Institut De Radioastronomie Millimetrique Research Articles

Simultaneous Millimetre-wave and X-ray monitoring of the Seyfert galaxy NGC 7469

ABSTRACT We report on daily monitoring of the Seyfert galaxy ngc 7469, around 95 and 143 GHz, with the iram (Institut de Radioastronomie Millimetrique) 30- m radio telescope, and with the Swift X-ray and UV/optical telescopes, over an overlapping period of 45 d. The source was observed on 36 d with iram, and the flux density in both mm bands was on average ∼10 mJy, but varied by $\pm 50{{\ \rm per\ cent}}$, and by up to a factor of 2 between days. The present iram variability parameters are consistent with earlier monitoring, which had only 18 data points. The X-ray light curve of ngc 7469 over the same period spans a factor of 5 in flux with small uncertainties. Similar variability in the mm band and in the X-rays lends support to the notion of both sources originating in the same physical component of the active galactic nucleus (AGN), likely the accretion disc corona. Simultaneous monitoring in eight UV/optical bands shows much less variability than the mm and X-rays, implying this light originates from a different AGN component, likely the accretion disc itself. We use a tentative 14-d lag of the X-ray light curve with respect to the 95 GHz light curve to speculate on coronal implications. More precise mm-band measurements of a sample of X-ray-variable AGN are needed, preferably also on time-scales of less than a day where X-rays vary dramatically, in order to properly test the physical connection between the two bands.

Variable millimetre radiation from the colliding-wind binary Cygnus OB2 #8A

In the colliding-wind region of massive binaries, non-thermal radio emission occurs. This non-thermal radio emission (due to synchrotron radiation) has so far been observed at centimetre wavelengths. At millimetre wavelengths, the stellar winds and the colliding-wind region emit more thermal free-free radiation, and it is expected that any non-thermal contribution will be difficult or impossible to detect. We aim to determine if the material in the colliding-wind region contributes substantially to the observed millimetre fluxes of a colliding-wind binary. We also try to distinguish the synchrotron emission from the free-free emission. We monitored the massive binary Cyg OB2 #8A at 3 mm with the NOrthern Extended Millimeter Array (NOEMA) interferometer of the Institut de Radioastronomie Millimetrique (IRAM). The data were collected in 14 separate observing runs (in 2014 and 2016), and provide good coverage of the orbital period. The observed millimetre fluxes range between 1.1 and 2.3 mJy, and show phase-locked variability, clearly indicating that a large part of the emission is due to the colliding-wind region. A simple synchrotron model gives fluxes with the correct order of magnitude, but with a maximum that is phase-shifted with respect to the observations. Qualitatively this phase shift can be explained by our neglect of orbital motion on the shape of the colliding-wind region. A model using only free-free emission results in only a slightly worse explanation of the observations. Additionally, on the map of our observations we also detect the O6.5 III star Cyg OB2 #8B, for which we determine a 3 mm flux of 0.21 +- 0.033 mJy. The question of whether synchrotron radiation or free-free emission dominates the millimetre fluxes of Cyg OB2 #8A remains open. More detailed modelling of this system, based on solving the hydrodynamical equations, is required to give a definite answer.

The NIKA2 Instrument, A Dual-Band Kilopixel KID Array for Millimetric Astronomy

NIKA2 (New IRAM KID Array 2) is a camera dedicated to millimeter wave astronomy based upon kilopixel arrays of Kinetic Inductance Detectors (KID). The pathfinder instrument, NIKA, has already shown state-of-the-art detector performance. NIKA2 builds upon this experience but goes one step further, increasing the total pixel count by a factor $\sim$10 while maintaining the same per pixel performance. For the next decade, this camera will be the resident photometric instrument of the Institut de Radio Astronomie Millimetrique (IRAM) 30m telescope in Sierra Nevada (Spain). In this paper we give an overview of the main components of NIKA2, and describe the achieved detector performance. The camera has been permanently installed at the IRAM 30m telescope in October 2015. It will be made accessible to the scientific community at the end of 2016, after a one-year commissioning period. When this happens, NIKA2 will become a fundamental tool for astronomers worldwide.

Electronically Tuned Local Oscillators for the NOEMA Interferometer

We present an overview of the electronically tuned local oscillator (LO) system developed at the Institut de RadioAstronomie millimetrique (IRAM) for the superconductor–insulator–superconductor (SIS) receivers of the NOrthern Extended Millimeter Array interferometer (NOEMA). We modified the frequency bands and extended the bandwidths of the LO designs developed by the National Radio Astronomy Observatory (NRAO) for the Atacama Large Millimeter Array (ALMA) project to cover the four NOEMA LO frequency ranges 82–108.3 GHz (Band 1), 138.6–171.3 GHz (Band 2), 207.7–264.4 GHz (Band 3), and 283–365 GHz (Band 4). The NOEMA LO system employs commercially available MMICs and GaAs millimeter MMICs from NRAO which are micro-assembled into active multiplied chain (AMC) and power amplifier (PA) modules. We discuss the problem of the LO spurious harmonics and of the LO signal directly multiplied by the SIS mixers that add extra noise and lead to detections of unwanted spectral lines from higher order sidebands. A waveguide filter in the LO path is used to reduce the higher order harmonics level of the LO at the output of the final frequency multiplier, thus mitigating the undesired effects and improving the system noise temperature.

Dense-gas properties in Arp 220 revealed by isotopologue lines

We present observations of isotopologue lines of dense-gas tracers at 3 mm and 1 mm towards the nearest ultra-luminous infrared galaxy Arp 220. The 3-mm and 1-mm observations were performed with the Institut de Radioastronomie Millimetrique 30-m telescope and the Atacama Pathfinder Experiment 12-m telescope, respectively. We detected (HCN)-C-13 and (HNC)-C-13 in 1-0 and 3-2, and (HCN)-N-15 1-0, among which (HCN)-N-15 1-0 and (HNC)-C-13 1-0 are detected in Arp 220 for the first time. The (HCO+)-C-13 1-0 and 3-2 lines are unlikely to be detected because of the confusion of SiO lines. We find that the ratio of the line brightness temperatures of (HNC)-C-13 3-2/1-0 is 2.4, which is significantly higher than that of (HCN)-C-13 3-2/1-0 (0.73). This indicates that (HNC)-C-13 and HNC molecules are in denser regions than (HCN)-C-13 and HCN molecules. With the line ratio of (HCN)-C-13 1-0 and (HCN)-N-15 1-0, the N-14/N-15 ratio was estimated to be 440(-82)(+140), which is larger than that of the local interstellar medium.

MOLECULAR GAS PROPERTIES UNDER ICM PRESSURE IN THE CLUSTER ENVIRONMENT

We present 12CO (2-1) data for four spiral galaxies (NGC 4330, NGC 4402, NGC 4522, NGC 4569) in the Virgo cluster that are undergoing different ram pressure stages. The goal is to probe the detailed molecular gas properties under strong intra-cluster medium (ICM) pressure using high-resolution millimeter data taken with the Submillimeter Array (SMA). Combining this with Institut de RadioAstronomie Millimetrique (IRAM) data, we also study spatially resolved temperature and density distributions of the molecular gas. Comparing with multi-wavelength data (optical, Hi, UV , Hα), we discuss how molecular gas properties and star formation activity change when a galaxy experiences Hi stripping. This study suggests that ICM pressure can modify the physical and chemical properties of the molecular gas significantly even if stripping does not take place. We discuss how this affects the star formation rate and galaxy evolution in the cluster environment.

Probing the turbulent ambipolar diffusion scale in molecular clouds with spectroscopy

We estimate the turbulent ambipolar diffusion length-scale and magnetic field strength in the massive dense cores CygX-N03 and CygX-N53, located in the Cygnus-X star-forming region. The method we use requires comparing the velocity dispersions in the spectral line profiles of the coexistent ion and neutral pair H13CN and H13CO+ (J = 1 → 0) at different length-scales. We fit Kolmogorov-type power laws to the lower envelopes of the velocity dispersion spectra of the two species. This allows us to calculate the turbulent ambipolar diffusion scale, which in turn determines the plane-of-the-sky magnetic field strength. We find turbulent ambipolar diffusion length-scales of 3.8 ± 0.1 and 21.2 ± 0.4 mpc, and magnetic field strengths of 0.33 and 0.76 mG for CygX-N03 and CygX-N53, respectively. These magnetic field values have uncertainties of a factor of a few. Despite a lower signal-to-noise ratio of the data in CygX-N53 than in CygX-N03, and the caveat that its stronger field might stem in part from projection effects, the difference in field strengths suggests different fragmentation efficiencies of the two cores. Even though the quality of our data, obtained with the Institut de Radio Astronomie Millimetrique (IRAM) Plateau de Bure Interferometer, is somewhat inferior to previous single-dish data, we demonstrate that this method is suited also for observations at high spatial resolution.

Latest NIKA Results and the NIKA-2 Project

NIKA (New IRAM KID Arrays) is a dual-band imaging instrument installed at the IRAM (Institut de RadioAstronomie Millimetrique) 30-meter telescope at Pico Veleta (Spain). Two distinct Kinetic Inductance Detectors (KID) focal planes allow the camera to simultaneous image a field-of-view of about 2 arc-min in the bands 125 to 175 GHz (150 GHz) and 200 to 280 GHz (240 GHz). The sensitivity and stability achieved during the last commissioning Run in June 2013 allows opening the instrument to general observers. We report here the latest results, in particular in terms of sensitivity, now comparable to the state-of-the-art Transition Edge Sensors (TES) bolometers, relative and absolute photometry. We describe briefly the next generation NIKA-2 instrument, selected by IRAM to occupy, from 2015, the continuum imager/polarimeter slot at the 30-m telescope.

A search for pre-substellar cores and proto-brown dwarf candidates in Taurus: multiwavelength analysis in the B213-L1495 clouds

In an attempt to study whether the formation of brown dwarfs (BDs) takes place as a scaled-down version of low-mass stars, we conducted IRAM30m/MAMBO-II observations at 1.2 mm in a sample of 12 proto-BD candidates selected from Spitzer/IRAC data in the B213-L1495 clouds in Taurus. Subsequent observations with the CSO at 350 micron, VLA at 3.6 and 6 cm, and IRAM30m/EMIR in the 12CO(1-0), 13CO(1-0), and N2H+(1-0) transitions were carried out toward the two most promising Spitzer/IRAC source(s), J042118 and J041757. J042118 is associated with a compact (<10 arcsec or <1400 AU) and faint source at 350 micron, while J041757 is associated with a partially resolved (~16 arcsec or ~2000 AU) and stronger source emitting at centimetre wavelengths with a flat spectral index. The corresponding masses of the dust condensations are ~1 and ~5 Mjup for J042118 and J041757, respectively. In addition, about 40 arcsec to the northeast of J041757 we detect a strong and extended submillimetre source, J041757-NE, which is not associated with NIR/FIR emission down to our detection limits, but is clearly detected in 13CO and N2H+ at ~7 km/s, and for which we estimated a total mass of ~100 Mjup, close to the mass required to be gravitationally bound. In summary, our observational strategy has allowed us to find in B213-L1495 two proto-BD candidates and one pre-substellar core candidate, whose properties seem to be consistent with a scaled-down version of low-mass stars.

Cold and warm molecular gas in the outflow of 4C 12.50

We present deep observations of the 12CO(1-0) and (3-2) lines in the ultra-luminous infrared and radio galaxy 4C12.50, carried out with the 30m telescope of the Institut de Radioastronomie Millimetrique. Our observations reveal the cold molecular gas component of a warm molecular gas outflow that was previously known from Spitzer Space Telescope data. The 12CO(3-2) profile indicates the presence of absorption at -950 km/s from systemic velocity with a central optical depth of 0.22. Its profile is similar to that of the HI absorption that was seen in radio data of this source. A potential detection of the (0-1) absorption enabled us to place an upper limit of 0.03 on its central optical depth, and to constrain the excitation temperature of the outflowing CO gas to >=65K assuming that the gas is thermalized. If the molecular clouds fully obscure the background millimeter continuum that is emitted by the radio core, the H2 column density is >=1.8*10^22 /cm^2. The outflow then carries an estimated cold H2 mass of at least 4.2*10^3 M_sun along the nuclear line of sight. This mass will be even higher when integrated over several lines of sight, but if it were to exceed 3*10^9 M_sun, the outflow would most likely be seen in emission. Since the ambient cold gas reservoir of 4C12.50 is 1.0*10^10 M_sun, the outflowing-to-ambient mass ratio of the warm gas (37%) could be elevated with respect to that of the cold gas.

Survival of molecular gas in Virgo’s hot intracluster medium: CO near M 86

We carried out CO(1-0) and CO(2-1) observations of 21 different regions in the vicinity of M86, NGC4438, and along the 120 kpc-long, Ha-emitting filamentary trail that connects them, aiming to test whether molecular gas can survive to be transferred from a spiral to an elliptical galaxy in Virgo's 10^7K intracluster medium (ICM). We targeted Ha-emitting regions that could be associated with the interface between cold molecular clouds and the hot ionized ICM. The data, obtained with the 30m telescope of the Institut de Radioastronomie Millimetrique, led to the detection of molecular gas close to M86. CO gas with a recession velocity that is similar to that of the stars, -265 km/s, and with a corresponding H2 mass of 2*10^7 M_sun, was detected ~10 kpc southeast of the nucleus of M86, near the peak of its HI emission. We argue that it is possible for this molecular gas either to have formed in situ from HI, or to have been stripped from NGC4438 directly in molecular form. In situ formation is nonetheless negligible for the 7*10^6 M_sun of gas detected at 12:26:15.9+12:58:49, at ~10 kpc northeast of M86, where no (strong) HI emission is present. This detection provides evidence for the survival of molecular gas in filaments for timescales of ~100 Myr. An amount equivalent to 5*10^7 M_sun of H2 gas that could be lost to the ICM or to neighboring galaxies was also discovered in the tidal tail northwest of NGC4438. A scenario of gas being alternatively brought to M86 from NGC4388 on its south was also examined but it was considered unlikely due to the non detection of CO below or at the HI stream velocities, 2000-2700 km/s.

Upgrade of EMIR's Band 3 and Band 4 Mixers for the IRAM 30 m Telescope

The Eight Mixer Receiver (EMIR) is a multi-band millimeter wave receiver installed since 2009 at the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM) at Pico Veleta in Spain. In autumn 2011, Band 3 and Band 4 will be equipped with state-of-the-art sideband-separating mixers with 8 GHz intermediate frequency (IF) bandwidth. For Band 3 a side- band-separating mixer has been developed within the European project AMSTAR+ in order to replace the currently employed single-sideband mixer. The new mixer has an IF bandwidth twice as large as the current one, and two sidebands, thus doubling the continuum sensitivity of the receiver. The mixer currently installed in Band 4 will simply be upgraded to 8 GHz IF bandwidth by changing its IF components.

HOT MOLECULAR CORES IN INFRARED DARK CLOUDS

We present high angular resolution continuum images and molecular line spectra obtained at 345 GHz with the Submillimeter Array (SMA) toward two massive cores that lie within Infrared Dark Clouds (IRDCs): G034.43+00.24 MM1 and G024.33+00.11 MM1. Both of these cores contain bright, unresolved (<2'') objects that have previously been imaged in the millimeter/submillimeter continuum with the Institut de RadioAstronomie Millimetrique (IRAM) Plateau de Bure Interferometer and SMA and show complex molecular line chemistry. The new, higher angular resolution SMA continuum images reveal that both cores contain massive (8, 26 M ☉), unresolved (06; ~3000 AU) continuum emission features and emission from many complex molecular transitions, which confirm that these are hot molecular cores, an early stage in the formation of a high-mass star. Because these hot cores are located within IRDCs, they may well represent the very earliest phases in the formation of high-mass protostars and, hence, their detailed study may reveal the initial conditions within high-mass star-forming cores, before they are shredded apart by stellar winds and radiation.

DETECTION OF A MOLECULAR DISK ORBITING THE NEARBY, “OLD,” CLASSICAL T TAURI STAR MP MUSCAE

We have used the Atacama Pathfinder Experiment 12 m telescope to detect circumstellar CO emission from MP Muscae (MP Mus; K1 IVe), a nearby (D ∼ 100 pc), actively accreting, ∼7 Myr old pre-main-sequence (pre-MS) star. The CO emission line profile measured for MP Mus is indicative of an orbiting disk with radius ∼120 AU, assuming that the central star mass is 1.2 M☉ and the disk inclination is i ∼ 30°. The inferred disk molecular gas mass is ∼3 M⊕. MP Mus thereby joins TW Hya and V4046 Sgr as the only late-type (low-mass), pre-MS star systems within ∼100 pc of Earth that are known to retain orbiting, molecular disks. We also report the nondetection (with the Institut de Radio Astronomie Millimetrique 30 m telescope) of CO emission from another 10 nearby (D ≲ 100 pc), dusty, young (age ∼10–100 Myr) field stars of spectral type A–G. We discuss the implications of these results for the timescales for stellar and Jovian planet accretion from, and dissipation of, molecular disks around young stars.

Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550

We present observations of the CO(1-0) emission in the central 750 pc (10 arcsec) of the counter-rotating disc galaxy NGC 4550, obtained at the Institut de Radioastronomie Millimetrique (IRAM) Plateau de Bure Interferometer. Very little molecular gas is detected, only 1 x 10^7 solar masses, and its distribution is lopsided, with twice as much molecular gas observed at positive relative velocities than at negative relative velocities. The velocity gradient in the CO(1-0) emission shows that the molecular gas rotates like the thicker of the two stellar discs, which is an unexpected alignment of rotations if the thinner disc was formed by a major gas accretion event. However, a simulation shows that the gas rotating like the thicker disc naturally results from the coplanar merger of two counter-rotating disc galaxies, demonstrating the feasibility of this scenario for the formation of NGC 4550. We investigate various star formation tracers to determine whether the molecular gas in NGC 4550 is currently forming stars. UV imaging data and optical absorption linestrengths both suggest a recent star formation episode; the best-fitting two population model to the UV-optical colours yields a mass of young stars of 5.9 x 10^7 solar masses with an age of 280 Myr. The best information on the current star formation rate is a far infrared-based upper limit of only 0.02 solar masses per year. We are thus witnessing NGC 4550 either in a dip within a bursty star formation period or during a more continuous low-level star formation episode.

Molecules in the disk orbiting the twin young suns of V4046 Sagittarii

Context. Direct information concerning the physical conditions and chemistry within the Jovian planet-building zones of circumbi- nary disks surrounding pre-main sequence stars is essentially nonexistent, especially for the more evolved pre-MS systems in which planets may already be forming or may have formed. Aims. We searched for a gaseous component within the dusty circumbinary disk around the nearby (D ∼ 70 pc), 12 Myr-old system V4046 Sgr - a tight (9 Rseparation), short-period (P ∼ 2.42 day) binary with nearly equal component masses of ∼0.9 M� -s o as to assess the mass, chemistry, and kinematics of this gaseous disk. Methods. We conducted a mm-wave molecular line survey of V4046 Sgr with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We use these data to investigate the kinematics, gas mass, and chemical constituents of the V4046 Sgr disk. Results. We detected rotational transitions of 12 CO 13 CO, HCN, CN, and HCO + . The double-peaked CO line profiles of V4046 Sgr are well fit by a model invoking a Keplerian disk with outer radius of ∼250 AU that is viewed at an inclination i = 35 ◦ . We infer mini- mum disk gas and dust masses of ∼13 and ∼20 Earth masses from the V4046 Sgr CO line and submm continuum fluxes, respectively. The actual disk gas mass could be much larger if the gas-phase CO is highly depleted and/or 13 CO is very optically thick. Conclusions. The overall similarity of the circumbinary disk of V4046 Sgr to the disk orbiting the single, ∼ 8M yr-old star TW Hya -as tar/disk system often regarded as representative of the early solar nebula - indicates that gas giant planets are likely common- place among close binary star systems. Given the relatively advanced age and proximity of V4046 Sgr, these results provide strong motivation for future high-resolution imaging designed to ascertain whether a planetary system now orbits its twin suns.

Molecules in the circumstellar disk orbiting BP Piscium

Context. BP Psc is a puzzling late-type, emission-line field star with large infrared excess. The star is encircled and enshrouded by a nearly edge-on, dusty circumstellar disk, and displays an extensive jet system similar to those associated with pre-main sequence (pre-MS) stars. However, the photospheric absorption features of the star itself appear more consistent with post-main sequence status. Aims. We seek to characterize the molecular gas component of the BP Psc disk, so as to compare the properties of its molecular disk with those of well-studied pre-main sequence stars. Methods. We conducted a mm-wave molecular line survey of BP Psc with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We use these data to investigate the kinematics, gas mass, and chemical constituents of the BP Psc disk. Results. We detected lines of 12 CO and 13 CO and, possibly, very weak emission from HCO + and CN; HCN, H 2 CO, and SiO are not detected. The CO line profiles of BP Psc are well fit by a model invoking a disk in Keplerian rotation. The mimumum disk gas mass, inferred from the 12 CO line intensity and 13 CO/ 12 CO line ratio, is ∼0.1 Jupiter masses. Conclusions. The weakness of HCO + and CN (relative to 13 CO) stands in sharp contrast to the strong HCO+ and CN emission that characterizes most low-mass, pre-main sequence stars that have been the subjects of molecular emission-line surveys, and is suggestive of a very low level of X-ray-induced molecular ionization within the BP Psc disk. These results lend some support to the notion that BP Psc is an evolved star whose circumstellar disk has its origins in a catastrophic interaction with a close companion.

Molecular content of a Type Ia supernova host galaxy at z= 0.6

We study the properties and the molecular content of the host of a Type Ia supernova (SN1997ey). This z = 0.575 host is the brightest submillimetre source of the sample of Type Ia supernova hosts observed at 450 and 850 μm by Farrah et al. Observations were performed at the Institut de Radioastronomie Millimetrique 30 m telescope (IRAM 30 m) to search for CO(2-1) and CO(3-2) lines in good weather conditions but no signal was detected. The star formation rate cannot exceed 50 M ⊙ yr -1 . These negative results are confronted with an optical analysis of a Keck spectrum and other data archives. We reach the conclusion that this galaxy is a late-type system (0.7L B * ), with a small residual star formation activity (0.2 M ⊙ yr -1 ) detected in the optical. No source of heating (active galactic nucleus or starburst) is found to explain the submillimetre-continuum flux and the non-CO detection excludes the presence of a large amount of cold gas. We thus suggest that either the star formation activity is hidden in the nucleus (with A v ∼ 4) or this galaxy is passive or anemic, and this flux might be associated with a background galaxy.

A New Water Vapor Megamaser

We report on the detection of a new megamaser, the 31, 3-22, 0 H2O line (ν0 = 183.310 GHz) in Arp 220, using the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescope. The line is about 350 km s-1 wide with a total luminosity of ~2.5 × 108 K km s-1 pc2. Although OH megamasers were first discovered in this source, no emission is seen in the 61, 6-52, 3 H2O transition (ν0 = 22.235 GHz), a line otherwise detected as a megamaser in about 50 sources to date. This fact puts interesting constraints on the physical conditions of the central region of Arp 220 that are further strengthened by the HCN and HNC J = 3-2 and J = 1-0 luminosities [in the range (1.5-10) × 108 K km s-1 pc2]. A scenario with ~106 star-forming cores similar to those found in Sgr B2 in the central kiloparsec of Arp 220 would be compatible with these data and would explain the lack of 22 GHz H2O emission. This result opens up the possibility of using the 183 GHz H2O line as an additional tool to explore the physical conditions in luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs, respectively) and their starburst or active galactic nucleus (AGN) nature, with a potential interest for high angular resolution observations with the Atacama Large Millimeter Array (ALMA).

An interferometric CO survey of luminous submillimetre galaxies

In this paper, we present results from an Institut de Radio Astronomie Millimetrique (IRAM) Plateau de Bure millimetre-wave Interferometer (PdBI) survey for carbon monoxide (CO) emission towards radio-detected submillimetre galaxies (SMGs) with known optical and near-infrared spectroscopic redshifts. Five sources in the redshift range z ∼ 1-3.5 were detected, nearly doubling the number of SMGs detected in CO. We summarize the properties of all 12 CO-detected SMGs, as well as six sources not detected in CO by our survey, and use this sample to explore the bulk physical properties of the submillimetre galaxy (SMG) population as a whole. The median CO line luminosity of the SMGs is = (3.8 ± 2.0) × 10 10 K km s -1 pc 2 . Using a CO-to-H 2 conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas mass = (3.0 ± 1.6) x 10 10 M ○. within an ∼ 2 kpc radius, approximately 4 times greater than the most luminous local ultraluminous infrared galaxies (ULIRGs) but comparable to that of the most extreme high-redshift radio galaxies (HzRGs) and quasi-sellar objects (QSOs). The median CO FWHM linewidth is broad, (FWHM) = 780 ± 320 km s -1 , and the SMGs often have double-peaked line profiles, indicative of either a merger or a disc. From their median gas reservoirs (∼ 3 x 10 10 M ○. ) and star formation rates (≥ 700 M ○. yr -1 ), we estimate a lower limit on the typical gas-depletion time-scale of ≥ 40 Myr in SMGs. This is marginally below the typical age expected for the starbursts in SMGs and suggests that negative feedback processes may play an important role in prolonging the gas consumption time-scale. We find a statistically significant correlation between the far-infrared and CO luminosities of the SMGs, which extends the observed correlation for local ULIRGs to higher luminosities and higher redshifts. The non-linear nature of the correlation implies that SMGs have higher far-infrared to CO luminosity ratios and possibly higher star formation efficiencies (SFEs), than local ULIRGs. Assuming a typical CO source diameter of θ ∼ 0.5 arcsec (D ∼ 4kpc), we estimate a median dynamical mass of ≃ (1.2 ± 1.5) x 10 11 M ○. for the SMG sample. Both the total gas and stellar masses imply that SMGs are very massive systems, dominated by baryons in their central regions. The baryonic and dynamical properties of these systems mirror those of local giant ellipticals and are consistent with numerical simulations of the formation of the most massive galaxies. We have been able to impose a lower limit of ≥ 5 x 10 -6 Mpc -3 to the comoving number density of massive galaxies in the redshift range z ∼ 2-3.5, which is in agreement with results from recent spectroscopic surveys and the most recent model predictions.