The paper by D. Downes and P. M. Solomon investigates the molecular gas distribution in ultraluminous infrared galaxies (ULIGs) using CO interferometer data. The authors find that the molecular gas is concentrated in rotating nuclear disks or rings, with radii ranging from 300 to 800 parsecs. The CO brightness temperatures, double-peaked line profiles, and constraints on thermal continuum flux indicate that the CO lines are subthermally excited and moderately opaque. They fit kinematic models where most of the CO flux comes from a moderate-density, warm intercloud medium rather than self-gravitating clouds. The derived gas masses are significantly lower than those estimated using standard CO-to-mass ratios, suggesting that the CO luminosity traces the geometric mean of the gas mass and dynamical mass. The study identifies compact extreme starburst regions within these ULIGs, with characteristic sizes of only 100 parsecs and masses of about $10^9$ M$_\odot$. These regions produce most of the HCN luminosity and 1mm dust continuum, while the diffuse gas in the nuclear disks dominates the CO luminosity. The authors conclude that the far-IR luminosity in ULIGs is powered by these extreme starbursts, not by dust-enshrouded quasars. The paper also discusses the rotation curves, disk geometry, and the ratio of gas mass to dynamical mass for several specific galaxies, including VII Zw 31, Arp 193, and Mrk 273.The paper by D. Downes and P. M. Solomon investigates the molecular gas distribution in ultraluminous infrared galaxies (ULIGs) using CO interferometer data. The authors find that the molecular gas is concentrated in rotating nuclear disks or rings, with radii ranging from 300 to 800 parsecs. The CO brightness temperatures, double-peaked line profiles, and constraints on thermal continuum flux indicate that the CO lines are subthermally excited and moderately opaque. They fit kinematic models where most of the CO flux comes from a moderate-density, warm intercloud medium rather than self-gravitating clouds. The derived gas masses are significantly lower than those estimated using standard CO-to-mass ratios, suggesting that the CO luminosity traces the geometric mean of the gas mass and dynamical mass. The study identifies compact extreme starburst regions within these ULIGs, with characteristic sizes of only 100 parsecs and masses of about $10^9$ M$_\odot$. These regions produce most of the HCN luminosity and 1mm dust continuum, while the diffuse gas in the nuclear disks dominates the CO luminosity. The authors conclude that the far-IR luminosity in ULIGs is powered by these extreme starbursts, not by dust-enshrouded quasars. The paper also discusses the rotation curves, disk geometry, and the ratio of gas mass to dynamical mass for several specific galaxies, including VII Zw 31, Arp 193, and Mrk 273.