Simon J. E. Radford

The Molecular Interstellar Medium in Ultraluminous Infrared Galaxies

We present observations with the IRAM 30 m telescope of CO in a large sample of ultraluminous IR galaxies out to redshift z = 0.3. Most of the ultraluminous galaxies in this sample are interacting, but not completed, mergers. The CO(1-0) luminosity of all but one of the ultraluminous galaxies is high, with values of log (L

Dense molecular gas and starbursts in ultraluminous galaxies

{′}{r CO}

MOLECULAR GAS IN EXTREME STAR-FORMING ENVIRONMENTS: THE STARBURSTS Arp 220 AND NGC 6240 AS CASE STUDIES

-->/K km s-1 pc2) = 9.92 ? 0.12. The extremely small dispersion of only 30% is less than that of the far-infrared luminosity. The integrated CO line intensity is strongly correlated with the 100 ?m flux density, as expected for a blackbody model in which the mid- and far-IR radiation is optically thick. We use this model to derive sizes of the FIR- and CO-emitting regions and the enclosed dynamical masses. Both the IR and CO emission originate in regions a few hundred parsecs in radius. The median value of LFIR

Molecular gas mass and far-infrared emission from distant luminous galaxies

{r FIR}

CO: Twenty-Five Years of Millimeter-Wave Spectroscopy

-->/L

A Fourier Transform Spectrometer for Measurement of Atmospheric Transmission at Submillimeter Wavelengths

{′}{r CO}

CO excitation and H2 masses of infrared-luminous galaxies

-->=160 L?/K km s-1 pc2, within a factor of 2 or 3 of the blackbody limit for the observed far-IR temperatures. The entire ISM is a scaled-up version of a normal galactic disk with the ambient densities a factor of 100 higher, making even the intercloud medium a molecular region. We compare three different techniques of H2 mass estimation and conclude that the ratio of gas mass to CO luminosity is about a factor of 4 times lower than for giant molecular clouds (GMCs) but that the gas mass is a large fraction of the dynamical mass. Our analysis of CO emission from ultraluminous galaxies reduces the H2 mass from previous estimates of 2-5 ? 1010 M? to 0.4-1.5 ? 1010 M?, which is in the range found for molecular gas-rich spiral galaxies. A collision involving a molecular gas-rich spiral could lead to an ultraluminous galaxy powered by central starbursts triggered by the compression of infalling preexisting GMCs. The extremely dense molecular gas in the center of an ultraluminous galaxy is an ideal stellar nursery for a huge starburst.

New Observations and a New Interpretation of CO(3-2) in IRAS F10214+4724

With the IRAM 30 m telescope we observed HCN(1→0) emission from five ultraluminous galaxies, three lower luminosity interacting systems, and two gas-rich normal galaxies. There are huge masses of high-density gas (2x10 10 M ⊙ ) in the ultraluminous galaxies, which shows star formation, rather than active galactic nuclei, generates their infrared luminosity. HCN traces H 2 at a much higher density, ∼10 4 cm −3 , than CO (∼500 cm −3 )

Stability of the Submillimeter Brightness of the Atmosphere above Mauna Kea, Chajnantor, and the South Pole

We report single-dish multitransition measurements of the ^(12)CO, HCN, and HCO^+ molecular line emission as well as HNC J = 1-0 and HNCO in the two ultraluminous IR galaxies Arp 220 and NGC 6240. Using this new molecular line inventory, in conjunction with existing data in the literature, we compiled the most extensive molecular line data sets to date for such galaxies. The many rotational transitions, with their different excitation requirements, allow the study of the molecular gas over a wide range of different densities and temperatures with significant redundancy, and thus allow good constraints on the properties of the dense gas in these two systems. The mass (~ (1-2)×10^(10) M_☉ ) of dense gas (≳10^(5–6) cm^(–3)) found accounts for the bulk of their molecular gas mass, and is consistent with most of their IR luminosities powered by intense starbursts while self-regulated by O, B star cluster radiative pressure onto the star-forming dense molecular gas. The highly excited HCN transitions trace a gas phase ~ (10-100)× denser than that of the subthermally excited HCO^+ lines (for both galaxies). These two phases are consistent with an underlying density-size power law found for Galactic giant molecular clouds (but with a steeper exponent), with HCN lines tracing denser and more compact regions than HCO^+. Whether this is true in IR-luminous, star-forming galaxies in general remains to be seen, and underlines the need for observations of molecular transitions with high critical densities for a sample of bright (U)LIRGs in the local universe—a task for which the HI-FI instrument on board Herschel is ideally suited to do.

The Q/U Imaging ExperimenT Instrument

Molecular line observations suggest the central few hundred parsecs of ultraluminous infrared galaxies have high mean gas densities, n(H 2 )∼3×10 3 to 10 4 cm -3 , unlike the centers of normal galaxies and very different from the disks of spiral galaxies. The CO line emission may not trace an ensemble of gravitationally bound gas clouds but instead a medium bound by the total potential of the Galactic center (gas and stars). This means the CO luminosity no longer measures gas mass alone, as in normal galaxies, but instead the geometric mean of the gas mass and the dynamical mass