Molecular Gas in Redshift 6 Quasar Host Galaxies

Abstract

We report our new observations of redshifted carbon monoxide emission from six z~6 quasars, using the PdBI. 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 GBT and the VLA, and dust continuum from five of these sources with the SHARC-II bolometer camera at the CSO. 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 10^10 M_sun 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 $1.4\leq z\leq5$ quasars and submillimeter galaxies. Most of the 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 results place important constraints on the formation and evolution of the most massive SMBH-spheroidal host systems at the highest redshift. Comment: 34 pages, 8 figures, accepted for publication in ApJ

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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
ABSTRACT
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,
China
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
11Hubble Fellow

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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
1. Introduction
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

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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;

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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
2.1.Sample
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

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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.
2006).
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