arXiv:1002.1561v1 [astro-ph.CO] 8 Feb 2010
Molecular Gas in Redshift 6 Quasar Host Galaxies
Ran Wang1,2, Chris L. Carilli2, R. Neri3, D. A. Riechers4,11Jeff Wagg2,5, Fabian Walter6,
Frank Bertoldi7, Karl M. Menten8, Alain Omont9, Pierre Cox3, Xiaohui Fan10
We report our new observations of redshifted carbon monoxide emission from
six z∼6 quasars, using the IRAM Plateau de Bure Interferometer. CO (6-5)
or (5-4) line emission was detected in all six sources. Together with two other
previous CO detections, these observations provide unique constraints on the
molecular gas emission properties in these quasar systems close to the end of
the cosmic reionization. Complementary results are also presented for low-J CO
lines observed at the Green Bank Telescope and the Very Large Array, and dust
continuum from five of these sources with the SHARC-II bolometer camera at the
Caltech Submillimeter Observatory. We then present a study of the molecular
gas properties in our combined sample of eight CO-detected quasars at z∼6. The
detections of high-order CO line emission in these objects indicates the presence
of highly excited molecular gas, with estimated masses on the order of 1010M⊙
within the quasar host galaxies. No significant difference is found in the gas
mass and CO line width distributions between our z ∼ 6 quasars and samples
of CO-detected 1.4 ≤ z ≤ 5 quasars and submillimeter galaxies. Most of the
1Purple Mountain Observatory,China Academy of Science, No. 2 Beijing West Road, Nanjing, 210008,
2National Radio Astronomy Observatory, PO Box 0, Socorro, NM, USA 87801
3Institute de Radioastronomie Millimetrique, St. Martin d’Heres, F-38406, France
4California Institute of Technology, 1200 E. California Blvd., Pasadena, CA, 91125, USA
5European Southern Observatory, Alonso de C´ ordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
6Max-Planck-Institute for Astronomy, K¨ onigsstuhl 17, 69117 Heidelberg, Germany
7Argelander-Institut f¨ ur Astronomie, University of Bonn, Auf dem H¨ ugel 71, 53121 Bonn, Germany
8Max-Planck-Institut f¨ ur Radioastronomie, Auf dem H¨ ugel 71, 53121 Bonn, Germany
9Institut d’Astrophysique de Paris, CNRS and Universite Pierre et Marie Curie, Paris, France
10Steward Observatory, The University of Arizona, Tucson, AZ 85721
– 2 –
CO-detected quasars at z∼6 follow the far infrared-CO luminosity relationship
defined by actively star-forming galaxies at low and high redshifts. This suggests
that ongoing star formation in their hosts contributes significantly to the dust
heating at FIR wavelengths. The result is consistent with the picture of galaxy
formation co-eval with supermassive black hole (SMBH) accretion in the earliest
quasar-host systems. We investigate the black hole–bulge relationships of our
quasar sample, using the CO dynamics as a tracer for the dynamical mass of
the quasar host. The median estimated black hole-bulge mass ratio is about
fifteen times higher than the present-day value of ∼0.0014. This places important
constraints on the formation and evolution of the most massive SMBH-spheroidal
host systems at the highest redshift.
Subject headings: galaxies: quasars — galaxies: high-redshift — galaxies: star-
burst — molecular data — galaxies: active — radio lines: galaxies
Optically bright and massive quasar systems have been discovered at z∼6 through large
optical surveys (e.g., Fan et al. 2006a; Willott et al. 2007, 2009a, 2009b; Cool et al. 2006;
Jiang et al. 2008). The Gunn-Peterson absorption seen in their rest-frame UV spectra indi-
cates that they exist close to the epoch of the cosmic reionization, and thus, are among the
first generation of the most luminous objects in the universe (Fan et al. 2006b; Fan et al.
2006c). About 30% of the z∼6 quasars have been detected in dust continuum emission at 250
GHz, indicating large far-infrared (FIR) luminosities (3×1012L⊙to 10×1012L⊙, Wang et al.
2008b) arising from dust with temperatures of 30 to 60 K (e.g., Benford et al. 1999; Petric et
al. 2003; Bertoldi et al. 2003a; Beelen et al. 2006; Wang et al. 2007, 2008b). Studies of these
objects’ optical-to-radio spectral energy distributions (SEDs) and luminosity correlations ar-
gue for dust heating by star formation in the host galaxies (Bertoldi et al. 2003a; Beelen et al.
2006; Carilli et al. 2004; Riechers et al. 2007; Wang et al. 2008b). High resolution imaging of
the dust continuum and [C II] λ158µm line emission in the brightest z∼6 millimeter source,
the z=6.42 quasar SDSS J114816.64+525150.3 (hereafter J1148+5251), suggest a high star
formation surface density in the quasar host of ∼ 1000M⊙yr−1kpc−2(Walter et al. 2009).
It is likely that we are witnessing an early galaxy evolutionary stage in these FIR luminous
quasar systems at z∼6, in which a massive starburst is ongoing co-evally with a luminous
central active galactic nuclei (AGN). These objects therefore may also provide crucial insight
into our understanding of the origin of the correlation between bulge mass/luminosity/stellar
velocity dispersion and supermassive black hole (SMBH) mass in nearby galaxies (Tremaine
– 3 –
et al. 2002; Marconi & Hunt 2003; Hopkins et al. 2007a), which suggests that the forma-
tion of the SMBH and its spheroidal host are tightly coupled (Kauffmann & Haehnelt 2000;
Hopkins et al. 2007b).
Molecular CO line emission has been widely detected and studied in similar FIR lumi-
nous quasar systems at z> 1 to 5 (e.g., Brown & Vanden Bout 1992; Omont et al. 1996a;
Carilli et al. 2002; Cox et al. 2002; Solomon & Vanden Bout 2005, hereafter SV05; Riechers
et al. 2006; Maiolino et al. 2007; Coppin et al. 2008a). These CO detected quasars show a
FIR-to-CO luminosity correlation similar to local starburst spiral galaxies, Ultra Luminous
Infra Red Galaxies (ULIRGs), and high-z submillimeter galaxies (SMGs, SV05; Greve et al.
2005; Riechers et al. 2006). The derived molecular gas masses are on the order of ∼ 1010M⊙,
which are also comparable to the typical values found in SMGs (Greve et al. 2005; Carilli &
Wang 2006; Coppin et al. 2008a) and the z∼1.5 massive star forming disk galaxies (Daddi et
al. 2008). The molecular gas can provide the requisite fuel for massive star formation which
is suggested by the FIR emission redshifted to submillimeter and millimeter wavelengths
(Benford et al. 1999; Omont et al. 1996b, 2003; Priddey et al. 2003; Robson et al. 2004;
Beelen et al. 2006; Wang et al. 2008a).
The CO line emission detected in FIR luminous quasars provides estimates of the dy-
namical properties of the spheroidal bulges (i.e. bulge mass and velocity dispersion). Shields
et al. (2006) investigated the relation between black hole mass (MBH) and bulge velocity
dispersion (σ) in high-z quasar systems using the observed CO line widths. They found that
the massive quasars (MBH> 109to 1010M⊙) at z>3 appear to have much narrower σ values
compared to what is expected from the local MBH–σ relationship (e.g., Tremaine et al. 2002).
Moreover, Coppin et al. (2008a) found that the average black hole–bulge mass ratios for
CO and FIR luminous quasar samples at z∼2 are likely to be an order of magnitude higher
than the local value (MBH/Mbulge ∼ 0.0014, Marconi & Hunt 2003). Less evolved stellar
bulges were also indicated by high resolution imaging of the CO emission in two z>4 quasars
(Riechers et al. 2008a, 2008b). These results argue for the scenario that the formation of
the SMBHs occurs prior to that of the stellar bulge, which is also suggested by the near-IR
imaging of high-z quasar host galaxies (Peng et al. 2006a, 2006b).
CO line emission was previously searched in four z∼6 quasars, and detected in two of
these (Walter et al. 2003; Bertoldi et al. 2003b; Carilli et al. 2007; Maiolino et al. 2007).
The two CO-detected quasars, J1148+5251 and J0927+2001, are the brightest millimeter
sources among a sample of thirty-three z∼6 quasars that have published millimeter dust
continuum observations (Petric et al. 2003; Bertoldi et al. 2003a; Wang et al. 2007, 2008a).
The detected CO transitions indicate highly excited molecular gas in the quasar host galaxies
(line flux density spectral energy distributions peaking at J=6 or higher, Carilli et al. 2007;
– 4 –
Riechers et al. 2009) and the implied molecular gas masses are all ∼ 2 × 1010M⊙. High
resolution VLA imaging has resolved the CO emission in J1148+5251 to kpc scale, suggesting
a dynamical mass of Mdynsin2i ∼ 4.5 × 1010M⊙within a radius of 2.5 kpc, where i is the
inclination angle (Walter et al. 2004; Riechers et al. 2009) of the molecular disk. This
provides the first direct constraint on the mass of the quasar host galaxy at the highest
redshift, which suggests a high black hole–bulge mass ratio similar to that found with the
z∼2 and z>4 quasars.
In this work, we extend the CO observations to all z∼6 quasars with known 250 GHz
continuum flux densities of S250GHz≥ 1.8mJy (Wang et al. 2007, 2008b). We aim to study
their general molecular gas properties and investigate possible constraints on the SMBH-host
evolution with these earliest quasars. We describe the sample selection and observations in
Section 2, and present the results in Section 3. The CO emission and gas properties of these
z∼6 quasars are analyzed in Section 4. Based on our results, we present a brief discussion
on the constraints of the black hole-bulge evolution in Section 5, and summarize the main
conclusions in Section 6. A Λ-CDM cosmology with H0= 71km s−1Mpc−1, ΩM= 0.27 and
ΩΛ= 0.73 is adopted throughout this paper (Spergel et al. 2007).
2. Sample and observations
There are thirty-three quasars at z∼6 that have published observations of dust con-
tinuum at 250 GHz that were made with the Max-Planck Millimeter Bolometer Array
(MAMBO, Kreysa et al. 1998) on the IRAM 30m-telescope, and ten of these have been
detected (Bertoldi et al. 2003a; Petric et al. 2003; Willott et al. 2007; Wang et al. 2007,
2008b). The two brightest MAMBO detections (i.e. S250GHz ∼ 5mJy, LFIR ∼ 1013L⊙),
J1148+5251 and J0927+2001, have all been detected in strong molecular CO line emission
(J1148+5251: CO (3-2), Walter et al. 2003; CO (7-6) and (6-5), Bertoldi et al. 2003b;
J0927+2001: CO (6-5) and (5-4), Carilli et al. 2007). In this work, we present new ob-
servations of CO (6-5) and/or (5-4) line emission in another six z∼6 quasars (J0840+5624,
J1044−0125, J1048+4637, J1335+3533, J1425+3254, J2054−0005), using the Plateau de
Bure Interferometer (PdBI). They all have 250 GHz flux density of S250GHz≥ 1.8mJy and
corresponding FIR luminosities of LFIR> 5×1012L⊙(using the estimation given in Wang et
al. 2008a and 2008b). We also report our observations of the CO (2-1) line in J0840+5624
and J0927+2001 with the Green Bank Telescope (GBT), CO (3-2) in J1048+4637 with the
Very Large Array (VLA), and 350 µm dust continuum in J0840+5624, J0927+2001, J1044-
0125, J1335+3533, and J1425+3254 with the SHARC-II bolometer camera at the Caltech
– 5 –
Submillimeter Observatory (CSO).
We list the basic information of all the eight FIR luminous z∼6 quasars in Table 1,
including their redshifts from the discovery papers (Fan et al. 2000, 2003, 2006a, Jiang et
al. 2008, Cool et al.2006), optical magnitudes, continuum flux densities at 250 GHz and 1.4
GHz (Bertoldi et al. 2003a; Carilli et al. 2004; Wang et al. 2007; Wang et al. 2008b), as well
as available measurements of the black hole masses and AGN bolometric luminosities (Jiang
et al. 2006). These objects are mainly optically selected with rest-frame 1450˚ A (m1450)
absolute magnitude of −27.8 < M1450 < −26.0, indicating AGN bolometric luminosity of
Lbol > 1013L⊙ (Jiang et al. 2006), which are among the brightest objects in the quasar
population. The black hole masses of J1148+5251 and J1044−0125 have been determined
from the measurements of the AGN UV broad line emission (Willotts et al. 2003; Jiang et
al. 2006) which are a few 109M⊙, indicating Eddington ratios of 0.5 to 1.0 (Jiang et al.
2.2.Molecular CO observations
The observations of the CO (6-5) and (5-4) lines were carried out with the new generation
3 mm receiver on the PdBI, which provides a bandwidth of 1 GHz in dual polarization. The
sources were observed between 2007 and 2009, in the compact D configuration (FWHM
resolution ∼ 5′′). We generally used two frequency setups for the first two tracks on each
target to cover a continuous bandwidth of 1.8 GHz, to account for the possible uncertainties
between the optical and systemic (CO) redshifts. In the case of marginal detections, we
added one more track with both polarization bands centered at the likely line frequency to
confirm the line at > 3σ significance. The original frequency resolution is 2.5 MHz which
was subsequently binned to 23 MHz (∼ 70kms−1). Phase calibration was performed every
20 min with observations of 0954+658, 1055+018, 1150+497, 1308+326, and 2134+004. We
observed MWC 349 as the flux calibrator. The typical rms after eight hours of observing
time is about 0.5mJybeam−1per 70kms−1wide bin. The data were reduced with the IRAM
GILDAS software package (Guilloteau & Lucas 2000).
We also conducted observations of CO (2-1) line emission in two of the CO-detected
z∼6 quasars, J0840+5624 and J0927+2001, during the winter months of 2007-2008, using the
Ka-band receiver on the 100-m GBT. The observations employed the subreflector nodding
mode with half-cycle times between 9.0 to 22.5 seconds. The tunings were based on the CO
(6-5)/(5-4) redshifts (Bertoldi et al. 2003b; Carilli et al. 2007). We set up the spectrometer
in low resolution mode with two single-polarization 800 MHz windows centered at offsets
of −100MHz and +100MHz from the observed line frequency. Thus, the two polarizations
– 6 –
cover the redshifted CO (2-1) line with a bandwidth of 600 MHz simultaneously.
Flux calibration was performed using 3C147, and pointing was checked regularly on
known radio sources. Data were analyzed using the data reduction routines in the GBTIDL
software package. The uncertainties of the flux calibration are typically 10% to 20%. We
have spent 17.7 hours of observing time (including overhead due to subreflector movement)
on J0840+5624, leading to an rms noise of about 0.13 mJy per 50 km s−1wide channel,
measured from the line-free channels. However, there is a small-scale baseline structure
close to the observed line frequency, which cannot be removed in the calibration, and thus,
precluded detection of weak line emission with peak flux density of a few hundred µJy (see
Wagg et al. 2008). We also spent 10.7 hours on J0927+2001, and the CO (2-1) line is not
detected with a 1σ rms sensitivity of 0.15 mJy per 50 km s−1channel.
Finally, we report a tentative VLA Q-band detection of CO (3-2) line emission from
J1048+4637. These observations were conducted in 2004 in BC, C, and CD configurations.
The source was observed in continuum mode, i.e. two polarizations, two Intermediate Fre-
quency bands ”IFs”, and 50 MHz bandwidth (∼310 km s−1in velocity) per IF for each
frequency setup. We searched the CO (3-2) line emission in the frequency range from 47.715
GHz to 48.165 GHz (corresponding to z=6.179 to 6.247 in redshift) by observing the source
repeatedly with different frequency setups.
We reduced the data and made images using the standard VLA wide field data reduction
software AIPS and the source is detected in the 50 MHz channel centered at 47.865 GHz
(z=6.224, ∆z = 0.008) at ?4σ with a 1σ rms of 70 µJybeam−1. The spatial resolution on
the final map is 0.5′′(FWHM). The 1σ rms values for the other channels are much higher,
i.e. from 120 to 200 µJy beam−1(due to less observing time) and no signal is detected at
the quasar postion. We excluded the data observed at 47.865 GHz and 47.815 GHz which
are close to the CO (3-2) line frequency for z=6.2284 (i.e. the redshift determined with the
PdBI CO (6-5) detection), and merged the data of all the other channels to constrain the
source continuum at 48 GHz. The 1σ rms noise on the combined image is 65 µJy beam−1
and no continuum emission is detected.
2.3. SHARC-II 350 µm dust continuum observations
We obtained new 350 µm observations of the dust continuum emission from five z∼6
quasars, using the SHARC-II bolometer camera on the CSO 10.4 m telescope. The SHARC-
II camera is a 12×32-element array with a field of view of 2.6′×1.0′and a beam size of 8.5′′.
The observations were conducted in February 2008 during excellent weather conditions on
– 7 –
Mauna Kea (opacity at 225 GHz < 0.06). We adopted a scan pattern similar to our previous
SHARC-II observations of z∼6 quasars, i.e. a Lissajous patten with amplitudes of ±45′′and
±12′′in azimuth and elevation respectively. This provides a uniform coverage of ∼ 65′′×34′′.
Pointing, focus, and flux calibrations were done on Mars, Saturn, and a number of secondary
calibrators (OH231.8+4.2, CRL618, IRC10216, Arp220, and 3C345). The final calibration
uncertainties are within 20%.
The on-source integration time for each of the five objects is listed in Table 2. Data
reduction was performed with the CRUSH data reduction package version 1.63 (Kov´ acs et
al. 2006a) and the rms noise values in the final maps are < 5 mJy beam−1for all targets.
With these new observations, we confirm a previous 350 µm detection of J0927+2001 and
marginally detect the dust continuum toward J0840+5624 (2.9σ). The measurements and
upper limits for the other three sources are listed in the next section.
We have detected CO (6-5) or (5-4) line emission at ≥ 5σ in all six quasars observed with
the PdBI. The CO emission is unresolved for all the six objects with the 5′′synthesized beam
(∼ 29.2kpc at z∼6), and only one source, J1048+4637, has a clear detection of the 3 mm
continuum in the line-free channels. We fit the line spectra with a single Gaussian profile to
determine the line widths (FWHM), host-galaxy redshifts, and corresponding uncertainties.
The line spectra with the Gaussian-fit profiles are presented in the left panel of Figure
1. The line fluxes are obtained by integrating the data over the line emitting channels.
The averaged-intensity maps are presented in the right panel of Figure 1. The CO emission
peaks on the intensity maps show offsets of 0.3′′to 1.8′′from the optical quasar positions (see
details for each sources below), which are all within the relative astrometric uncertainties of
the observations. We summarize the line and continuum parameters for the PdBI detections
in Table 2. The CO (3-2) detection in J1048+4637, CO (2-1) upper-limits, and the 350
µm dust continuum measurements are also listed in this table. We show the 350 µm dust
continuum maps of J0840+5624 and J0927+2001 in Figure 2. The VLA Q-band 47.865 GHz
image of the CO (3-2) line detection in J1048+4637 is shown in the left panel of Figure 3.
An average image of the line-free channels is also presented (right panel), which constrains
the continuum emission from this object at 48 GHz.
– 8 –
3.1. Note for individual objects
J0840+5624 Both the CO (6-5) and (5-4) transitions were detected, peaking at 08h40m35.08s,
+56◦24′20.5′′, 0.6′′from the optical quasar position. The integrated line flux densities are
0.60±0.07 Jy km s−1and 0.72±0.15 Jy km s−1, respectively. A single Gaussian line profile
fitted to the combined spectrum of the two CO transitions yields a host galaxy redshift of
5.8441±0.0013 and a FWHM line width of 860±190 km s−1. This is the largest CO line
width that we have detected among the z∼6 quasars, and the line profile is double-peaked.
A fit with two Gaussian components to the combined spectrum gives peak velocity offsets
of −300kms−1and 230kms−1, and FWHM line widths of ∼300 kms−1and 410 kms−1for
the two components, respectively. Continuum is not detected in the line-free channels and
the 3σ upper limits are 0.15 mJy at 85 GHz and 0.30 mJy at 101 GHz.
The GBT observation of the CO (2-1) transition gives an rms of about 0.13 mJy per
50 km s−1channel. Assuming a velocity range similar to the CO (5-4) line, We estimated
a 3σ line flux upper limit of about 0.1 Jy km s−1for this object1. However, we emphasize
that there are large uncertainties in this measurement due to a small-scale baseline structure
close to the expected line frequency (Wagg et al. 2008).
The 350 µm dust continuum emission is marginally detected in this object at ∼ 2.9σ.
The continuum source is un-resolved and peaks at RA=08h40m35.32sDec=56d24′17.5′′,
which is 3.1′′away from the optical quasar (the left panel of figure 2). The peak value
is 9.3±3.2 mJy beam−1on the final map and we adopt this as the tentative 350 µm flux
density. Further 350 µm observations with better sensitivity are necessary to confirm this
tentative detection. On the SHARC-II map, there is also a 4σ peak (14.8±3.7 mJy) at the
position of RA=08h40m34.35sDec=56d24′45.0′′(i.e. northwest to the quasar, see the left
panel of Figure 2). We checked our previous VLA 1.4 GHz continuum observation (observed
in A array with a resolution of FWHM∼ 1.4′′, Wang et al. 2007) and there is a radio source
with S1.4GHz= 44± 9µJy close to this position, i.e. at RA=08h40m34.25sDec=56d24′44.7′′.
The offset is only 0.9′′, well within the position uncertainty of the SHARC-II observation.
There is no further identification information for this field source yet.
J1044−0125 We detected the CO (6-5) line in this source, peaking at the position of
10h44m33.02, −01◦25′02.3′′, 0.3′′away from the optical quasar. The FWHM of the CO line
fitted with a single Gaussian profile is 160±60 km s−1, which represents the narrowest CO
1The 3σ upper limits on the CO (2-1) line emission are estimated as 3σchannel(∆vchannel∆vline)1/2(in
Jy km s−1), where ∆vlineis the expected CO line width in km s−1, ∆vchannelis the channel width in km
s−1, and σchannelis the corresponding rms noise value in Jy (Seaquist et al. 1995; Wagg et al. 2007). We
adopt the full line width at zero intensity from the PdBI spectrum as ∆vlinein the calculation.
– 9 –
line width among all the CO-detected z∼6 quasars. The integrated line flux is 0.21±0.04 Jy
km s−1. The 3σ upper limit on the continuum emission is 0.15 mJy at 102 GHz.
The 350 µm dust continuum emission has not been detected in our SHARC-II observa-
tions, and the 1σ rms is 4.5 mJy beam−1. This yields a 3σ flux density upper limit of 13.5
J1048+4637 We detected the CO (6-5) line emission in this source, peaking at 10h48m45.08,
46◦37′18.7′′, 0.5′′away from the optical quasar position. The CO redshift is 6.2284±0.0017,
which is close to the value of 6.23 estimated from the Lyα+NV emission in the quasar rest-
frame UV spectrum (Fan et al. 2003), but higher than the value of 6.19∼6.20 measured
from the Mg II λ2798˚ A emission (Iwamuro et al. 2004; Maiolino et al. 2004). The 3 mm
dust continuum emission is detected along with the CO line emission. A simultaneous fit
of the CO lines and the continuum gives a continuum flux density of 0.13 ± 0.03mJy at 96
GHz. The integrated line flux is 0.27±0.05 Jy km s−1, and the FWHM line width from single
Gaussian fit is 370±130 km s−1. We have also detected CO (6-5) line emission from a source
∼ 28′′away, to the northeast of the quasar (J1048NE). The CO spectrum yields a redshift
of 6.2259±0.0019 for this NE source, indicating the presence of a companion galaxy ∼ 160
kpc away from the optical quasar. The observed peak flux density is 0.53±0.14 mJy, which
gives an integrated flux of 0.23±0.05 Jy km s−1. Considering an attenuation factor of ∼ 0.4
for the primary beam (FWHM ∼ 52′′) at the source position, we estimate the intrinsic line
flux to be 0.58±0.13 Jy km s−1(see Figure 1).
The CO (3-2) line emission from this object was searched using the Q-band receiver
on the VLA, and we detected the source at 0.30 ± 0.07mJy in a 50 MHz wide channel
centered at 47.865 GHz. The data averaged over the other channels present a 3σ upper limit
of 0.195mJy for the continuum emission at 48 GHz, indicating that the 0.3mJy detection
at 47.865 GHz are not due to nonthermal quasar contniuum emission. An optically thin
graybody model with a dust temperature of Tdust= 47K and an emissivity index of β = 1.6
(Beelen et al. 2006; Wang et al. 2008b) fitted to the continuum measurements at 250 GHz
and 96GHz suggests a continuum flux density of only 0.017 mJy at the observed frequency
of 47.865 GHz. Thus the 0.30mJy detection in this channel are likely to be from the CO
(3-2) line emission. The source on the VLA image is marginally resolved by the 0′′.5 × 0′′.5
synthesized beam, and a fit to a 2D-Gaussian component yields a source size of 0′′.9 × 0′′.4.
The integrated line intensity detected in the channel is 0.09 ± 0.02Jykms−1. According
to the line profile and source redshift of z=6.2284±0.0017 determined from the CO (6-5)
transition, the VLA observation is likely to be centered at the blue part of the CO (3-2) line
emission and covers a bandwidth of 310 km s−1in velocity. Thus we adopt the measurement
as a lower limit of the CO (3-2) line flux for this object.
– 10 –
J1335+3533 The CO (6-5) line was detected in this source, peaking at the position of
13h35m50.75s, 35◦33′15.8′′, 0.7′′away from the optical quasar position. The CO FWHM
fitted with a single Gaussian profile is 310±50 km s−1. The integrated line intensity is
0.53±0.07 Jy km s−1, and the 3σ upper limit on the continuum emission is 0.15 mJy at 100
No 350 µm dust continuum is detected in our SHARC-II observations of this object.
The 1σ rms on the final map is 4.4 mJy beam−1, and the corresponding 3σ upper limit is
13.2 mJy for the 350 µm flux density.
J1425+3254 The CO (6-5) line was detected, peaking at the position of 14h25m16.26s,
32◦54′09.3′′, 0.6′′away from the optical quasar. The CO FWHM fitted with a single Gaussian
profile is 690±180 km s−1, and the line flux of 0.59±0.11 Jy km s−1. The 3σ upper limit for
the continuum at 101 GHz is 0.12 mJy.
This source was not detected in our SHARC-II observations of the 350 µm dust contin-
uum with a 1σ rms of 2.6 mJy beam−1. This yields a 3σ upper limit of 7.8 mJy for the 350
µm flux density.
J2054−0005 The CO (6-5) line was detected in this source, peaking at the position of
20h54m06.45s, −00◦05′13.1′′, 1.8′′away from the optical quasar. The CO line FWHM ob-
tained from a fit to a single-Gaussian line profile is 360±110 km s−1. The derived integrated
intensity is 0.34±0.07 Jy km s−1, and the 3σ upper limit on the continuum emission at 98
GHz is 0.15 mJy.
J0927+2001 We have observed the CO (2-1) line in this object for 10.7 hours with the
GBT, and the final rms is 0.15 mJy per 50 km s−1channel. We adopt a line width of ∼900
km s−1(full width at zero intensity) based on the previous PdBI detections of the CO (6-5)
and (5-4) transitions (Carilli et al. 2007), and derive a 3σ upper-limit to the integrated line
intensity of ∼0.1 Jy km s−1.
The 350 µm dust continuum emission from this object was previously detected by our
SHARC-II observations in 2007 (17.7±5.7 mJy, Wang et al. 2008a). The new data we
presented here confirms the detection with better sensitivity, i.e. a 350 µm flux density of
11.7±2.4 mJy. This 350 µm dust continuum peak is quite consistent with the optical quasar
position (offset of < 1′′). We cannot confirm the ∼ 3σ peak 15′′away from the quasar
position reported in our previous work (Wang et al. 2008a).
– 11 –
We compare the redshifts determined from CO and UV emission lines in Table 3 for
all CO detected z∼6 quasars, including the previous CO-detections in J1148+5251 and
J0927+2001 (Bertoldi et al. 2003b, Carilli et al. 2007). The C IV λ1549˚ A redshifts of
J0840+5624 and J1044−0125 are blue-shifted by about 3000 and 1600kms−1compared to
the values determined from the CO lines, respectively. This is consistent with the fact that
the C IV emission is typically blue-shifted from the quasar systematic redshift by up to a
few thousand kms−1(Richards et al. 2002; Goodrich et al. 2001; Ryan-Weber et al. 2009).
There is no systematic offset between the CO redshifts and those determined from other UV
emission lines. In particular, the CO redshifts of J1044−0125 and J1148+5251 are in good
agreement with the values derived from the CIII] λ1909˚ A and SiIV λ1400˚ A broad emission lines
with high-quality near-infrared spectroscopy (Jiang et al. 2007; Ryan-Weber et al. 2009).
The largest offset between CO and Lyα redshifts is ∼0.04, which is reasonable since the Lyα
redshifts are always poorly constrained (with uncertainties of 0.02∼0.05)2. Three objects
have redshifts determined from the Mg II λ2798˚ A line. The CO redshifts of J1044−0125
and J1148+5251 are consistent with the Mg II measurements within the uncertainties, while
a large offset of ∼ 0.03 is seen in J1048+4637 between the CO redshift and the Mg II ones
quoted in Iwamuro et al. (2004) and Maiolino et al. (2004).
Our PdBI observations have a 100% CO detection rate among the millimeter (FIR) lumi-
nous quasars in the early universe. Together with the previous CO detections in J1148+5251
and J0927+2001 (Bertoldi et al. 2003b; Walter et al. 2003, 2004; Carilli et al. 2007), all
eight z∼6 quasars with published 250 GHz flux densities of S250GHz≥ 1.8mJy (Bertoldi et al.
2003a; Wang et al. 2007; Wang et al. 2008b) have now been detected in CO (6-5) and/or (5-
4) line emission. In this section, we present a study of the CO emission line properties in the
earliest quasar host galaxies with this sample. We compare their molecular gas masses and
CO line width distributions to that of the CO-observed quasars and submillimeter galaxies
at lower redshift. A list of fifteen CO detected quasars at 1.4 ≤ z ≤ 4.7 was summarized in
SV05. Another quasar SDSS J0338+0021 at z=5.0267 was detected in CO (5-4) line emission
by Maiolino et al. (2007). Additionally, Coppin et al. (2008a) presented six CO-detected
quasars at 1.7 ≤ z ≤ 2.6 (one of these is also in the SV05’s sample). These give a sample of
2Note that the Lyα redshift uncertainty of 0.004 quoted in Jiang et al. (2008) is only an error from their
– 12 –
twenty-one CO-detected quasars with redshifts from 1.4 to 5. We also consider the a sample
of fourteen CO-detected SMGs at 1.1 ≤ z ≤ 3.4 from Greve et al. (2005) and Tacconi et al.
4.1. Molecular gas masses
The mass of the molecular gas in J1148+5251 is well determined from the CO (3-2)
data (Walter et al. 2003, 2004; Riechers et al. 2009). The high-order CO transitions (J ≥ 5)
detected in SMGs and high-redshift quasars are usually not thermalized (e.g., Weiß et al.
2005a, 2007; Riechers et al. 2009). Thus, for the other seven CO-detected z∼6 quasars,
We adopt the line ratios of L′
multi CO transition large velocity gradient (LVG) modeling of J1148+5251 (Riechers et al.
2009) and use these numbers to calculate the CO (1-0) line luminosities (L′
estimate the molecular gas masses (Mgas= M[H2+ He]) within the CO emitting region of
the quasar host galaxies using Mgas= αL′
conversion factor of α = 0.8M⊙(Kkms−1pc2)−1, appropriate for ULIRGs is adopted here
(Solomon et al. 1997; Downes & Solomon 1998; SV05). The derived molecular gas masses
are in the range, 0.7×1010to 2.5×1010M⊙(Table 4) with a median value of 1.8×1010M⊙.
CO(1−0)≈ 0.78 and L′
CO(1−0)≈ 0.88 from the
CO(1−0)). We then
CO(1−0). An integrated CO intensity-to-gas mass
We plot the molecular gas mass distribution of the z∼6 quasars, CO-observed SMGs,
and 1.4 ≤ z ≤ 5 quasars in the left panel of Figure 4. The range of gas masses of the z∼6
quasars (0.7 × 1010to 2.5 × 1010M⊙) appears narrower but still comparable to the typical
values found in the other two samples. We performed the standard Kolmogorov-Smirnov
test between the z∼6 quasars and the other two samples. The probabilities that the data
are drawn from the same parent population are 19% for the z∼6 and 1.4 ≤ z ≤ 5 quasar
samples, and 16% for the z∼6 quasar and SMG samples. This agrees with the results from
previous studies that the high-z FIR and CO luminous quasars show molecular gas mass
distribution similar to that of the SMGs (Carilli et al. 2006; Coppin et al. 2008a).
4.2. CO line widths
The CO line widths (FWHM) of the z∼6 quasar sample are spread over a wide range,
from 160kms−1to 860kms−1, with a median value3of 360kms−1. The broadest CO line de-
3The median value is the average of the two middle values if an even number of data points are presented
in the sample.
– 13 –
tection (J0840+5624) appears to be double-peaked with peak-velocity offsets of ∼ 270kms−1
(see Figure 1). Similar line profiles are widely observed in samples of SMGs (Greve et al.
2005; Weiß et al. 2005a; Tacconi et al. 2006), which may be due to either uncoalesced
molecular gas components in a galaxy merger or simply reflect a large inclination angle of
an extended gas disk relative to the sky plane.
In the right panel of Figure 4, we compare the CO line-width distribution of the z∼6
quasar sample to that of the CO-detected SMGs (Greve et al. 2005; Tacconi et al. 2006) and
1.7 ≤ z ≤ 5 quasars (SV05; Coppin et al. 2008a). The Kolmogorov-Smirnov tests return
probabilities of 13% for z∼6 quasars and SMGs, and 71% for z∼6 and 1.7 ≤ z ≤ 5 quasars,
i.e. there is no significant difference in the line-width distributions. This result confirms
that the observed line-width distribution of high-z CO and FIR quasars is comparable to
that of the SMGs as was found in Coppin et al. (2008a). The systemic difference reported
in previous works (Greve et al. 2005; Carilli & Wang 2006) is likely to be due to selection
effects and small sample size.
4.3. Molecular CO excitation
The detection of CO (6-5) line emission in all eight z∼6 quasars in our sample, implies
highly excited molecular gas in their host galaxies. Four of the CO-detected z∼6 quasars,
J0840+5624, J0927+2001, J1048+4637, and J1148+5251, have observations of multiple CO
line transitions (Bertoldi et al. 2003b; Carilli et al. 2007; this work). The CO ”excitation
ladder” of J1148+5251 has been studied by Bertoldi et al. (2003b) and Riechers et al. (2009),
and the best LVG model fitted to the data suggests CO emission from a single gas component
with kinetic temperature of Tkin= 50K and a molecular gas density of ρgas(H2) = 104.2cm−3.
Similar CO excitation conditions are also found in the CO and FIR luminous quasar BR
1202-0725 at z=4.69 (Carilli et al. 2002; Riechers et al. 2006) and some nearby starburst
galaxies (e.g., the high excitation component in M82, Weiß et al. 2005b). In Figure 5, we plot
the CO excitation ladders of J0840+5624, J0927+2001, and J1048+4637, together with the
LVG models of J1148+5251, BR 1202-0725, and the high excitation component in M82. The
line intensity ratios of ICO(5−4)/ICO(6−5)for J0840+5624 and lower limit of ICO(3−2)/ICO(6−5)
for J1048+4637 are consistent with the model values. J0927+2001 shows a lower value of
ICO(5−4)/ICO(6−5)compared to the model, which, however, is still marginally consistent given
the large uncertainties in the measurements of both transitions. These results suggest that
such a warm, highly excited, single component gas model is a reasonable description of the
molecular gas in all of these quasar host galaxies. Further observations of additional CO
transitions in the CO-detected z∼6 quasars will address how the CO excitation properties
– 14 –
vary among these objects.
4.4. FIR-to-CO luminosity relationship
The FIR luminosities, LFIR, of the eight CO-detected quasars at z∼6 are derived using
the submillimeter and millimeter continuum measurements from the literature and the CO
observations in this work (Bertoldi et al. 2003a; Beelen et al. 2006; Wang et al. 2007; 2008a,
2008b; Carilli et al. 2007), as listed in Table 4. A dust emissivity index of β = 1.6 (Beelen
et al. 2006) is adopted in these calculations. J1148+5251 has a fitted dust temperature,
Tdust, of 56 K (Beelen et al. 2006) and J0927+2001 has Tdust = 46K, from fits to the
submillimeter and millimeter measurements (this work; Wang et al. 2007, 2008a; Carilli et
al. 2007; Riechers et al. 2009). We adopt the average dust temprature of 47 K found in
high-z quasar host galaxies (Beelen et al. 2006) for the other six objects (see Wang et al.
We plot LFIRversus L′
local spiral, infrared luminous galaxies (LIRGs, Gao & Solomon 2004), ULIRGs (Solomon
et al. 1997), the 1.4 ≤ z ≤ 5 quasar and SMG samples discussed above. For the high-z
samples, LFIRis re-calculated using (sub)mm measurements from the literature (Smail et
al. 2002; Kov´ acs et al. 2006b; Chapman et al. 2005; Omont et al. 2003; Benford et al.
1999; Beelen et al. 2006), using the β value mentioned above. We assume Tdust= 47K for
the objects with less than three (sub)mm measurements in the quasar sample (Beelen et al.
2006), while Tdust= 35K is used for the SMGs, which is typical for the Tdustvalues found
in previous (sub)mm studies (Kov´ acs et al. 2006b; Coppin et al. 2008b). The z∼6 quasars
fall along the trend defined by all the other samples systems within the scatter (see the open
stars in Figure 6).
CO(1−0)for the eight z∼6 quasars in Figure 6, and compare with
A fit to the samples of local spirals, LIRGs, ULIRGs, and SMGs using the Ordinary
Least Squares Bisector method (Isobe et al. 1990) yields a relationship of logLFIR= 1.67 ×
z∼6 quasars all lie above this relationship. There are a number of undetermined parameters
for individual objects, e.g., unknown AGN contributions to the FIR emission, different dust
temperatures and CO line ratios, which may account for the offsets and scatters. Further
observations at infrared, submillimeter, and millimeter wavelengths will be important to
directly probe the infrared SEDs and better constrain the dust temperatures and AGN
contributions for these objects. For a rough estimation, we derive the AGN contributions
to the FIR emission for our sample with the quasar rest-frame 1450˚ A continuum (Table 1)
CO(1−0)−4.87 for these typical star-forming systems, which is consistent with the result of
CO(1−0)−5 quoted in SV05 with similar samples. We also notice that the
– 15 –
and a FIR-to-1450˚ A luminosity ratio of LFIR/νLν,1450˚
quasar template (Elvis et al. 1994). The estimated AGN-dominated FIR emission is 0.9–
4.4 × 1012L⊙ for the eight sources, which accout for about 30% of the FIR luminosities we
calculated above with the (sub)mm observations on the average. We subtract these from the
original LFIRvalues (Table 1) and the corrected data points (see the filled stars in Figure
6) show a better agreement with the FIR-to-CO luminosity relationship for star-forming
A= 0.14 from the local radio quiet
5.1.Star forming activity in the millimeter and CO-detected quasars at z∼6
CO emission has been detected toward the eight z∼6 quasars with published 250 GHz
dust continuum flux densities of S250GHz≥ 1.8mJy, indicating ? 1010M⊙of molecular gas
in the host galaxies of the FIR luminous quasars at the earliest epoch. The derived FIR and
CO (1-0) luminosities follow the LFIR− L′
at low and high redshifts, suggesting a star-formation origin of a dominant fraction of the
FIR emission in most of these objects, i.e. the excess FIR dust emission is powered by star
forming activity in the circumnuclear region, not the central AGN (Bertoldi et al. 2003a;
Wang et al. 2008b; Walter et al. 2009). This is consistent with the idea of co-eval star
formation with SMBH accretion in the massive z∼6 quasar systems which are bright at both
UV-optical and FIR wavelengths (eg. Bertoldi et al. 2003a, 2003b; Wang et al. 2007, 2008a,
2008b; Walter et al. 2003, 2004, 2009).
COrelationship defined by star forming systems
The measurements of (sub)mm dust continuum and molecular CO line emission from
the quasar host galaxies provide the first constraint on the star forming activities in these
objects. Using the AGN contribution-removed FIR luminosities derived in the previous
section (Table 1), we estimate the star formation rates (SFRs)4for the eight z∼6 quasars to
be ∼ 530 to 2380M⊙yr−1(Kennicutt 1998). The ratios between SFR and Mgas(i.e. SFR
per solar mass of molecular gas) provide a measure of the star formation efficiency, which are
about 5 × 10−8yr−1to 1 × 10−7yr−1. This is comparable to that of the SMGs, 1.4 ≤ z ≤ 5
CO-detected quasars, and local ULIRGs (SV05), and systematically higher than that of the
local normal spiral disks (Kennicutt 1998), and high-z star-forming disk galaxies (e.g., the
z∼1.5 BzK galaxies, Daddi et al. 2008, 2009), suggesting a high star formation efficiency in
4Calculated with SFR ∼ 1.7 × 10−10LIR(M⊙yr−1), where LIRis the infrared luminosity (8-1000µm) in
units of L⊙. We assume LIRis ∼ 1.5LFIRfor 40∼60 K warm dust emission.
– 16 –
the host galaxies of these FIR and CO luminous z∼6 quasars similar to that of the extreme
starburst systems (Tacconi et al. 2006; Walter et al. 2009). The inverse gives the gas
depletion time scales of τdep= Mgas/SFR ∼ 1–2 × 107yr.
5.2. Black hole–bulge evolution
Massive bursts of star formation are likely to be on-going in these FIR and CO bright
quasars at z∼6. In this section, we investigate the possible constraints of the black hole-
bulge masses and velocity dispersion relationships in these z∼6 quasar systems using the
available CO measurements. The available SMBH masses and AGN bolometric luminosities
(Lbol) from the literature are listed in Table 1. For other objects, we adopt the Lbolvalues
estimated in Wang et al. (2008b) from M1450, corrected to the CO redshifts, and estimated
the black hole masses assuming Lbol/LEDD= 1 and MBH= Lbol(ergss−1)/1.26 × 1038M⊙.
The estimated MBHvalues are all of the order of 109M⊙
The dynamical masses (Mdyn) of the eight z∼6 quasars can be expressed as Mdyn ≈
2.3 × 105vcir2R (e.g., Neri et al. 2003; Walter et al. 2004; SV05; Narayanan et al. 2008),
where R is the disk radius in kpc and vcir is the maximum circular velocity of the gas
disk in km s−1. High-resolution CO observations of J1148+5251 give a gas disk radius
of ∼2.5 kpc and a dynamical mass of 4.5×1010M⊙. In the absence of spatially resolved
measurement for our sample quasars, we here assume a disk radius of 2.5 kpc also for the
other seven objects, and estimate vcirusing 1/2 of the full width at 20% maximum (Ho 2007a,
2007b), which corresponds to about 3/4 of the CO FWHM for a single-Gaussian profile i.e.
vcir= 3FWHM/4sini, where i is the inclination angle of the molecular gas disk. We leave
sini as an unknown factor in these calculations. The derived Mdynsin2i are from 8.4 × 109
to 2.4 × 1011M⊙(Table 4). For 0840+5624, if we adopt the average peak-velocity offset of
270 kms−1as the disk circular velocity, the Mdynsin2i will reduce from 2.4 × 1011M⊙ to
4.2 × 1010M⊙.
We then estimate the masses of the stellar components (Mbulge) in the spheroidal bulges
as Mbulge= Mdyn− Mgas. We adopt a series of inclination angle values, i.e. i=1◦, 5◦, 10◦,
20◦, 30◦, 40◦, 50◦, 60◦, and 90◦, and derive Mdynand Mbulgeaccordingly. The corresponding
black hole-bulge mass ratios (MBH/Mbulge) versus different inclination angles are ploted in
Figure 7, compared to the local mass relationship of MBH∼ 0.0014Mbulge. The plot shows
– 17 –
that if the sources were falling on the local black hole-bulge mass ratio of 0.0014 this would
require inclination angles from < 5◦to ∼ 25◦, or < 5◦to 15◦when the two broadest line
objects are excluded.
However, the observed distribution of CO line widths of the z∼6 quasar sample is
similar to that of the SMGs, which argues against systematically smaller inclination angle
values. Moreover, these z∼6 quasars are all among the brightest objects in the quasar
population with AGN bolometric luminosities of Lbol≥ 1013L⊙. Recent studies showed that
the unobscured fraction of quasars is increasing with quasar luminosity, and for objects with
Lbol≥ 1013L⊙, this fraction is probably larger than 50% (Simpson 2005; Treister et al. 2008).
This is consistent with the receding torus model and implies a very large opening angle of
the unobscured cone in these most luminous quasars, i.e. the central AGNs can be directly
seen over an inclination angel range of i from 0◦to probably ≥ 60◦(Elitzur 2008). Thus,
though we cannot rule out small inclination angle values for individual objects, it is highly
unlikely that more than half of these CO detected z∼6 quasars are viewed in the extreme
inclination angle range of i < 15◦. The CO estimated bulge dynamical masses probably
reflect intrinsically smaller MBH/Mbulgefor these z∼6 quasars compared to that of the local
We emphasize that accurate calculations of the bulge dynamical and stellar masses
still require high-resolution measurements of the disk size and geometry for each object. To
roughly constrain the average black hole-bulge mass ratio of these FIR and CO luminous z∼6
quasars here, we simply adopt an average inclination angle of 40◦based on the assumption
of uniformly distributed i between 0◦and 60◦. This results in a median MBH/Mbulgeratio
of 0.022 for our sample, which is about fifteen times higher than the present day value of
0.0014. This is in good agreement with the picture suggested by other high-z MBH− Mbulge
studies (e.g., Walter et al. 2004; Coppin et al. 2008a; Riechers et al. 2008a, 2008b; Peng et
al. 2006a, 2006b), i.e. that the SMBH accumulates most its mass before the formation of
the stellar bulge.
For further consideration, if these z∼6 quasars will finally evolve into systems with black
hole-bulge relationships identical to that in the local universe, the final bulge stellar mass
should be around 1012M⊙with black hole masses on the order of 109M⊙. The MBH/Mbulge
ratio we derived above suggests that only ?10% of the stellar component has been formed
in these FIR and CO bright z∼6 quasars on the average. On the other hand, the detected
molecular gas mass of ∼ 1010M⊙in these objects can only account for < 3% of the mature
bulge mass if the gas will all be converted to stars. Thus, large amounts of gas supplied from
external sources is required to form the 1012M⊙stellar bulge by z=0.
– 18 –
We investigate the black hole mass-bulge velocity dispersion relation with the eight
CO-detected z∼6 quasars in Figure 8. The supermassive black hole mass (MBH) and bulge
velocity dispersion (σ) in local galaxies follow a relationship of log(MBH/M⊙) = 8.13 +
4.02log(σ/200kms−1) (Tremaine et al. 2002). We first roughly estimate σ for the eight z∼6
quasars with the CO line widths, using the empirical relation σ ≈ FWHM/2.35 (Shields
et al. 2006; Nelson et al. 2000). The results are plotted in Figure 8, together with the
local active and inactive galaxies from Tremaine et al. (2002) and CO-detected 1.4 ≤ z ≤ 5
quasars that have available SMBH mass measurements (Shields et al. 2006; Coppin et al.
2008a). Most of the high-z CO-detected quasars, especially the objects with MBH≥ 109M⊙,
are above the local MBH− σ relationship with offsets of more than one order of magnitude
in black hole mass, as was found in Shields et al. (2006). The median value of the ratios
between MBHand the expected black hole masses from the local MBH–σ relation (MBH,σ) for
our z∼6 sample is ?MBH/MBH,σ?median∼ 40.
This is consistent with the high MBH/Mbulgeratios and the idea of prior SMBH forma-
tions we discussed in the previous section (§5.2.1). However, the systemic offset between the
high-z objects and the local MBH− σ relation (i.e. ?MBH/MBH,σ?median) can be reduced if
we include assumptions of inclination angles in our estimations of σ, following the empirical
correlations between σ and inclination angle-corrected CO line width from the literature (eg.
Ho 2007a, 2007b, Wu 2007). In particular, if we consider the possible gas disk inclination
angle range for these z∼6 quasars and adopt an average value of i = 40◦( as was discussed
in §5.2.1), the derived ?MBH/MBH,σ?medianwill reduce to ∼26 and 4 following the methods
described in Wu (2007) and Ho (2007a), respectively.
We also notice that our sample exhibits significant scatter on the MBH–σ plot with
derived MBH/MBH,σvalues over three orders of magnitude. This is mainly due to the intrinsic
scatter and offset between the real bulge velocity dispersions and the CO estimated values
based on the empirical relationships of low-z samples. Indeed, how the observed CO line
widths trace the bulge velocity dispersions in the high-z CO-detected quasars is still an
open question. These objects are the most massive SMBH-host systems in the universe with
SMBH masses of a few 109to 1010M⊙and experiencing massive star formation in the central
kpc scale regions (Walter et al. 2009). We will expect the sensitive interferometer arrays
(such as ALMA and the EVLA) to directly probe the gas properties e.g., the gas distribution,
geometry, dynamics (virialized or not), line profiles, and contributions from turbulence, etc.
in their spheroidal hosts and finally address the MBH−σ correlation in these high-z FIR and
CO luminous quasars.
– 19 –
We present new observations of molecular CO line emission and 350 µm dust continuum
emission in quasar host galaxies at z∼6. Our most important finding is that high-order CO
transitions are detected in all six of the z∼6 quasars observed with the 3 mm receiver on the
PdBI. The new CO detections all have observed 250 GHz dust continuum flux densities of
S250GHz≥ 1.8mJy. These results, together with previous CO-detections in another two ob-
jects, reveal an extremely high CO detection rate in the FIR luminous quasars at z∼6. With
the final sample of eight CO-detected z∼6 quasars, we study the molecular gas properties in
the earliest quasar host galaxies, and the main results are summarized as follows:
The CO emission indicates molecular gas masses of 0.7 to 2.5 × 1010M⊙in the quasar
host galaxies. The observed CO line widths are spread over a wide range from 160 to
860kms−1, with a median value of about 360kms−1. The gas mass and CO line width
distributions of the z∼6 quasars are consistent with samples of CO-observed SMGs and
quasars at 1.4 ≤ z ≤ 5.
The CO and FIR luminosities of the eight z∼6 quasars follow the LFIR−L′
derived for local spirals, LIRGs, ULIRGs, high-z SMGs, and CO-detected quasars, though
the weakest CO detection has the largest offset from the trend. This is consistent with the
idea of co-eval star formation with rapid growth of the supermassive black hole in the early
quasar-host systems. The derived SFRs are from ∼ 530 to 2380M⊙yr−1. The corresponding
star formation efficiencies indicated by the ratios of SFR/Mgasare consistent with the extreme
starburst systems at low and high redshifts.
We investigate the black hole-bulge correlations of these FIR and CO luminous quasars
at z∼6 using the CO measurements. Based on certain assumptions of the molecular gas disk
size, average inclination angle, and σ-CO line width relation, we estimate the bulge dynamical
masses and velocity dispersions for our sample and compare them to the local black hole-
bulge relationships. The results suggest that the black hole masses of these z∼6 quasars
are typically an order of magnitude higher than the values expected from the present-day
relationships, which is consistent with the idea that the formation of the SMBHs occurs prior
to that of the stellar bulges in the massive high-z quasar-galaxy systems. However, we also
recognize that there are large uncertainties in the estimations of Mbulgeand σ for individual
objects due to unknown gas distribution, disk inclination, and dynamics. Further high-
resolution observations of quasar host galaxies should focus on these FIR and CO luminous
z∼6 quasars to fully understand the black hole-bulge evolution at the highest redshift.
This work is based on observations carried out with the IRAM Plateau de Bure In-
– 20 –
terferometer, the Green Bank Telescope (NRAO), the Very Large Array (NRAO), and the
SHARC-II bolometer camera at the Caltech Submillimeter observatory (CSO). IRAM is sup-
ported bu INSU/CNRS (France), MPG (Germany) and IGN (Spain). The CSO is supported
by the NSF under AST-0540882. The National Radio Astronomy Observatory (NRAO) is a
facility of the National Science Foundation operated under cooperative agreement by Asso-
ciated Universities, Inc. We thank the anonymous referee for useful comments. Ran Wang
thank Dr. Alexandre Beelen at Institut d’Astrophysique Spatiale for helpful discussions and
suggestions. We acknowledge support from the Max-Planck Society and the Alexander von
Humboldt Foundation through the Max-Planck-Forschungspreis 2005. Dominik A. Riechers
acknowledges support from from NASA through Hubble Fellowship grant HST-HF-51235.01
awarded by the Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. Ran
Wang acknowledges support of the National Natural Science Foundation of China grant
10833006 and grant 0816341034.
Facilities: IRAM:30m (MAMBO), VLA, Sloan (SDSS) CSO (SHARC-II)
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– 25 –
Table 1: Optical and the millimeter measurements of the sample
(9)(1) (2)(3)(4) (7)
−15 ± 24
7 ± 13
35 ± 10
Note – We list the SDSS name in Column (1), the redshifts, AB magenitudes at rest-frame 1450˚ A from the discovery paper in
Column (2), (3), and (4). The 250 GHz and 1.4 GHz continuum measurements from the literatures are summarized and list in
Column (5) and (6). Column (7) gives the references for the optical, millimeter, and radio data, with  -Fan et al. (2006a);
-Wang et al. (2007); -Wang et al. (2008b); -Fan et al. (2000); -Petric et al. (2003); -Fan et al. (2003); -Bertoldi
et al. (2003a); -Carilli et al. (2004); -Cool et al. (2006); -Jiang et al. (2008). The last two columns list the available
measurements of AGN bolometric luminosities and black masses from Jiang et al. (2006).
aDerived with the absolute AB magnitude at rest-frame 1450˚ A from Cool et al. 2006.
Table 2: Measurements of the CO emission
Jy km s−1
J1048NE6−5 6.2259±0.0019 30.1–––
Note – Column (1), source names; Column (2), CO transitions observed in this work; Column (3), Column (4), and Column
(5), redshifts, line widths, and flux measured with the CO detection; Column (6), on-source time; Column (7), dust continuum
measurements under the CO line spectrum observed with PdBI; Column (8) and (9), dust continuum measurements and on-
source integration time with SHARC-II at 350 µm.
aUpper limits derived with the channel-to-channel rms noise. One should be cautious with this value as there is a small-scale
structure close to the line frequency which preclude detection of weak line emission with peak flux density of a few hundred
µJy (Wagg et al. 2008);bCarilli et al. (2007), measured with the CO (6-5) and (5-4) line emission;cline width calculated with
the 50 MHz channel width;dWang et al. (2008a);ePrimary beam attenuation-corrected line flux for J1048NE (see Figure 1).
– 26 –
Table 3: Redshift measurements
5.78 (3)5.745±0.030(4) 5.80±0.02
J1048+4636 6.2284±0.0017 6.23±0.05(6) 6.203
aRedshift measured from the CO (7-6) line (Bertoldi et al. 2003b).
bRedshift measured from the CO (6-5) line (Bertoldi et
cRedshift measured from other UV emission lines, such as the the CIII] λ1909˚ A, OI+SiII λ1302˚ A, and SiIV+OIV
References: (1) Fan et al. 2006a; (2) Ryan-Weber et al. 2009; (3) Freudling et al. 2003; (4) Goodrich et al. 2001; (5) Fan et
al. 2000; (6) Fan et al. 2003; (7) Iwamuro et al. 2004; (8) Maiolino et al. 2004a; (9) Maiolino et al. 2004b; (10) Willott et al.
2003; (11) Cool et al. 2006; (12) Jiang et al. 2007; (13) Jiang et al. 2008.
Table 4: Derived parameters
Note – Column (1), source name; Column (2), line luminosities of a. the CO (5-4) transition (Carilli et al. 2007; this work), b.
the CO (6-5) transition (this work), and c. the CO (3-2) transition (Walter et al. 2003; Riechers et al. 2009) in 1010Kkms−1pc2
(see e.g., SV05 for the calculation), which are used to derive L′
CO(1−0)in the next column (See §4.1 for details); Column (3),
the derived CO (1-0) luminosity; Column (4), FIR luminosity derived with the submillimeter and millimeter continuum data
from the literature, and the values quoted in brackets are the AGN-corrected FIR luminosity (See §4.4 for details); Column (5)
star formation rate derived with the AGN-corrected FIR luminosity; Column (6) dynamical masses derived with the observed
CO line widths as was described in §5.2.1. The two values for J0840+5624 are derived with the single-Gaussian fitted FWHM
of 860kms−1and the average peak offset of 270kms−1, respectively. We adopt the CO line widths of FWHM = 600kms−1
for J0927+2001 from Carilli et al. (2007), and FWHM = 297kms−1from Walter et al. (2009) for J1148+5251. Column (7),
molecular gas masses derived with L′
– 27 –
Fig. 1.— The CO line spectra (left) and velocity-integrated images (right) of the six CO-
detected quasars at z∼6. The top abscissa of the left plots give the redshift range of the
spectra windows, and the zero velocities correspond to the redshifts of z=5.844, 5.778, 6.200,
5.950, 5.877, and 6.070. The error bars in the spectra denote the 1σ rms noise in each
channel, and the solid lines show Gaussian fits to the line emission. The third spectrum for
J0840+5624 is a combined spectrum obtained by merging the data of the two detected CO
transitions. In the spectrum of the NE source close to J1048+4637, the gray line indicates
the primary beam attenuation-corrected line profile, assuming a line width of 370 km s−1.
The crosses on the images indicate the optical quasar position, and the dashed circles shows
the primary beam of the PdBI. On the map of J1044-0125, the spurious pattern to the east
and west of the source is caused by the substantial sidelobes due to the strong signal from
the central source.
– 28 –
Fig. 1.— Continued.
– 29 –
(left) and SHARC-II detected source J09227+2001 (right), smoothing to a beam size of
FWHM = 12.4′′. The contour levels are (2,3,4,5,6)×2.2mJybeam−1for J0840+5624, and
(2,3,4)×2.7mJybeam−1for J0927+2001. The crosses denote the optical quasar positions.
2.— The 350 µm maps of the SHARC-II marginally detected source J0840+5624
RIGHT ASCENSION (J2000)
10 48 45.1545.1045.05 45.0044.95
46 37 19.0
RIGHT ASCENSION (J2000)
10 48 45.1545.1045.0545.0044.95
46 37 19.0
Fig. 3.— Left–The VLA Q band image of the CO (3-2) line emission from J1048+4637
detected at 47.865 GHz (Left) and a 48 GHz image averaged over the line-free channels
(right). The 1σ rms values are 70 µJy beam−1and 65 µJy beam−1for the left and right
images, respectively. The contour levels are (-2, 2, 3, 4) × 60 µJy beam−1for both images.
The ellipses indicate the beam sizes of 0.5′′×0.5′′(FWHM) for the left plot and 0.4′′× 0.4′′
for the right one, and the cross marks the optical quasar position.
– 30 –
Fig. 4.— The molecular gas mass (left) and CO line width (right) distributions of the
CO-detected z∼6 quasars, the SMGs, and 1.4 ≤ z ≤ 5 quasars.
– 31 –
Fig. 5.— CO spectral energy distributions of J0840+5624 (blue circles), J0927+2001 (red
triangles), and J1048+4637 (black squares). The arrow with black square represents the
VLA detection of the CO (3-2) line emission from J1048+4637 in a 50 MHz channel, which
provides a lower limit of the total line intensity due to the narrow bandwidth. The error bars
show the 1σ uncertainties of the integrated line intensities. The solid line and dashed line
show the LVG models of J1148+5251 with Tkin= 50K, ρgas(H2) = 104.2cm−3from Riechers
et al. (2009) and BR 1202-0725 with Tkin= 60K, ρgas(H2) = 104.1cm−3from Riechers et
al. (2006), respectively. The dotted line represent the LVG model of the high excitation
component in the nearby starburst galaxy M82 with Tkin= 50K, ρgas(H2) = 104.2cm−3from
Weiß et al. (2005b) We also plotted the CO excitation ladder of the Milky Way inner disk
region for comparison (diamonds with light dashed line, Fixsen et al. 1999). The CO line
intensities are normalized to the CO (3-2) line for the Milky Way, and CO (6-5) for other
sources and models.
– 32 –
Fig. 6.— LFIR vs. CO luminosity (L′
Gao & Solomon (2004), Greve et al. (2005), Tacconi et al. (2006), Solomon et al. (1997),
SV05, Riechers et al. (2006), Maiolino et al. (2007) and Coppin et al. (2008a). The
dashed line represents the relationship LFIR∝ L′
LIRGs, ULIRGs and SMGs. The open stars represent the eight CO detected quasars at z∼6
with LFIRestimated from (sub)mm observations, and the filled stars denote the values when
contributions from AGN are subtracted (see §4.4 for details).
CO). The data of samples at z≤5 are taken from
1.67fitted to the samples of local spirals,
– 33 –
Fig. 7.— The derived ratios between the black hole and the bulge stellar masses versus
the inclination angles of the molecular disk. The bulge dynamical masses (Mdynsin2i) are
derived using the CO FWHMs from single-Gaussian spectral fitting, and the bulge stellar
masses (Mbulge) are estimated with Mdyn−Mgas. The plot shows how MBH/Mbulgecompares
to present day value of 0.0014 (dashed line) with different assumptions of inclination angles.
– 34 –
Fig. 8.— The black hole masses of the z∼6 quasars versus their bulge velocity dispersion
(σ). The dashed line denotes the local MBH− σ relationship of log(MBH/M⊙) = 8.13 +
4.02log(σ/200kms−1) (Tremaine et al. 2002). The filled circles represent the local galaxies
from Tremaine et al. (2002). The filled squares and open diamonds are for the z∼6 and
1.4 ≤ z ≤ 5 quasar samples, respectively, with σ derived from the observed CO line width
using σ ≈ FWHM/2.35. The open squares show the σ values derived with the method
described in Ho (2007a) assuming an average inclination angle of 40◦for the z∼6 quasars.