magnitude, line widths, luminosities, and EWs of Ly
α
and C
iv
, 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 C
iv
emitters seen in
z
= 6–7 galaxy samples. Further observations of other metal emission lines in the rest-frame UV or optical, along with X-ray follow-up observations of the
z
= 6–7 narrow-line quasars, are needed for more robust diagnostics and to determine their nature.
PDF
