Millimeter-wave thermal dust emission from luminous mergers

We have observed several infrared-luminous galaxies which are in the process of merging (Mkn 231, Arp 299, NGC 6240, NGC 520, and IRAS 17208-0014) at 1.2mm with the multi-channel bolometer at the IRAM 30-meter telescope. Such observations of the long-wavelength thermal dust emission provide an alternative to CO as a measure of the gas mass. Comparing the masses of gas determined from the dust emission and from CO observations of these galaxies, we find that the conversion factor N(H2)/ICO(1−0) < ∼ 0.5 1020 cm−2(K km s−1)−1. Our calculations are conservative. We have chosen a ν2 emissivity; other emission laws result in higher dust temperatures. We have made our calculations assuming a metallicity of 2Z , which is low for a galactic nucleus. We have not assumed any enhancement of the dust emission through agglomeration of dust particles nor destruction of grains. Finally, we have assumed that all of the gas associated with 1.2mm thermal dust emission is molecular. Expected deviations from these assumptions would all decrease the gas masses estimated from the thermal dust emission, resulting in even lower values for the N(H2)/ICO conversion factor.