Annu. Rev. Astron. Astrophys. 2005. 43: 677-725
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2.1. Luminosities: Basic Relations

The calculation of high-redshift source properties from the observation of molecular emission lines requires care with respect to the cosmology assumed. This is important when comparing published source properties, as different cosmologies can lead to significantly different values for properties such as luminosity, size, mass. In this review we have assumed a cosmology with Omegam = 0.3, OmegaLambda = 0.7, and H0 = 70 km s-1 Mpc-1.

The CO line luminosity can be expressed in several ways. From energy conservation, the monochromatic luminosity, observed flux density, and luminosity distance are related by nurest L(nurest) = 4 pi DL2 nuobs S(nuobs), yielding

Equation 1 (1)

where the CO line luminosity, LCO, is measured in Lodot; the velocity integrated flux, SCO Delta v, in Jy km s-1; the rest frequency, nurest = nuobs (1 + z), in GHz; and the luminosity distance, DL, in Mpc. 1

The CO line luminosity is often expressed (Solomon et al. 1997) in units of K km s-1pc2 as the product of the velocity integrated source brightness temperature, Tb Delta v, and the source area, Omegas DA2, where Omegas is the solid angle subtended by the source. The observed integrated line intensity, ICO = integ Tmbdv, measures the beam diluted brightness temperature, which decreases with redshift, Tb Deltav Omegas = ICO Omegas ⋆ b (1 + z), where Omegas ⋆ b is the solid angle of the source convolved with the telescope beam. Then the line luminosity L'CO = Tb Deltav Omegas DA2 = Omegas ⋆ b DL2 ICO (1 + z)-3, or

Equation 2 (2)

where L'CO is measured in K km s-1 pc2, Omegas ⋆ b in arcsec2, DL in Mpc, and ICO in K km s-1 . If the source is much smaller than the beam, then Omegas ⋆ b approx Omegab.

The line luminosity, L'CO, can also be expressed for a source of any size in terms of the total line flux, L'CO = (c2 / 2k) SCO Deltav nuobs-2 DL2 (1 + z)-3, or

Equation 3 (3)

Because L'CO is proportional to brightness temperature, the L'CO ratio for two lines in the same source is equal to the ratio of their intrinsic brightness temperatures averaged over the source. These ratios provide important constraints on physical conditions in the gas. For thermalized optically thick CO emission the intrinsic brightness temperature and line luminosity are independent of J and of rest frequency. For example, L'CO (J = 3-2) = L'CO (J = 1-0).

By observing CO emission from higher J transitions for high- redshift galaxies researchers can maintain the same approximate observed frequency as redshift increases. Equations 2 and 3 show that for fixed line luminosity (L'CO) and a fixed observed frequency (or a fixed beam size), the observed integrated line intensity and the integrated flux do not scale as the inverse square of luminosity distance (DL-2), but rather as (1 + z)3 DL-2 . This substantial negative K-correction (Solomon, Downes & Radford 1992a, b) is one of the reasons the relatively clear 3-mm atmospheric window, with instruments developed for observation of CO(1-0) in the local Universe, has been the most important wavelength band for observations of CO from EMGs at z geq 2.

A significant fraction of the EMGs are gravitationally imaged by an intervening galaxy. The luminosities L and L' calculated without correction for the magnification by the gravitational lens are, therefore, only apparent luminosities. If a model of the gravitational lens is available, the intrinsic luminosities can be calculated from Lint = Lapp / µ and L'int = L'app / µ, where µ is the area magnification factor of the gravitational lens. Wiklind & Alloin (2002) have reviewed gravitational lensing of EMGs.

2.2. From CO Luminosity to Molecular Mass

Observation of emission from CO rotational transitions is the dominant means of tracing interstellar molecular clouds, which consist almost entirely of molecular hydrogen, H2. Molecular hydrogen rather than atomic hydrogen is the principal component of all interstellar clouds with density n > 100 cm-3 owing to a balance between formation on dust and self-shielding of H2 from photodissociation (Solomon & Wickramasinghe 1969) by the interstellar radiation field. This transition from atomic to molecular hydrogen at a moderate interstellar density means that all dense clouds are molecular. Molecular clouds are the raw material for star formation and a critical component in the evolution of galaxies. The first generation of stars must have formed, in the absence of heavy elements, from HI with only trace amounts of H2 available to provide essential cooling. However, the huge IR luminosity seen in ULIRGs and EMGs is clearly emitted by interstellar dust, and we can expect all dense, dusty clouds to be molecular. H2 has strongly forbidden rotational transitions, and the H2 vibration-rotation lines require high temperature to be produced, for example, by UV excitation or shocks. In the absence of these special circumstances, the H2 is invisible.

CO emission is the best tracer of molecular hydrogen for two reasons. It is a very stable molecule and the most abundant molecule after H2. Second, a weak dipole moment (µe = 0.11 Debye) means that CO rotational levels are excited and thermalized by collisions with H2 at relatively low molecular hydrogen densities. Strong CO emission from interstellar gas dominated by H2 is ubiquitous. The critical density necessary to produce substantial excitation of a rotational transition is given approximately by n(H2) geq A/C where A is the Einstein coefficient for spontaneous decay and C is the collisional rate coefficient. The A coefficient scales as µ2 nu3 where µ is the dipole moment and nu(J, J - 1) = 2BJ for a simple rotational ladder, is the frequency of the transition . In practice the critical density is lowered by line trapping for CO emission and for emission from other optically thick tracers such as HCN and CS. The full multi level excitation problem must be solved usually using the LVG (large velocity gradient) approximation (Scoville and Solomon 1974; Goldreich and Kwan 1974). The effective density for strong CO emission ranges from n(H2) approx 300 cm-3 for J = (1-0) to approx 3000 cm-3 for J = (4-3) or (5-4). Of course the higher J transitions also require a minimum kinetic temperature for collisional excitation.

For high-z galaxies there is another obvious requirement for strong CO emission. The large quantities of dust and molecular gas observed in EMGs clearly indicate not only ongoing star formation but also substantial enrichment by previous star formation. Researchers have known for some time that many quasar emission line regions show substantial metallicity; EMGs have not only a high metallicity, but also a huge mass of enriched interstellar matter much larger and more extensive than that of a quasar emission line region.

The H2 mass-to-CO luminosity relation can be expressed as

Equation 4 (4)

where M(H2) is defined to include the mass of He, so that M(H2) = Mgas, the total gas mass, for molecular clouds. For the Galaxy, three independent analyses yield the same linear relation between the gas mass and the CO line luminosity: (a) correlation of optical/IR extinction with 13CO in nearby dark clouds (Dickman 1978); (b) correlation of the flux of gamma rays, produced by cosmic ray interactions with protons, with the CO line flux for the Galactic molecular ring (Bloemen et al. 1986, Strong et al. 1988); and (c) the observed relations between virial mass and CO line luminosity for Galactic giant molecular clouds (GMCs) (Solomon et al. 1987), corrected for a solar circle radius of 8.5 kpc. All these methods indicate that in our Galaxy, alpha ident Mgas / L'CO = 4.6 Modot(K km s-1 pc2)-1 (Solomon & Barrett 1991). (Some authors use X rather than alpha as a symbol for this conversion factor, even though X by convention relates H2 column density and line-integrated CO intensity.)

For a single cloud or an ensemble of nonoverlapping clouds, the gas mass determined from the virial theorem, Mgas, and the CO line luminosity, L'CO, are related by

Equation 5 (5)

where m' is the mass of an H2 molecule multiplied by 1.36 to account for He, n(cm-3) is the average H2 number density in the clouds, and Tb(K) is the intrinsic (rest-frame) brightness temperature of the CO line. Mgas is in Modot and L'CO is in K km s-1 pc2 (Dickman, Snell & Schloerb 1987; Solomon et al. 1987). This is the physical basis for deriving gas mass from CO luminosity. The existence of gravitationally bound clouds is confirmed by the agreement between alpha determined from application of the virial theorem, using measured velocity dispersions and sizes for the Milky Way clouds, and alpha determined from the totally independent methods (a) and (b) discussed above.

Use of the Milky Way value for the molecular gas mass to CO luminosity ratio, alpha = 4.6 Modot(K km s-1 pc2)-1, overestimates the gas mass in ULIRGs and probably in EMGs. After high-resolution maps were produced for a few ULIRGs (Scoville, et al. 1991) it became apparent that the molecular gas mass calculated using the Milky Way value for alpha was comparable to and in some cases greater than the dynamical mass of the CO-emitting region. This contradiction led to a new model (Downes, Solomon & Radford 1993; Solomon et al. 1997) for CO emission in ULIRGs. Unlike Galactic clouds or gas distributed in the disks of galaxies, most of the CO emission in the centers of ULIRGs may not come from many individual virialized clouds, but from a filled intercloud medium, so the linewidth is determined by the total dynamical mass in the region (gas and stars), that is, DeltaV2 = G Mdyn / R . The CO luminosity depends on the dynamical mass as well as the gas mass. The CO line emission may trace a medium bound by the total potential of the galactic center, containing a mass Mdyn consisting of stars, dense clumps, and an interclump medium; the interclump medium containing the CO emitting gas with mass Mgas.

Defining f ident Mgas / Mdyn , the usual CO to H2 mass relation becomes (Downes, Solomon & Radford 1993)

Equation 6

Equation 6


Equation 6 (6)

where bar{n} is the gas density averaged over the whole volume. The quantity alpha L'CO measures the geometric mean of total mass and gas mass. It underestimates total mass and overestimates gas mass. Hence if the CO emission in ULIRGs comes from regions not confined by self-gravity, but instead from an intercloud medium bound by the potential of the galaxy, or from molecular gas in pressure, rather than gravitational equilibrium, then the usual relation Mgas / L'CO = alpha must be changed. The effective alpha is lower than 2.6n1/2 / Tb.

Extensive high-resolution mapping of CO emission from ULIRGs shows that the molecular gas is in rotating disks or rings. Kinematic models (Downes & Solomon 1998) in which most of the CO flux comes from a moderate density warm intercloud medium have been used to account for the rotation curves, density distribution, size, turbulent velocity, and mass of these molecular rings. Gas masses were derived from a model of radiative transfer rather than the use of a standard conversion factor. The models yield gas masses of ~ 5 × 109 Modot, approximately five times lower than the standard method, and a ratio Mgas / L'CO approx 0.8 Modot (K km s-1 pc2)-1. The ratio of gas to dynamical mass Mgas / Mdyn approx 1/6 and a maximum ratio of gas to total mass surface density µ / µtot = 1/3. This effective conversion factor alpha = 0.8 Modot (K km s-1 pc2)-1 for ULIRGs has been adopted for EMGs by many observers of high-z CO emission and we use it throughout this review. However, until a significant number of EMGs are observed with sufficient angular resolution to enable a calibration of alpha, the extrapolation in the use of alpha = 0.8 to EMGs from ULIRGs must be regarded as tentative.

2.3. Classification of the EMGs

The list of 36 EMGs reported in the literature at the time of this review are given in Table 1, together with their derived properties. The gas masses were calculated using the luminosity of the lowest available CO transition and alpha = 0.8 (see Section 2.2). All quantities assume the cosmology adopted for this review. Appendices 1, 2, and 3 at the end of this article give the observed properties from which the quantities in Table 1 were calculated. 2 The overwhelming majority of these detections were made with the IRAM interferometer. Lists of EMGs have been constructed by Cox et al. (2002), Carilli et al. (2004), Hainline et al. (2004), and Beelen ( The sources are listed in all tables and appendices in order of redshift. No blind survey for high-z CO emission has been done because of its prohibitive cost in observing time with present instruments. Were such a blind survey to be done eventually by ALMA, it could result in additional types of EMGs. Figure 1 shows the number of EMGs by type as a function of redshift. Despite the selection effects that attend the detection of EMGs, one can see that the current flux-limited sample broadly reflects the epoch where most star formation in the Universe is currently thought to occur.

Table 1: EMG Properties: line & FIR luminosities, gas & dust masses, star formation rate

EMG Redshift Transition L'(app.) LFIR(app.) Lens L'(int.) LFIR(int.) Mgas Mdust SFR tauSF
  z   (1010 L'*) a (1012 Lodot) Mag. (1010 L'*) a (1012 Lodot) (1010 Modot) (108 Modot) (Modot y-1) (106 y)

SMM J02396 1.062 CO 2-1 5.1 ± 0.5 16.3 2.5 2.0 6.5 1.6   975 16
Q0957+561 1.414 CO 2-1 0.9 ± 0.5 14 1.6 0.6 6 0.4 2.5 900 4
HR10 1.439 CO 1-0 6.5 ± 1.1 6.5 ? - - 5.2µ-1 6.8µ-1    
IRAS F10214 2.286 CO 3-2 11.3 ± 1.7 60 17 0.7 3.6 0.6   540 11
SMM J16371 2.380 CO 3-2 3.0 ± 0.6 - ? - - 2.4µ-1      
SMM J16368 2.385 CO 3-2 6.9 ± 0.6 16 ? - - 5.5µ-1      
53W002 2.393 CO 3-2 3.6 ± 0.4 - 1 3.6 - 2.9      
SMM J16366 2.450 CO 3-2 5.6 ± 0.9 20 ? - - 4.5µ-1      
SMM J04431 2.509 CO 3-2 4.5 ± 0.6 13 4.4 1.0 3 0.8   450 18
SMM J16359 2.517 CO 3-2 18.9 ± 0.8 45 45 0.4 1 0.3 2 150 20
Cloverleaf 2.558 CO 3-2 44 ± 1 59 11 4.0 5.4 3.2 1.5 810 40
SMM J14011 2.565 CO 3-2 9.4 ± 1.0 20 5-25 0.4-1.9 0.8-4.0 0.3-1.5 0.13-0.65 120-600 25
VCV J1409 2.583 CO 3-2 7.9 ± 0.7 35 ? - - 6.3µ-1 38µ-1    
LBQS 0018 2.620 CO 3-2 5.4 ± 0.9 33 ? -   4.3µ-1      
MG0414 2.639 CO 3-2 9.2 32 ? -   7.4µ-1      
MS1512-cB58 2.727 CO 3-2 1.4 ± 0.3 3.1 32 0.043 0.1 0.03   15 20
LBQS 1230 2.741 CO 3-2 3.0 ± 1.0 36 ? - - 2.4µ-1 11µ-1    
RX J0911.4 2.796 CO 3-2 11.3 ± 4.3 51 22 0.52 2.3 0.4   345 12
SMM J02399 2.808 CO 3-2 12.2 ± 1.6 11 2.5 4.9 4.4 3.9 6-8 660 59
SMM J04135 2.846 CO 3-2 22 ± 5 31 1.3 17 24 13.0 18 3600 36
B3 J2330 3.092 CO 4-3 3.4 ± 0.8 28 1 3.4 28 2.7   1950 14
SMM J22174 3.099 CO 3-2 3.7 ± 0.9 12 ? -   3.0   1800 17
MG 0751 3.200 CO 4-3 16 ± 1 49 17 1.0 2.9 0.8   435 18
SMM J09431 3.346 CO 4-3 3.2 ± 0.3 20 1.2 2.7 12 2.2   1800 12
SMM J13120 3.408 CO 4-3 5.2 ± 0.9 12 ? -   4.2µ-1      
TN J0121 3.520 CO 4-3 5.4 ± 1.0 7 1 5.4 7 4.3   1050 41
6C1908 3.532 CO 4-3 5.2 ± 1.0 9.8 1 5.2 9.8 4.2   1470 29
4C60.07b 3.791 CO 1-0 8.7 ± 1.7 13 1 8.7 13 7.0   1950 36
4C60.07r 3.791 CO 1-0 5.2 ± 0.6   1 5.2   4.2      
4C60.07b   4-3 6.0 ± 0.9   1 6.0          
4C60.07r   4-3 3.0 ± 0.2   1 3.0          
4C41.17R 3.796 CO 4-3 4.3 ± 0.5 20 1 4.3 20 3.4 4.6 3000 11
4C41.17B 3.796 CO 4-3 2.2 ± 0.5   1 2.2   1.8      
APM 08279 3.911 CO 1-0 9.1 ± 2.7 200 7 1.3 29 1.0 5.8 4350 2
PSS J2322 4.119 CO 1-0 12 ± 5 23 2.5 5.0 9.3 4.0   1800 22
BRI 1335N 4.407 CO 2-1 3.3 ± 1.1   ? - - 2.6µ-1      
BRI 1335S 4.407 CO 2-1 4.8 ± 1.1   ? - - 3.8µ-1      
BRI 1335   CO 5-4 8.2 ± 0.9 28 ? - -   17µ-1    
BRI 0952 4.434 CO 5-4 2.8 ± 0.3 9.6 4 0.7 2.4 0.5 0.7 360 19
BR 1202N 4.692 CO 2-1 5.2 ± 1.0   ? -   7  
BR 1202S 4.695 CO 2-1 4.6 ± 0.8   ? -        
BR 1202   CO 4-3 7.6 ± 1.5 71 ? - -   19µ-1    
TN J0924 5.203 CO 1-0 8.2 ± 1.6   1 8.2   6.6      
SDSS J1148 6.419 CO 3-2 2.6 ± 0.6 27 ? - - 2.1 2.4µ-1    

Figure 1

Figure 1. Distribution in redshift of the 36 known EMGs: 16 quasi-stellar objects (QSOs), 11 submillimeter galaxies (SMGs), 7 radio galaxies (RGs), one Lyman Break galaxy (LBG), and one extremely red object (ERO). Despite the large selection effects of the flux-limited sample, the distribution broadly reflects the current understanding of when most of the star formation in the Universe occured.

With recent improvements in millimeter bolometers, large numbers of quasi-stellar objects (QSOs) have been observed in 1.2-mm continuum emission. Approximately 30% of the bright QSOs at all redshifts z > 2 are strong millimeter/submillimeter continuum emitters with a typical inferred rest-frame luminosity of LFIR ~ 1013 Lodot (Izaak et al 2002, Omont et al. 1996a). The percentage of submillimeter detections is higher (60%) for gravitationally lensed quasars (Barvainis & Ivison 2002). Identifying the redshift appropriate for a CO emission search can be difficult because the molecular gas in the host galaxy may have a significantly different redshift from the broad optical emission line region of the QSO. A key question for the EMGs identified with QSOs is whether the FIR luminosity is powered by rapid star formation (starbursts) in the molecular clouds or by the active galactic nucleus (AGN) that may be accreting molecular gas.

In SMGs, unlike the optically selected quasars, the total luminosity is completely dominated by their (rest-frame) FIR emission. The surveys at 850 µm, primarily carried out with the Submillimetre Common-User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT) have found several hundred galaxies, or about 1 arcmin-1 (see, for example, Scott et al. 2002). They represent a substantial part of the FIR background and may contribute as much as half of all star formation at high z. Although many SCUBA galaxies harbor active galactic nuclei (AGNs), the AGNs contribute only a small fraction of the bolometric luminosity, which is dominated by star formation (Alexander et al. 2004). Only a small subset of about 15 blank-field submillimeter sources have been observed in CO emission.

A relatively small proportion (19%) of EMGs are identified with radio galaxies. Radio galaxies are a rare population and are not selected for being gravitationally lensed. However, seven IR-luminous radio galaxies have been observed in CO emission, and these include some of the more interesting examples.

The identification of a set of EMGs with LBGs would be significant in that it would tie the EMGs to a huge population of early Universe objects. However, only a single LBG has been detected in CO emission (Baker et al. 2004). The low CO line luminosity of this object compared with the other EMGs suggests that LBGs form a different class of early Universe galaxies, something that remains to be confirmed using ALMA.

2.4. Examples of EMGs

This section presents and discusses EMGs by type and historically within each type.

2.4.1. IRAS F10214     In 1991, IRAS FSC10214+4724 was shown to be an extraordinarily luminous high-redshift IR source (Rowan-Robinson et al. 1991). With a redshift of z = 2.3 it was by far the most luminous IR galaxy yet found, more than 30 times as luminous as local ULIRGs. Shortly after IRAS F10214 was identified, the first high-z CO emission was searched for and found in the (3-2), (4-3), and (6-5) lines (Brown & Vanden Bout 1991, 1992; Solomon, Downes & Radford 1992a). Allowing for the negative K-correction, Solomon, Downes & Radford (1992b) found the CO line luminosity, L'CO, calculated from the flux measured at the IRAM 30m Telescope, to be 100 times less than first estimated, but still about an order of magnitude greater than that in any galaxy in the local Universe, yielding a molecular gas mass of 1011 Modot, equal to the baryonic mass of an entire large galaxy. (Agreement between the 12-m and 30-m measured fluxes was obtained with new observations at the 12-m by Radford et al. (1996)). The strong CO(6-5) line, originating from a rotational level 116 K above the ground state, and the (6-5)/(3-2) line ratio indicates the presence of moderately dense gas substantially warmer than most of the molecular mass in Milky Way GMCs or normal spiral galaxies.

Optical and near-IR spectroscopy show both narrow and broad emission line systems, with the narrow lines indicating a Seyfert 2 nucleus (Lawrence et al. 1993) and the broad lines observed in polarized light indicating the presence of an obscured quasar (Goodrich et al. 1996).

High-resolution optical and near-IR imaging (Broadhurst & Leh'ar 1995, Graham & Liu 1995, Matthews et al. 1994) clearly show that F10214 is gravitationally lensed. The 2.2-µm image shows a compact 0.7" diameter source superposed on a weaker 1.5" arc. CO maps of the (6-5) line with the IRAM interferometer show an elongated structure that was modeled as a CO arc convolved with the interferometer beam and fit to the CO data (Downes, Solomon & Radford 1995). From the length of the CO arc, the apparent CO luminosity, the linewidth, and the intrinsic brightness temperature of the line (deduced from line ratios), Downes, Solomon & Radford (1995) derived a magnification µ = 10 fv, where fv is the velocity filling factor, or fraction of the full line width intercepted by a typical line of sight. This magnification reduced the intrinsic CO line luminosity and molecular mass to that of local ULIRGs. The radius of the molecular ring was found to be 600 / fv pc, much larger than that of the AGN torus and similar to that in ULIRGs, but much less than that of a full galactic disk. The magnification for the FIR radation was 13, and for the mid-IR it was 50.

Recent improved high-resolution maps of CO(3-2), (6-5), and (7-6) (Downes & Solomon, manuscript in preparation) show that the size of the lensed CO image is 1.6" × leq 0.3" (2.7 × leq 0.5 kpc). More importantly, a velocity gradient is observed along the arc and line profiles show two distinct kinematic components at the east and west sides, demonstrating that the molecular emission originates in a rotating disk around the quasar. Positions, sizes, and linewidths are the same in all three lines, indicating that they originate in the same volume with the same kinematic distribution. The line ratios indicate a mean emission-weighted kinetic temperature of 50 K and a mean H2 density of 3000 cm-3. A search for 13CO emission yields a ratio of 12CO/13CO geq 21, which is similar to high values found in ULIRGs but higher than those of nearby spiral galaxies, indicating a modest opacity for 12CO. The true size of the molecular ring, the CO luminosity, molecular mass, and the excitation of the CO ladder all look similar to those observed in local ULIRGs.

Vanden Bout, Solomon & Maddalena (2004) observed strong HCN(1-0) emission from F10214 with an intrinsic line luminosity similar to that in local ULIRGs such as Mrk 231 and Arp 220. HCN emission traces dense gas generally associated with the star-forming cores of GMCs (see Section 3.1). The very high ratio of HCN to CO luminosities L'CO / L'HCN = 0.18 is characteristic of starbursts in the local Universe. All galaxies with global HCN/CO luminosity ratios greater than 0.07 were found to be luminous (LFIR > 1011 Lodot) starbursts (Gao & Solomon 2004). F10214 contains both a dust-enshrouded quasar responsible for the mid-IR luminosity and a much larger molecular ring starburst responsible for a substantial fraction of the FIR luminosity.

2.4.2. Cloverleaf     Hazard et al. (1984) found the quasar H1413+1143 (better known as the Cloverleaf), a broad absorption line QSO at a redshift of z = 2.55. It was subsequently identified optically as a lensed object with four bright image components (Magain et al. 1988). Barvainis, Antonucci & Coleman (1992) discovered strong FIR and submillimeter radiation from the Cloverleaf, indicating a substantial dust component with a FIR spectral energy distribution (SED) similar to that of IRAS F10214. This was the first indication that some bright optical high-z quasars also are extremely IR luminous.

Redshifted strong CO(3-2) emission was observed using both the IRAM 30-m Telescope and Plateau de Bure Interferometer (Barvainis et al. 1994) with an apparent line luminosity about three times greater than that from F10214. Barvainis et al. (1997) observed three additional rotational lines (4-3), (5-4), and (7-6) were observed at the IRAM 30m Telescope and their line ratios used to constrain the physical conditions of the gas and the CO to H2 conversion factor. These measurements showed L'CO (4-3) > L'CO (3-2), indicating a high kinetic temperature and low optical depths. More recent measurements (Weiß et al. 2003) show a higher (3-2) flux and a lower line ratio (4-3)/(3-2) indicative of lower kinetic temperatures and subthermal excitation. The Cloverleaf CO emission lines have a higher flux density than do the lines from any other high-z source, owing to both powerful intrinsic line luminosities and magnification. As a result, they can be successfully imaged at high angular resolution. The lensing also magnifies the scale of the emission making it possible to deduce true source size at scales below the instrumental resolution.

Using the millimeter array at the Owens Valley Radio Observatory (OVRO), Yun et al. (1997) obtained an interferometric map of the Cloverleaf in which the CO(7-6) emission was partially resolved. They used Hubble Space Telescope (HST) images to model a lens with an elliptical potential and an external sheer. This model constrained the intrinsic size of the CO(7-6) source, which has a radius of approximately 1100 pc. Separation of the red and blue line wings showed a kinematic structure consistent with a rotating disk. Alloin et al. (1997) obtained a high-resolution map (0.5'') with the IRAM interferometer that clearly resolved the emission into four spots similar to the lensed optical radiation. Figure 2 shows an image of the CO(7-6) emission contructed by Venturini & Solomon (2003) from their data. A model based on HST and Very Large Array (VLA) images gave an upper limit to the source radius of approximately 1200 pc. Kneib et al. (1998) used enhanced IRAM CO(7-6) images and HST images to construct two lens models using a truncated elliptical mass distribution with an external shear (galaxy + cluster). From the separation of the kinematic components and the HST-based lens model they deduced a CO radius of only 100 pc and a magnification of 30. This size scale is characteristic of an AGN torus.

Figure 2

Figure 2. Image of the Cloverleaf in CO(7-6) emission taken with the IRAM interferometer (constructed by Venturini & Solomon 2003 from the data of Alloin et al. 1997). The high observing frequency of 226 GHz provides the angular resolution (0.5") needed to construct a gravitational lens model based on CO data.

Venturini and Solomon (2003) fit a two-galaxy lensing model directly to the IRAM CO(7-6) map rather than to the optical HST image. The fit obtained by minimizing the difference between the map produced by the lensed model and the IRAM CO(7-6) image yielded a source with disklike structure and a characteristic radius of 800 pc, a value similar to that of the CO-emitting regions present in nearby starburst ULIRGs. The model reproduces the geometry as well as the brightness of the four images of the lensed quasar. The large size of the CO source seems to rule out a scenario in which the molecular gas is concentrated in a very small region around the central AGN. With the magnification of 11 found from this model and the CO(3-2) flux given by Weiß et al. (2003), the total molecular mass is 3.2 × 1010 Modot, with a molecular surface density of 104 Modot pc-2. Weiß et al. (2003) argue that using L'CO (3-2) rather than L'CO (1-0) has only a 10% effect on the calculated molecular mass. The dynamical mass of the rotating disk is Mdyn sin2i = 2.5 × 1010 Modot.

HCN emission traces dense gas generally associated with the star-forming cores of GMCs. Strong HCN(1-0) emission has been observed from the Cloverleaf (Solomon et al. 2003) with an intrinsic line luminosity slightly higher than that in local ULIRGs, such as Mrk 231 and Arp 220, and 100 times greater than that of the Milky Way. To put this in perspective, the intrinsic HCN luminosity of the Cloverleaf is 10 times greater than the CO luminosity of the Milky Way, indicating the presence of 1010 Modot of dense star-forming molecular gas.

The molecular and IR luminosities for the Cloverleaf show that the large mass of dense molecular gas indicated by the HCN luminosity could account for a substantial fraction (from star formation), but not all, of the IR luminosity from this quasar. If Arp 220 is used as a standard for the luminosity ratio LFIR / L'HCN, star formation in the dense molecular gas could account for 5 × 1012 Lodot , or about 20% of the total intrinsic IR luminosity. Using the highest ratio for a ULIRG gives an upper limit of 40%.

The model by (Weiß et al. (2003) of the IR spectral energy distribution of the Cloverleaf has two distinct components: one with a warm dust temperature Td = 115 K responsible for the mid-IR, and the other much more massive component with Td = 50 K that produces the FIR. The model FIR luminosity, 22% of the total, may correspond to the luminosity generated by star formation and the mid-IR to heating by the AGN. Using the model LFIR yields LFIR /L'HCN = 1700, comparable to that of ULIRGs and only a factor of 2 higher than that for normal spiral galaxies (Gao & Solomon (2004). The star formation rate per solar mass of dense gas is then similar to that in ULIRGs and only slightly higher than that in normal spirals.

2.4.3. VCV J1409+5628     This EMG is an optically luminous radio-quiet quasar with the strongest 1.2-mm flux density found in the survey by Omont et al. (2003). It has been observed in both CO(3-2) and CO(7-6) emission (Beelen et al. 2004). The line luminosity of L'CO(app.) = (7.9 ± 0.7) × 1010 K km s-1 pc2 leads to a gas mass of Mgas = 6.3 × 1010 µ-1 Modot, which is ~ 20% of Mdyn for reasonable inclinations. If the extent of the radio continuum, from a VLA image at 1.4 GHz, represents the extent of the CO emission, the molecular gas is confined to a torus or disk of diameter 1-5 kpc. This is similar both to the molecular gas extents inferred from lens models of F10214 and the Cloverleaf and to what is observed in ULIRGs.

2.4.4. PSS J2322+1944     This EMG is an IR-luminous quasar. The extent of its molecular gas has been inferred from a remarkable gravitationally lensed image of the CO emission - a so-called Einstein Ring. Carilli et al. (2003) studied this lensed system on sub-kiloparsec scales with the 0.6" resolution of the VLA at 43 GHz, where the CO(2-1) line from this z = 4.12 object is redshifted. The VLA image is shown in Figure 3. The data are consistent with a dynamical mass of Mdyn = 3 × 1010 sin-2i Modot and confinement of the molecular gas in a disk of diameter 2.2 kpc. The radio continuum is co-spatial with the molecular gas and the star formation rate is ~ 900 Modot year-1. PSS J2322+1944 is the fourth EMG to be observed in [C I] emission. This object provides strong evidence for the presence of active star formation in the host galaxy of a luminous high-redshift quasar.

Figure 3

Figure 3. The Einstein Ring in PSS 2322, observed in CO(2-1) emission using the VLA at a resolution of 0.6" (Carilli et al. 2003).

2.4.5. BR 1202-0725     This is an optically bright radio-quiet quasar, the third EMG to be discovered (Omont et al. 1996b), and the first to show multiple components. Whether these two components, separated by 4", are companion objects or the result of gravitational lensing remains an issue. High-resolution imaging Carilli et al. 2002a) using the VLA of the CO(2-1) emission has shown that the southern component is roughly twice as massive as the northern component, and there is a significant difference in the velocity widths of the CO lines of the two components. This finding provides evidence against the presence of a gravitational lens. However, the total molecular gas mass exceeds the dynamical mass of the system unless an unreasonably low value of alpha is used to calculate Mgas. Magnification by a gravitational lens would allow for more reasonable values of alpha.

2.4.6. APM 08279+5245     This extremely luminous broad absorption line quasar was accidently discovered in a survey for cool carbon stars (Irwin et al. 1998). The high redshift of z = 3.9 would have made it the most luminous known object in the Universe were it not for the magnification of a gravitational lens (Egami et al. 2000). The magnification at optical wavelengths can be as large as µ = 100; for CO emission it is much less, µ = 7 (Downes et al. 1999, Lewis et al. 2002). The CO (4-3) and (9-8) emission was first observed in APM08279 with the IRAM interferometer Downes et al. 1999). The strong (9-8) emission indicates the presence of hot dense gas with a kinetic temperature of approximately 200 K. The observed ratio of LFIR / L'CO is twice that of other EMGs. In addition to the central molecular emission region, observed in four CO transitions, high-resolution images of the CO(2-1) emission with the VLA reveal two emission regions lying to the north and northeast, 2-3" distant from the central region (Papadopoulos et al. 2001). If real, these could be companion galaxies. The nuclear CO(1-0) emission is imaged in a (partial) Einstein Ring (Lewis et al. 2002).

2.4.7. SDSS J1148+5251     This is the most distant known quasar, with a redshift of z = 6.42. It was shown to be an EMG via the observations of CO(3-2) emission using the VLA, and CO(6-5) and CO(7-6) emission using the IRAM interferometer (Bertoldi et al. 2003b, Walter et al. 2003). The CO observations imply a mass of molecular gas Mgas = 2.1 × 1010 µ-1 Modot. The thermal dust emission (Bertoldi et al. 2003a) leads to a star formation rate of ~ 3000 µ-1 Modot year-1. This is clear evidence for the presence of vast amounts of molecular gas, composed of heavy elements, only ~ 850 million years following the Big Bang. High-resolution (0.17" × 0.13" leq 1 kpc) imaging of the CO(3-2) emission using the VLA (Walter et al. 2004), shown in Figure 4, suggest that this source may be a merger of two galaxies.

Figure 4

Figure 4. SDSS J1148, a quasar at z = 6.4 imaged in CO(3-2) emission using the VLA at a resolution of 0.17" × 0.13" (Walter et al. 2004). This system is a possible merger of two components that resemble the ULIRGs of the more local Universe. The presence of CO in this system is evidence for substantial enrichment in heavy metals ~ 850 million years after the Big Bang.

2.4.8. SMM J02399-0136     This SMG was the first SCUBA source identified as an EMG (Frayer et al. 1999), using OVRO. It is the brightest galaxy detected in an early SCUBA survey of rich lensing clusters (Smail, Ivison & Blain 1997). J02399 harbors an AGN (Ivison et al. 1998). The observed integrated line strength of the CO(3-2) line, with the observed CO redshift of z = 2.808, leads to L'CO(app) = 12 × 1010 K km s-1 pc2. Correction for a cluster lens magnification of µ = 2.5 yields L'CO (int) = 4.9 × 1010 K k ms-1 pc2. This is comparable to CO luminosities for ULIRGs, and was the first evidence that SCUBA sources identified as EMGs may be similar in nature to ULIRGs. Higher resolution observations of the CO emission at IRAM confirmed the OVRO detection (Genzel et al. 2003). These data were fitted to a rotating disk model very similar but larger in size than that seen in ULIRGs: a molecular gas mass Mgas = 3.9 × 1010 Modot confined within a radius of 8 kpc. This source remains one of few EMGs with the potential for molecular gas to be extended in a disk with radius larger than 2 kpc.

2.4.9. SMM J14011+0252     This SMG was the second SCUBA source from the Lensing Cluster Survey (Smail, Ivison & Blain 1997) to be detected in CO emission; it has been heavily observed since being identified as an EMG. There is no evidence for the presence of an AGN in J14011. The detection of CO(3-2) emission (Frayer et al. 1999) at OVRO was followed by more interferometry to determine the location of the CO source among the 850-µ m peaks in the SCUBA image and its extent. From combined OVRO and Berkeley-Illinois-Maryland Association (BIMA) observations it was argued (Ivison et al. 2001) that the CO emission was extended on a scale of diameter 20 kpc, assuming a cluster magnification of µ = 2.5, well beyond what is seen in ULRIGs. Higher signal-to-noise observations at IRAM (Downes & Solomon 2003) did not confirm this extent, as the CO emission is confined to an observed disk of only 2.2", or a diameter leq 7 kpc for a magnification of 2.5.

2.4.10 SMM 16359+6612     This is a somewhat lower luminosity (LFIR = 1012 Lodot) SMG that nevertheless has been observed in CO(3-2) emission aided by a gravitational lens that provides a total magnification factor of µ = 45. The image obtained with the IRAM Interferometer (Kneib et al. 2005a), together with spectra of the three image components, is shown in Figure 5. CO observations of SMM J16359 have also been reported by Sheth et al. (2004). This is the third SMG reported to have spatially resolved CO emission. Here, the quality of the data together with the lens model of Kneib et al. (2004b) leads to an inferred disk size of 3 × 1.5 kpc. Whereas the FIR luminosity is comparable to that of Arp 220, the CO luminosity is approximately half that of Arp 220. The mass inferred from the CO luminosity is 30% or 60% of the calculated dynamical mass for a ring-disk structure or a merger, respectively.

Figure 5

Figure 5. The lower panel shows SMM J16359 in CO(3-2) emission that has been triply imaged by a gravitational lens (Kneib et al. 2004a). The total magnification is µ = 45, making possible this observation of CO in a somewhat less luminous SMG. The CO contours are superimposed on an HST image of Abell 2218, and show good registration with their optical counterparts. The synthesized CO beam ( ~ 6") is shown in the lower left corner. The SED in the range 450-3000 µm is shown in the upper right corner (Kneib et al. 2004b). The upper panel shows the CO spectra from each image together with the combined spectrum. The redshifts deduced from HST imaging and Halpha spectroscopy, shown as alpha and beta, are in close agreement with those of the CO emission peaks.

2.4.11. 4C41.17     This is one of only seven radio galaxies to be observed in CO emission. High-z radio galaxies (HzRGs) have been difficult to detect in CO emission because the candidates searched are not gravitationally lensed and the observed peak CO flux densities are small (~ 2 mJy). Stevens et al. (2003) have argued that HzRGs and their companions, revealed in deep 850-µm images, form central cluster ellipticals. Four of the seven HzRG examples cited by Stevens et al. (2003), including 4C41.17, are also EMGs. A position-velocity plot of the CO(4-3) emission (De Breuck et al. 2005), clearly reveals two components. Both are gas-rich systems, each with Mgas ~ 3 × 1010 Modot. Their velocity separation leads to a dynamical mass Mdyn ~ 6 × 1011 sin-2 i Modot, for the potential binding the components. The system could be two gas-rich galaxies merging to form a massive cD elliptical galaxy.

1 The rough dependence of the luminosity distance on redshift can be seen from the following: DL = DA (1 + z)2, where DA is the angular size distance. For the cosmology assumed in this review, DA rapidly increases with redshift, reaching a peak value at z approx 1.6, and then declines roughly as (1 + z)-1 for larger z. So for redshifts larger than z ~ 2, DL grows roughly as (1 + z). A calculator for computing luminosity and angular size distances in any cosmology can be found at Back.

2 Appendix 1 lists coordinates, redshift, galaxy type and magnification for each EMG. Appendix 2 gives velocity integrated flux densities (SDeltav), linewidths as full width at half-maximum (FWHM) (Delta v), peak line flux densities (S), line luminosities (L') for the CO transitions observed in the EMGs, and inferred molecular gas masses. The observed quantities listed are those reported in the references cited, after adjustment for the cosmology assumed in this review. Where lens models exist, intrinsic luminosities are listed, calculated using the magnifications given in Appendix 1. In addition to CO, data for detections of HCN are listed, as well as for CI whose fine-structure lines originate from interstellar molecular gas. Appendix 3 gives the observed continuum flux densities at various wavelengths of the EMGs, together with the inferred FIR luminosity, including the intrinsic luminosity where it is possible to correct for lens magnification. Brackets indicate the measurements that were included in the calculation of the listed luminosity values cited. Frequently, only a single measurement is used to estimate the luminosity, together with a set of assumptions, so the values listed should be regarded with caution. Back.

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