Dense molecular gas in ultraluminous and high redshift galaxies

Abstract

Molecular gas is the raw material for star formation and hence a crucial factor in galactic evolution. Ultraluminous infrared galaxies emit the bulk of their power in the far infrared, show disturbed morphologies indicative of recent mergers, and rival QSOs in their bolometric luminosities, but are more numerous in the local universe. Although they are as rich in molecular gas as the most gas rich normal spiral galaxies, they have elevated ratios of infrared luminosity to molecular mass that suggest they are undergoing bursts of very rapid and efficient star formation. A survey of HCN(1→0) emission from ten ultraluminous and normal galaxies shows far infrared emission correlates better with the amount of dense, n(H₂)>10⁴ cm⁻³, molecular gas than with the total amount of molecular gas. The star formation efficiency appears to depend on the fraction of the molecular gas reservoir at high density.

The galaxy IRAS 10214+4724 at z=2.286 is perhaps the most luminous object in the universe. Observations of its CO(6→5), CO(4→3), and CO(3→2) lines indicate this galaxy has as much molecular gas as the total mass of the Milky Way. The molecular gas in 10214+4724 is both warmer and denser than that in the Galaxy and the normal gas to dust ratio suggests the abundances are nearly Solar. In the Milky Way, CO (6→5) is only observed in regions of high-mass star formation, so its presence in 10214+4724 implies the occurance of active star formation there. A map of the CO(3→2) emission with 2.3″ resolution shows a small source slightly extended EW with a deconvolved size of (10×4)±4h ⁻¹ kpc. The mass of molecular gas is comparable to the dynamical mass. This extraordinary primeval galaxy appears to have most of its mass in molecular gas and to be undergoing an extreme starburst that is generating metals with close to Solar abundances.